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Microprocessors in B&O consumer products
Category: Bang & Olufsen
What is a “virtual” loudspeaker? Part 3
#91.3 in a series of articles about the technology behind Bang & Olufsen
In Part 1 of this series, I talked about how a binaural audio signal can (hypothetically, with HRTFs that match your personal ones) be used to simulate the sound of a source (like a loudspeaker, for example) in space. However, to work, you have to make sure that the left and right ears get completely isolated signals (using earphones, for example).
In Part 2, I showed how, with enough processing power, a large amount of luck (using HRTFs that match your personal ones PLUS the promise that you’re in exactly the correct location), and a room that has no walls, floor or ceiling, you can get a pair of loudspeakers to behave like a pair of headphones using crosstalk cancellation.
There’s not much left to do to create a virtual loudspeaker. All we need to do is to:
- Take the signal that should be sent to a right surround loudspeaker (for example) and filter it using the HRTFs that correspond to a sound source in the location that this loudspeaker would be in. REMEMBER that this signal has to get to your two ears since you would have used your two ears to hear an actual loudspeaker in that location.
- Send those two signals through a crosstalk cancellation processing system that causes your two loudspeakers to behave more like a pair of headphones.
One nice thing about this system is that the crosstalk cancellation is only there to ensure that the actual loudspeakers behave more like headphones. So, if you want to create more virtual channels, you don’t need to duplicate the crosstalk cancellation processor. You only need to create the binaurally-processed versions of each input signal and mix those together before sending the total result to the crosstalk cancellation processor, as shown below.
This is good because it saves on processing power.
So, there are some important things to realise after having read this series:
- All “virtual” loudspeakers’ signals are actually produced by the left and right loudspeakers in the system. In the case of the Beosound Theatre, these are the Left and Right Front-firing outputs.
- Any single virtual loudspeaker (for example, the Left Surround) requires BOTH output channels to produce sound.
- If the delays (aka Speaker Distance) and gains (aka Speaker Levels) of the REAL outputs are incorrect at the listening position, then the crosstalk cancellation will not work and the virtual loudspeaker simulation system won’t work. How badly is doesn’t work depends on how wrong the delays and gains are.
- The virtual loudspeaker effect will be experienced differently by different persons because it’s depending on how closely your actual personal HRTFs match those predicted in the processor. So, don’t get into fights with your friends on the sofa about where you hear the helicopter…
- The listening room’s acoustical behaviour will also have an effect on the crosstalk cancellation. For example, strong early reflections will “infect” the signals at the listening position and may/will cause the cancellation to not work as well. So, the results will vary not only with changes in rooms but also speaker locations.
Finally, it’s worth nothing that, in the specific case of the Beosound Theatre, by setting the Speaker Distances and Speaker Levels for the Left and Right Front-firing outputs for your listening position, then you have automatically calibrated the virtual outputs. This is because the Speaker Distances and Speaker Levels are compensations for the ACTUAL outputs of the system, which are the ones producing the signal that simulate the virtual loudspeakers. This is the reason why the four virtual loudspeakers do not have individual Speaker Distances and Speaker Levels. If they did, they would have to be identical to the Left and Right Front-firing outputs’ values.
What is a “virtual” loudspeaker? Part 2
#91.2 in a series of articles about the technology behind Bang & Olufsen
In Part 1, I talked at how a binaural recording is made, and I also mentioned that the spatial effects may or may not work well for you for a number of different reasons.
Let’s go back to the free field with a single “perfect” microphone to measure what’s happening, but this time, we’ll send sound out of two identical “perfect” loudspeakers. The distances from the loudspeakers to the microphone are identical. The only difference in this hypothetical world is that the two loudspeakers are in different positions (measuring as a rotational angle) as shown in Figure 1.
In this example, because everything is perfect, and the space is a free field, then output of the microphone will be the sum of the outputs of the two loudspeakers. (In the same way that if your dog and your cat are both asking for dinner simultaneously, you’ll hear dog+cat and have to decide which is more annoying and therefore gets fed first…)
IF the system is perfect as I described above, then we can play some tricks that could be useful. For example, since the output of the microphone is the sum of the outputs of the two loudspeakers, what happens if the output of one loudspeaker is identical to the other loudspeaker, but reversed in polarity?
In this example, we’re manipulating the signals so that, when they add together, you nothing at the output. This is because, at any moment in time, the value of Loudspeaker 2’s output is the value of Loudspeaker 1’s output * -1. So, in other words, we’re just subtracting the signal from itself at the microphone and we get something called “perfect cancellation” because the two signals cancel each other at all times.
Of course, if anything changes, then this perfect cancellation won’t work. For example, if one of the loudspeakers moves a little farther away than the other, then the system is broken, as shown below.
Again, everything that I’ve said above only works when everything is perfect, and the loudspeakers and the microphone are in a free field; so there are no reflections coming in and ruining everything.
We can now combine these two concepts:
- using binaural signals to simulate a sound source in a location (although this would normally be done using playback over earphones to keep it simple) and
- using signals from loudspeakers to cancel each other at some location in space as a
to create a system for making virtual loudspeakers.
Let’s suspend our adherence to reality and continue with this hypothetical world where everything works as we want… We’ll replace the microphone with a person and consider what happens. To start, let’s just think about the output of the left loudspeaker.
If we plot the impulse responses at the two ears (the “click” sound from the loudspeaker after it’s been modified by the HRTFs for that loudspeaker location), they’ll look like this:
What if were were able to send a signal out of the right loudspeaker so that it cancels the signal from the left loudspeaker at the location of the right eardrum?
Unfortunately, this is not quite as easy as it sounds, since the HRTF of the right loudspeaker at the right ear is also in the picture, so we have to be a bit clever about this.
So, in order for this to work we:
- Send a signal out of the left loudspeaker.
We know that this will get to the right eardrum after it’s been messed up by the HRTF. This is what we want to cancel… - …so we take that same signal, and
- filter it with the inverse of the HRTF of the right loudspeaker
(to undo the effects of the HRTF of the right loudspeaker’s signal at the right ear) - filter that with the HRTF of the left loudspeaker at the right ear
(to match the filtering that’s done by your head and pinna) - multiply by -1
(so that it will cancel when everything comes together at your right eardrum) - and send it out the right loudspeaker.
- filter it with the inverse of the HRTF of the right loudspeaker
Hypothetically, that signal (from the right loudspeaker) will reach your right eardrum at the same time as the unprocessed signal from the left loudspeaker and the two will cancel each other, just like the simple example shown in Figure 3. This effect is called crosstalk cancellation, because we use the signal from one loudspeaker to cancel the sound from the other loudspeaker that crosses to the wrong side of your head.
This then means that we have started to build a system where the output of the left loudspeaker is heard ONLY in your left ear. Of course, it’s not perfect because that cancellation signal that I sent out of the right loudspeaker gets to the left ear a little later, so we have to cancel the cancellation signal using the left loudspeaker, and back and forth forever.
If, at the same time, we’re doing the same thing for the other channel, then we’ve built a system where you have the left loudspeaker’s signal in the left ear and the right loudspeaker’s signal in the right ear; just like a pair of headphones!
However, if you get any of these elements wrong, the system will start to under-perform. For example, if the HRTFs that I use to predict your HRTFs are incorrect, then it won’t work as well. Or, if things aren’t time-aligned correctly (because you moved) then the cancellation won’t work.
What is a “virtual” loudspeaker? Part 1
#91.1 in a series of articles about the technology behind Bang & Olufsen
Without connecting external loudspeakers, Bang & Olufsen’s Beosound Theatre has a total of 11 independent outputs, each of which can be assigned any Speaker Role (or input channel). Four of these are called “virtual” loudspeakers – but what does this mean? There’s a brief explanation of this concept in the Technical Sound Guide for the Theatre (you’ll find the link at the bottom of this page), which I’ve duplicated in a previous posting. However, let’s dig into this concept a little more deeply.
To begin, let’s put a “perfect” loudspeaker in a free field. This means that it’s in a space that has no surfaces to reflect the sound – so it’s an acoustic field where the sound wave is free to travel outwards forever without hitting anything (or at least appear as this is the case). We’ll also put a “perfect” microphone in the same space.
We then send an impulse; a very short, very loud “click” to the loudspeaker. (Actually a perfect impulse is infinitely short and infinitely loud, but this is not only inadvisable but impossible, and probably illegal.)
That sound radiates outwards through the free field and reaches the microphone which converts the acoustic signal back to an electrical one so we can look at it.
There are three things to notice when you compare Figure 3 to Figure 2:
- The signal’s level is lower. This is because the microphone is some distance from the loudspeaker.
- The signal is later. This is because the microphone is some distance from the loudspeaker and sound waves travel pretty slowly.
- The general shape of the signals are identical. This is because I said that the loudspeaker and the microphone were both “perfect” and we’re in a space that is completely free of reflections.
What happens if we take away the microphone and put you in the same place instead?
If we now send the same click to the loudspeaker and look at the “outputs” of your two eardrums (the signals that are sent to your brain), these will look something like this:
These two signals are obviously very different from the one that the microphone “hears” which should not be a surprise: ears aren’t microphones. However, there are some specific things of which we should take note:
- The output of the left eardrum is lower than that of the right eardrum. This is largely because of an effect called “head shadowing” which is exactly what it sounds like. The sound is quieter in your left ear because your head is in the way.
- The signal at the right eardrum is earlier than at the left eardrum. This is because the left eardrum is not only farther away, but the sound has to go around your head to get there.
- The signal at the right eardrum is earlier than the output of the microphone output (in Figure 3) because it’s closer to the loudspeaker. (I put the microphone at the location of the centre of the simulated head.) Similarly the left ear output is later because it’s farther away.
- The signal at the right eardrum is full of spikes. This is mostly caused by reflections off the pinna (the flappy thing on the side of your head that you call your “ear”) that arrive at slightly different times, and all add together to make a mess.
- The signal at the left eardrum is “smoother”. This is because the head itself acts as a filter reducing the levels of the high frequency content, which tends to make things less “spiky”.
- Both signals last longer in time. This is the effect of the ear canal (the “hole” in the side of your head that you should NOT stick a pencil in) resonating like a little organ pipe.
The difference between the signals in Figures 2 and 4 is a measurement of the effect that your head (including your shoulders, ears/pinnae) has on the transfer of the sound from the loudspeaker to your eardrums. Consequently, we geeks call it a “head-related transfer function” or HRTF. I’ve plotted this HRTF as a measurement of an impulse in time – but I could have converted it to a frequency response instead (which would include the changes in magnitude and phase for different frequencies).
Here’s the cool thing: If I put a pair of headphones on you and played those two signals in Figure 5 to your two ears, you might be able to convince yourself that you hear the click coming from the same place as where that loudspeaker is located.
Although this sounds magical, don’t get too excited right away. Unfortunately, as with most things in life, reality tends to get in the way for a number of reasons:
- Your head and ears aren’t the same shape as anyone else’s. Your brain has lived with your head and your ears for a long time, and it’s learned to correlate your HRTFs with the locations of sound sources. If I suddenly feed you a signal that uses my HRTFs, then this trick may or may not work, depending on how similar we are. This is just like borrowing someone else’s glasses. If you have roughly the same prescription, then you can see. However, if the prescriptions are very different, you’ll get a headache very quickly.
- In reality, you’re always moving. So, even if the sound source is not moving, the specific details of the HRTFs are always changing (because the relative positions and angles to your ears are changing) but my system doesn’t know about this – so I’m simulating a system where the loudspeaker moves around you as you rotate your head. Since this never happens in real life, it tends to break the simulation.
- The stuff I showed above doesn’t include reflections, which is how you determine distance to sources. If I wanted to include reflections, each reflection would have to have its own HRTF processing, depending on its angle relative to your head.
However, hypothetically, this can work, and lots of people have tried. The easiest way to do this is to not bother measuring anything. You just take a “dummy head” -a thing that is the same size as an average human head (maybe with an average torso) and average pinnae* – but with microphones where the eardrums are – and you plunk it down in a seat in a concert hall and record the outputs of the two “ears”. You then listen to this over earphones (we don’t use headphones because we want to remove your pinnae from the equation) and you get a “you are there” experience (assuming that the dummy head’s dimensions and shape are about the same as yours). This is what’s known as a binaural recording because it’s a recording that’s done with two ears (instead of two or more “simple” microphones).
If you want to experience this for yourself, plug a pair of headphones into your computer and do a search for the “Virtual Barber Shop” video. However, if you find that it doesn’t work for you, don’t be upset. It just means that you’re different: just like everyone else.* Typically, recordings like this have a strange effect of things sounding very close in the front, and farther away as sources go to the sides. (Personally, I typically don’t hear anything in the front. All of the sources sound like they’re sitting on the back of my neck and shoulders. This might be because I have a fat head (yes, yes… I know…) and small pinnae (yes, yes…. I know…) – or it might indicate some inherent paranoia of which I am not conscious.)
* Of course, depressingly typically, it goes without saying that the sizes and shapes of commercially-available dummy heads are based on averages of measurements of men only. Neither women nor children are interested in binaural recordings or have any relevance to such things, apparently…
Beosound Theatre: Virtual loudspeakers
#90 in a series of articles about the technology behind Bang & Olufsen
Devices such as the ‘stereoscope’ for representing photographs (and films) in three-dimensions have been around since the 1850s. These work by presenting two different photographs with slightly different perspectives two the two eyes. If the differences in the photographs are the same as the differences your eyes would have seen had you ‘been there’, then your brain interprets into a 3D image.
A similar trick can be done with sound sources. If two different sounds that exactly match the signals that you would have heard had you ‘been there’ are presented at your two ears (using a binaural recording) , then your brain will interpret the signals and give you the auditory impression of a sound source in some position in space. The easiest way to do this is to ensure that the signals arriving at your ears are completely independent using headphones.
The problem with attempting this with loudspeaker reproduction is that there is ‘crosstalk’ or ‘bleeding of the signals to the opposite ears’. For example, the sound from a correctly-positioned Left Front loudspeaker can be heard by your left ear and your right ear (slightly later, and with a different response). This interference destroys the spatial illusion that is encoded in the two audio channels of a binaural recording.
However, it might be possible to overcome this issue with some careful processing and assumptions. For example, if the exact locations of the left and right loudspeakers and your left and right ears are known by the system, then it’s (hypothetically) possible to produce a signal from the right loudspeaker that cancels the sound of the left loudspeaker in the right ear, and therefore you only hear the left channel in the left ear. (Of course, the cancelling signal of the right loudspeaker also bleeds to the left ear, so the left loudspeaker has to be used to cancel the cancellation signal of the right loudspeaker in the left ear, and so on…)
Using this ‘crosstalk cancellation’ processing, it becomes (hypothetically) possible to make a pair of loudspeakers behave more like a pair of headphones, with only the left channel in the left ear and the right in the right. Therefore, if this system is combined with the binaural recording / reproduction system, then it becomes (hypothetically) possible to give a listener the impression of a sound source placed at any location in space, regardless of the actual location of the loudspeakers.
Theory vs. Reality
It’s been said that the difference between theory and practice is that, in theory, there is no difference between theory and practice, whereas in practice, there is. This is certainly true both of binaural recordings (or processing) and crosstalk cancellation.
In the case of binaural processing, in order to produce a convincing simulation of a sound source in a position around the listener, the simulation of the acoustical characteristics of a particular listener’s head, torso, and (most importantly) pinnae (a.k.a. ‘ears’) must be both accurate and precise. (For the same reason that someone else should not try to wear my glasses.)
Similarly, a crosstalk cancellation system must also have accurate and precise ‘knowledge’ of the listener’s physical characteristics in order to cancel the signals correctly; but this information also crucially includes the exact locations of the
loudspeakers and the listener (we’ll conveniently pretend that the room you’re sitting in does not exist).
In the end, this means that a system with adequate processing power can use two loudspeakers to simulate a ‘virtual’ loudspeaker in another location. However, the details of that spatial effect will be slightly different from person to person (because we’re all shaped differently). Also, more importantly, the effect will only be experienced by a listener who is positioned correctly in front of the loudspeakers. Slight movements (especially from side-to-side, which destroys the symmetrical time-of-arrival matching of the two incoming signals) will cause the illusion to collapse.
Beosound Theatre gives you the option to choose Virtual Loudspeakers that appear to be located in four different positions: Left and Right Wide, and Left and Right Elevated. These signals are actually produced using the Left and Right front-firing outputs of the device using this combination of binaural processing and crosstalk cancellation in the Dolby Atmos processing system. If you are a single listener in the correct position (with the Speaker Distances and Speaker Levels adjusted correctly) then the Virtual outputs come very close to producing the illusion of correctly-located Surround and Front Height loudspeakers.
However, in cases where there is more than one listener, or where a single listener may be incorrectly located, it may be preferable to use the ‘side-firing’ and ‘up-firing’ outputs instead.
Keep your needle clean
One of my jobs at Bang & Olufsen is to do the final measurements on each bespoke Beogram 4000c turntable before it’s sent to the customer. Those measurements include checking the end-to-end magnitude response, playing from a vinyl record with a sine sweep on it (one per channel), recording that from the turntable’s line-level output, and analysing it to make sure that it’s as expected. Part of that analysis is to very that the magnitude responses of the left and right channel outputs are the same (or, same enough… it’s analogue, a world where nothing is perfect…)
Today, I was surprised to see this result on a turntable that was being inspected part-way through its restoration process :
Taken at face value, this should have resulted in a rejection – or at least some very serious questions. This is a terrible result, with unacceptable differences in output level between the two channels. When I looked at the raw measurements, I could easily see that the left channel was behaving – it was the right channel that was all over the place.
The black curve looks very much like what I would expect to see. This is the result of playing a track that is a sine sweep from 20 Hz to 20 kHz, where the signal below 1 kHz follows the RIAA curve, whereas the signal above 1 kHz does not. This is why, after it’s been filtered using a RIAA preamp, the low frequency portion has a flat response, but the upper frequency band rolls off (following the RIAA curve).
Notice that the right channel (the red curve) is a mess…
A quick inspection revealed what might have been the problem: a small ball of fluff collected around the stylus. (This was a pickup that was being used to verify that the turntable was behaving through the restoration – not the one intended for the final customer – and so had been used multiple times on multiple turntables.)
So, we used a stylus brush to clean off the fluff and ran the measurement again. The result immediately afterwards looked like this:
which is more like it! A left-right channel difference of something like ± 0.5 dB is perfectly acceptable.
The moral of the story: keep your pickup clean. But do it carefully! That cantilever is not difficult to snap.
Beosound Theatre: Spatial control
#89 in a series of articles about the technology behind Bang & Olufsen
There are many cases where the number of input channels in the audio signal does not match the number of loudspeakers in your configuration. For example, you may have two loudspeakers, but the input signal is from a multichannel source such as a 7.1-channel stream or a 7.1.4-channel Blu-ray. In this case, the audio must be ‘downmixed’ to your two loudspeakers if you are to hear all components of the audio signal. Conversely, you may have a full surround sound system with 7 main loudspeakers and a subwoofer (a 7.1-channel system) and you would like to re-distribute the two channels from a CD to all of your loudspeakers. In this example, the signal must be ‘upmixed’ to all loudspeakers.
Bang & Olufsen’s True Image is a processor that accomplishes both of these tasks dynamically, downmixing or upmixing any incoming signal so that all components and aspects of the original recording are played using all of your loudspeakers.
Of course, using the True Image processor means that signals in the original recording are re-distributed. For example, in an upmixing situation, portions in the original Left Front signal from the source will be sent to a number of loudspeakers in your system instead of just one left front loudspeaker. If you wish to have a direct connection between input and output channels, then the Processing should be set to ‘Direct’, thus disabling the True Image processing.
Note that, in Direct mode, there may be instances where some input or output channels will not be audible. For example, if you have two loudspeakers but a multichannel input, only two of the input channels will be audible. These channels are dependent on the speaker roles selected for the two loudspeakers. (For example, if your loudspeakers’ roles are Left Front and Right Front, then only the Left Front and Right Front channels from the multichannel source will be heard.)
Similarly, in Direct mode, if you have a multichannel configuration but a two-channel stereo input, then only the loudspeakers assigned to have the Left Front and Right Front speaker roles will produce the sound; all other loudspeakers will be silent.
If True Image is selected and if the number of input channels and their channel assignments matches the speaker roles, and if all Spatial Control sliders are set to the middle position, then the True Image processing is bypassed. For example, if you have a 5.1 loudspeaker system with 5 main loudspeakers (Left Front, Right Front, Centre Front, Left Surround, and Right Surround) and a subwoofer, and the Spatial Control sliders are in the middle positions, then a 5.1 audio signal (from a DVD, for example) will pass through unaffected.
However, if the input is changed to a 2.0 source (i.e. a CD or an Internet radio stream) then the True Image processor will upmix the signal to the 5.1 outputs.
In the case where you wish to have the benefits of downmixing without the spatial expansion provided by upmixing, you can choose to use the Downmix setting in this menu. For example, if you have a 5.1-channel loudspeaker configuration and you wish to downmix 6.1- and 7.1-channel sources (thus ensuring that you are able to hear all input channels) but that two-channel stereo sources are played through only two loudspeakers, then this option should be selected. Note that, in Downmix mode, there are two exceptions where upmixing may be applied to the signal. The first of these is when you have a 2.0-channel loudspeaker configuration and a 1-channel monophonic input. In this case, the centre front signal will be distributed to the Left Front and Right Front loudspeakers. The second case is when you have a 6.1 input and a 7.1 loudspeaker configuration. In this case, the Centre Back signal will be distributed to the Left Back and Right Back loudspeakers.
The Beosound Theatre includes four advanced controls (Surround, Height, Stage Width and Envelopment, described below) that can be used to customise the spatial attributes of the output when the True Image processor is enabled.
Surround
The Surround setting allows you to determine the relative levels of the sound stage (in the front) and the surround information from the True Image processor.
Changes in the Surround setting only have an effect on the signal when the Processing is set to True Image.
Height
This setting determines the level of the signals sent to all loudspeakers in your configuration with a ‘height’ Speaker Role. It will have no effect on other loudspeakers in your system.
If the setting is set to minimum, then no signal will be sent to the ‘height’ loudspeakers.
Changes in the Height setting only have an effect on the signal when the Processing is set to True Image.
Stage Width
The Stage Width setting can be used to determine the width of the front images in the sound stage. At a minimum setting, the images will collapse to the centre of the frontal image. At a maximum setting, images will be pushed to the sides of the front sound stage. This allows you to control the perceived width of the band or music ensemble without affecting the information in the surround and back loudspeakers.
If you have three front loudspeakers (Left Front, Right Front and Centre Front), the setting of the Stage Width can be customised according to your typical listening position. If you normally sit in the ‘sweet spot’, at roughly the same distance from all three loudspeakers, then you should increase the Stage Width setting somewhat, since it is unnecessary to use the centre front loudspeaker to help to pull phantom images towards the centre of the sound stage. The further to either side of the sweet spot that you are seated, the more reducing the Stage Width value will improve the centre image location.
Changes in the Stage Width setting only have an effect on the signal when the Processing is set to True Image.
Envelopment
The Envelopment setting allows you to set the desired amount of perceived width or spaciousness from your surround and back loudspeakers. At its minimum setting, the surround information will appear to collapse to a centre back phantom location. At its maximum setting, the surround information will appear to be very wide.
Changes in this setting have no effect on the front loudspeaker channels and only have an effect on the signal when the Processing is set to True Image.
One last point…
One really important thing to know about the True Image processor is that, if the input signal’s configuration matches the output, AND the 4 sliders described above are in the middle positions, then True Image does nothing. In other words, in this specific case, it’s the same as ‘Direct’ mode.
However, if there is a mis-match between the input and the output channel configuration (for example, 2.0 in and 5.1 out, or 7.1.4 in and 5.1.2 out) then True Image will do something: either upmixing or downmixing. Also, if the input configuration matches the output configuration (e.g. 5.x in and 5.x out) but you’ve adjusted any of the sliders, then True Image will also do something…
Beosound Theatre and Dolby Atmos
#88 in a series of articles about the technology behind Bang & Olufsen
In 2012, Dolby introduced its Dolby Atmos surround sound technology in movie theatres with the release of the Pixar movie, ‘Brave’, and support for the system was first demonstrated on equipment for home theatres in 2014. However, in spite of the fact that it has been 10 years since its introduction, it still helps to offer an introductory explanation to what, exactly, Dolby Atmos is. For more in-depth explanations, https://www.dolby.com/technologies/dolby-atmos is a good place to start and https://www.dolby.com/about/support/guide/speaker-setup-guides has a wide range of options for loudspeaker configuration recommendations.
From the perspective of audio / video systems for the home, Dolby Atmos can most easily be thought of as a collection of different things:
- a set of recommendations for loudspeaker configuration that can include loudspeakers located above the listening position
- a method of supplying audio signals to those loudspeakers that not only use audio channels that are intended to be played by a single loudspeaker (e.g. Left Front or Right Surround), but also audio objects whose intended spatial positions are set by the mixing engineer, but whose actual spatial position is ‘rendered’ based on the actual loudspeaker configuration in the customer’s listening room.
- a method of simulating the spatial positions of ‘virtual’ loudspeakers
- the option to use loudspeakers that are intentionally directed away from the listening position, potentially increasing the spatial effects in the mix. These are typically called ‘up-firing’ and ‘side-firing’ loudspeakers.
In addition to this, Dolby has other technologies that have been enhanced to be used in conjunction with Dolby Atmos-encoded signals. Arguably, the most significant of these is an upmixing / downmixing algorithm that can adapt the input signal’s configuration to the output channels.
Note that many online sites state that Dolby’s upmixing / downmixing processor is part of the Dolby Atmos system. This is incorrect. It’s a separate processor.
1. Loudspeaker configurations
Dolby’s Atmos recommendations allow for a large number of different options when choosing the locations of the loudspeakers in your listening room. These range from a simple 2.0.0, traditional two-channel stereo loudspeaker configuration up to a 24.1.10 large-scale loudspeaker array for movie theatres. The figures below show a small sampling of the most common options. (see https://www.dolby.com/about/support/guide/speaker-setup-guides/ for many more possibilities and recommendations.)
Standard loudspeaker configuration for 5.x multichannel audio. The actual angles of the surround loudspeakers at 110 degrees shows the reference placement used at Bang & Olufsen for testing and tuning. Note that the placement of the subwoofer is better determined by your listening room’s acoustics, but it is advisable to begin with a location near the centre front loudspeaker.
Loudspeaker positions associated with the speaker roles available in the Beosound Theatre, showing a full 7.x.4 configuration.
2. Channels and Objects
Typically, when you listen to audio, regardless of whether it’s monophonic or a stereo (remember that ‘stereo’ merely implies ‘more than one channel’) signal, you are reproducing some number of audio channels that were mixed in a studio. For example, a recording engineer placed a large number of microphones around a symphony orchestra or a jazz ensemble, and then decided on the mix (or relative balance) of those signals that should be sent to a loudspeakers in the left front and right front positions. They did this by listening to the mix through loudspeakers in a standard configuration with the intention that you place your loudspeakers similarly and sit in the correct location.
Consequently, each loudspeaker’s input can be thought of as receiving a ‘pre-packaged’ audio channel of information.
However, in the early 2000s, a new system for delivering audio to listeners was introduced with the advent of powerful gaming consoles. In these systems, it was impossible for the recording engineer to know where a sound should be coming from at any given moment in a game with moving players. So, instead of pre-mixing sound effects (like footsteps, for example) in a fixed position, a monophonic recording of the effect (the footsteps) was stored in the game’s software, and then the spatial position could be set at the moment of playback. So, if the footsteps should appear on the player’s left, then the game console would play them on the left. If the player then turned, the footsteps could be moved to appear in the centre or on the right. In this way different sound objects could be ‘rendered’ instead of merely being reproduced. Of course, the output of these systems was still either loudspeakers or headphones; so the rendered sound objects were mixed with the audio channels (e.g. background music) before being sent to the outputs.
The advantage of a channel-based system is that there is (at least theoretically) a 1:1 match between what the recording or mastering engineer heard in the studio, and what you are able to hear at home. The advantage of an object-based system is that it can not only adapt to the listener’s spatial changes (e.g. the location and rotation of a player inside a game environment) but also to changes in loudspeaker configurations. Change the loudspeakers, and you merely tell the system to render the output differently.
Dolby’s Atmos system merges these two strategies, delivering audio content using both channel-based and object-based streams. By delivering audio channels that match older systems, it becomes possible to have a mix on a newly-released movie that is compatible with older playback systems. However, newer Dolby Atmos-compatible systems can render the object-based content as well, optimising the output for the particular configuration of the loudspeakers.
3. Virtual Loudspeakers
Dolby’s Atmos processing includes the option to simulate loudspeakers in ‘virtual’ locations using ‘real’ loudspeakers placed in known locations. Beosound Theatre uses this Dolby Atmos processing to generate the signals used to create four virtual outputs. (This is discussed in detail in another posting.)
4. Up-firing and Side-firing Loudspeakers
A Dolby Atmos-compatible soundbar or loudspeaker can also include output beams that are aimed away from instead of towards the listening position; either to the sides or above the loudspeakers.
These are commonly known as ‘up-firing’ and ‘side-firing’ loudspeakers. Although Beosound Theatre gives you the option of using a similar concept, it is not merely implemented with a single loudspeaker driver, but with a version of the Beam Width and Beam Direction control used in other high-end Bang & Olufsen loudspeakers. This means that, when using the up-firing and side-firing outputs, more than a single loudspeaker driver is being used to produce the sound. This helps to reduce the beam width, reducing the level of the direct sound at the listening position, which, in turn can help to enhance the spatial effects that can be encoded in a Dolby Atmos mix.
Upmixing and Downmixing
There are cases where an incoming audio signal was intended to be played by a different number of loudspeakers than are available in the playback system. In some cases, the playback uses fewer loudspeakers (e.g. when listening to a two-channel stereo recording on the single loudspeaker on a mobile phone). In other cases, the playback system has more loudspeakers (e.g. when listening to a monophonic news broadcast on a 5.1 surround sound system). When the number of input channels is larger than the number of outputs (typically loudspeakers), the signals have to be downmixed so that you are at least able to hear all the content, albeit at the price of spatially-distorted reproduction. (For example, instruments will appear to be located in incorrect locations, and the spaciousness of a room’s reverberation may be lost.) When the number of output channels (loudspeakers) is larger than the number of input channels, then the system may be used to upmix the signal.
The Dolby processor in a standard playback device has the capability of performing both of these tasks: either upmixing or downmixing when required (according to both the preferences of the listener). One particular feature included in this processing is the option for a mixing engineer to ‘tell’ the playback system exactly how to behave when doing this. For example, when downmixing a 5.1-channel movie to a two-channel output, it may be desirable to increase the level of the centre channel to increase the level of the dialogue to help make it more intelligible. Dolby’s encoding system gives the mixing engineer this option using ‘metadata’; a set of instructions defining the playback system’s behaviour so that it behaves as intended by the artist (the mixing engineer). Consequently, the Beosound Theatre gives you the option of choosing the Downmix mode, where this processing is done exclusively in the Dolby processor.
However, there are also cases where you may also wish to upmix the signal to more loudspeakers than there are input channels from the source material. For these situations, Bang & Olufsen has developed its own up-/down-mixing processor called True Image, which I’ll discuss in more detail in another posting.
Beosound Theatre: A very short history lesson
#87 in a series of articles about the technology behind Bang & Olufsen
Once upon a time, in the incorrectly-named ‘Good Old Days’, audio systems were relatively simple things. There was a single channel of audio picked up by a gramophone needle or transmitted to a radio, and that single channel was reproduced by a single loudspeaker. Then one day, in the 1930s, a man named Alan Blumlein was annoyed by the fact that, when he was watching a film at the cinema and a character moved to one side of the screen, the voice still sounded like it was coming from the centre (because that’s where the loudspeaker was placed). So, he invented a method of simultaneously reproducing more than one channel of audio to give the illusion of spatial changes. (See Patent GB394325A:Improvements in and relating to sound-transmission, sound-recording and sound-reproducing systems’) Eventually, that system was called stereophonic audio.
The word stereo’ first appeared in English usage in the late 1700s, borrowed directly from the French word ‘stéréotype’: a combination of the Greek word στερεó (roughly pronounced ‘stereo’) meaning ‘solid’ and the Latin ‘typus’ meaning ‘form’. ‘Stereotype’ originally meant a letter (the ‘type’) printed (e.g. on paper) using a solid plate (the ‘stereo’). In the mid-1800s, the word was used in ‘stereoscope’ for devices that gave a viewer a three-dimensional visual representation using two photographs. So, by the time Blumlein patented spatial improvements in audio recording and reproduction in the 1930s, the word had already been in used to mean something akin to ‘a three-dimensional representation of something’ for over 80 years.
Over the past 90 years, people have come to mis-understand that ‘stereo’ audio implies only two channels, but this is incorrect, since Blumlein’s patent was also for three loudspeakers, placed on the left, centre, and right of the cinema screen. In fact, a ‘stereo’ system simply means that it uses more than one audio channel to give the impression of sound sources with different locations in space. So it can easily be said that all of the other names that have been used for multichannel audio formats (like ‘quadraphonic’, ‘surround sound’, ‘multichannel audio’, and ‘spatial audio’, just to name a few obvious examples) since 1933 are merely new names for ‘stereo’ (a technique that we call ‘rebranding’ today).
At any rate, most people were introduced to `stereophonic’ audio either through a two-channel LP, cassette tape, CD, or a stereo FM radio receiver. Over the years, systems with more and more audio channels have been developed; some with more commercial success than others. The table below contains a short list of examples.
Some of the information in that list may be surprising, such as the existence of a 7-channel audio system in the 1950s, for example. Another interesting thing to note is the number of multichannel formats that were not `merely’ an accompaniment to a film or video format.
One problem that is highlighted in that list is the confusion that arises with the names of the formats. One good example is ‘Dolby Digital’, which was introduced as a name not only for a surround sound format with 5.1 audio channels, but also the audio encoding method that was required to deliver those channels on optical film. So, by saying ‘Dolby Digital’ in the mid-1990s, it was possible that you meant one (or both) of two different things. Similarly, although SACD and DVD-Audio were formats that were capable of supporting up to 6 channels of audio, there was no requirement and therefore no guarantee that the content be multichannel or that the LFE channel actually contain low-frequency content. This grouping of features under one name still causes confusion when discussing the specifics of a given system, as we’ll discuss below in the section on Dolby Atmos.
| Format | Introduced | Channels | Note |
| Edison phonograph cylinders | 1896 | 1 | |
| Berliner gramophone record | 1897 | 1 | |
| Wire recorder | 1898 | 1 | |
| Optical (on film) | ca. 1920 | 1 | |
| Magnetic Tape | 1928 | 2 | |
| Fantasound | 1940 | 3 / 54 | 3 channels through 54 loudspeakers |
| Cinerama | 1950s | 7.1 | Actually 7 channels |
| Stereophonic LP | 1957 | 2 | |
| Compact Cassette | 1963 | 2 | |
| Q-8 magnetic tape cartridge | 1970 | 4 | |
| CD-4 Quad LP | 1971 | 4 | |
| SQ (Stereo Quadraphonic) LP | 1971 | 4 | |
| IMAX | 1971 | 5.1 | |
| Dolby Stereo | 1975 | 4 | aka Dolby Surround |
| Compact Disc | 1983 | 2 | |
| DAT | 1987 | 2 | |
| Mini-disc | 1992 | 2 | |
| Digital Compact Cassette | 1992 | 2 | |
| Dolby Digital | 1992 | 5.1 | |
| DTS Coherent Acoustics | 1993 | 5.1 | |
| Sony SDDS | 1999 | 7.1 | |
| SACD | 1999 | 5.1 | Actually 6 full-band channels |
| Dolby EX | 1999 | 6.1 | |
| Tom Holman / TMH Labs | 1999 | 10.2 | |
| DVD-Audio | 2000 | 5.1 | Actually 6 full-band channels |
| DTS ES | 2000 | 6.1 | |
| NHK Ultra-high definition TV | 2005 | 22.2 | |
| Auro 3D | 2005 | 9.1 to 26.1 | |
| Dolby Atmos | 2012 | up to 24.1.10 |
Looking at the column listing the number of audio channels in the different formats, you may have three questions:
- Why does it say only 4 channels for Dolby Stereo? I saw Star Wars in the cinema, and I remember a LOT more loudspeakers on the walls.
- What does the `.1′ mean? How can you have one-tenth of an audio channel?
- Some of the channel listings have one or two numbers and some have three, which I’ve never seen before. What do the different numbers represent?
Input Channels and Speaker Roles
A ‘perfect’ two-channel stereo system is built around two matched loudspeakers, one on the left and the other on the right, each playing its own dedicated audio channel. However, when better sound systems were developed for movie theatres, the engineers (starting with Blumlein) knew that it was necessary to have more than two loudspeakers because not everyone is sitting in the middle of the theatre. Consequently, a centre loudspeaker was necessary to give off-centre listeners the impression of speech originating in the middle of the screen. In addition, loudspeakers on the side and rear walls helped to give the impression of envelopment for effects such as rain or crowd noises.
It is recommended but certainly not required, that a given Speaker Role should only be directed to one loudspeaker. In a commercial cinema, for example, a single surround channel is most often produced by many loudspeakers arranged on the side and rear walls. This can also be done in larger home installations where appropriate.
Similarly, in cases where the Beosound Theatre is accompanied by two larger front loudspeakers, it may be preferable to use the three front-firing outputs to all produce the Centre Front channel (instead of using the centre output only).
x.1 ?
The engineers also realised that it was not necessary that all the loudspeakers be big, and therefore requiring powerful amplifiers. This is because larger loudspeakers are only required for high-level content at low frequencies, and we humans are terrible at locating low-frequency sources when we are indoors. This meant that effects such as explosions and thunder that were loud, but limited to the low-frequency bands could be handled by a single large unit instead; one that handled all the content below the lowest frequencies capable of being produced by the other loudspeakers’ woofers. So, the systems were designed to rely on a powerful sub-woofer that driven by a special, dedicated Low Frequency Effects (or LFE) audio channel whose signals were limited up to about 120 Hz. However, as is discussed in the section on Bass Management, it should not be assumed that the LFE input channel is only sent to a subwoofer; nor that the only signal produced by the subwoofer is the LFE channel. This is one of the reasons it’s important to keep in mind that the LFE input channel and the subwoofer output channel are separate concepts.
Since the LFE channel only contains low frequency content, it has only a small fraction of the bandwidth of the main channels. (‘Bandwidth’ is the total frequency width of the signal. In the case of the LFE channel it is up to about 120 Hz. (In fact, different formats have different bandwidths for the LFE channel, but 120 Hz is a good guess.) In the case of a main channel, it is up to about 20,000 Hz; however these values are not only fuzzy but dependent on the specifications of distribution format, for example.) Although that fraction is approximately 120/20000, we generously round it up to 1/10 and therefore say that, relative to a main audio channel like the Left Front, the LFE signal is only 0.1 of a channel. Consequently, you’ll see audio formats with something like ‘5.1 channels’ meaning `5 main channels and an LFE channel’. (This is similar to the way rental apartments are listed in Montr\’eal, where it’s common to see a description such as a 3 1/2; meaning that it has a living room, kitchen, bedroom, and a bathroom (which is obviously half of a room).)
LFE ≠ Subwoofer
Many persons jump to the conclusion that an audio input with an LFE channel (for example, a 5.1 or a 7.1.4 signal) means that there is a ‘subwoofer’ channel; or that a loudspeaker configuration with 5 main loudspeakers and a subwoofer is a 5.1 configuration. It’s easy to make this error because those are good descriptions of the way many systems have worked in the past.
However, systems that use bass management break this direct connection between the LFE input and the subwoofer output. For example, if you have two large loudspeakers such as Beolab 50s or Beolab 90s for your Lf / Rf pair, it may not be necessary to add a subwoofer to play signals with an LFE channel. In fact, in these extreme cases, adding a subwoofer could result in downgrading the system. Similarly, it’s possible to play 2.0-channel signal through a system with two smaller loudspeakers and a single subwoofer.
Therefore, it’s important to remember that the ‘x.1’ classification and the discussion of an ‘LFE’ channel are descriptions of the input signal. The output may or may not have one or more subwoofers; and these two things are essentially made independent of each other using a bass management system.
x.y.4 ?
If you look at the table above, you’ll see that some formats have very large numbers of channels, however, these numbers can be easily mis-interpreted. For example, in both the ‘10.2’ and ‘22.2’ systems, some of the audio channels are intended to be played through loudspeakers above the listeners, but there’s no way to know this simply by looking at the number of channels. This is why we currently use a new-and-improved method of listing audio channels with three numbers instead of two.
- The first number tells you how many ‘main’ audio channels there are. In an optimal configuration, these should be reproduced using loudspeakers at the listeners’ ear heights.
- The second number tells you how many LFE channels there are.
- The third number tells you how many audio channels are intended to be reproduced by loudspeakers located above the listeners.
For example, you’ll notice looking at the table below, that a 7.1.4 channel system contains seven main channels around the listeners, one LFE channel, and four height channels.
| Speaker Role | 2.0 | 5.1 | 7.1 | 7.1.4 |
| Left Front | x | x | x | x |
| Right Front | x | x | x | x |
| Centre Front | x | x | x | |
| LFE | x | x | x | |
| Left Surround | x | x | x | |
| Right Surround | x | x | x | |
| Left Back | x | x | ||
| Right Back | x | x | ||
| Left Front Height | x | |||
| Right Front Height | x | |||
| Left Surround Height | x | |||
| Right Surround Height | x |
It is worth noting here that the logic behind the Bang & Olufsen naming system is either to avoid having duplicate letters for different role assignments, or to reserve options for future formats. For example, ‘Back’ is used instead of ‘Rear’ to prevent confusion with ‘Right, and ‘Height’ is used instead of ‘Top’ because another, higher layer of loudspeakers may be used in future formats. (The ‘Ceiling’ loudspeaker channel used in multichannel recordings from Telarc is an example of this.)
Beosound Theatre: Outputs
#86 in a series of articles about the technology behind Bang & Olufsen
Beosound Theatre has a total of 11 possible outputs, seven of which are “real” or “internal” outputs and four of which are “virtual” loudspeakers. As with all current Beovision televisions, any input channel can be directed to any output by setting the Speaker Roles in the menus.
Internal outputs
On first glance of the line drawing above it is easy to jump to the conclusion that the seven real outputs are easy to find, however this would be incorrect. The Beosound Theatre has 12 loudspeaker drivers that are all used in some combination of level and phase at different frequencies to all contribute to the total result of each of the seven output channels.
So, for example, if you are playing a sound from the Left front-firing output, you will find that you do not only get sound from the left tweeter, midrange, and woofer drivers as you might in a normal soundbar. There will also be some contribution from other drivers at different frequencies to help control the spatial behaviour of the output signal. This Beam Width control is similar to the system that was first introduced by Bang & Olufsen in the Beolab 90. However, unlike the Beolab 90, the Width of the various beams cannot be changed in the Beosound Theatre.
The seven internal loudspeaker outputs are
- Front-firing: Left, Centre, and Right
- Side-firing: Left and Right
- Up-firing: Left and Right
Looking online, you may find graphic explanations of side-firing and up-firing drivers in other loudspeakers. Often, these are shown as directing sound towards a reflecting wall or ceiling, with the implication that the listener therefore hears the sound in the location of the reflection instead. Although this is a convenient explanation, it does not necessarily match real-life experience due to the specific configuration of your system and the acoustical properties of the listening room.
The truth is both better and worse than this reductionist view. The bad news is that the illusion of a sound coming from a reflective wall instead of the loudspeaker can occur, but only in specific, optimised circumstances. The good news is that a reflecting surface is not strictly necessary; therefore (for example) side-firing drivers can enhance the perceived width of the loudspeaker, even without reflecting walls nearby.
However, it can be generally said that the overall benefit of side- and up-firing loudspeaker drivers is an enhanced impression of the overall width and height of the sound stage, even for listeners that are not seated in the so-called “sweet spot” (see Footnote 1) when there is appropriate content mixed for those output channels.
Virtual outputs
Devices such as the “stereoscope” for representing photographs (and films) in three-dimensions have been around since the 1850s. These work by presenting two different photographs with slightly different perspectives two the two eyes. If the differences in the photographs are the same as the differences your eyes would have seen had you “been there”, then your brain interprets into a 3D image.
A similar trick can be done with sound sources. If two different sounds that exactly match the signals that you would have heard had you “been there” are presented at your two ears (using a binaural recording) , then your brain will interpret the signals and give you the auditory impression of a sound source in some position in space. The easiest way to do this is to ensure that the signals arriving at your ears are completely independent using headphones.
The problem with attempting this with loudspeaker reproduction is that there is “crosstalk” or “bleeding of the signals to the opposite ears”. For example, the sound from a correctly-positioned Left Front loudspeaker can be heard by your left ear and your right ear (slightly later, and with a different response). This interference destroys the spatial illusion that is encoded in the two audio channels of a binaural recording.
However, it might be possible to overcome this issue with some careful processing and assumptions. For example, if the exact locations of the left and right loudspeakers and your left and right ears are known by the system, then it’s (hypothetically) possible to produce a signal from the right loudspeaker that cancels the sound of the left loudspeaker in the right ear, and therefore you only hear the left channel in the left ear. (see Footnote 2)
Using this “crosstalk cancellation” processing, it becomes (hypothetically) possible to make a pair of loudspeakers behave more like a pair of headphones, with only the left channel in the left ear and the right in the right. Therefore, if this system is combined with the binaural recording / reproduction system, then it becomes (hypothetically) possible to give a listener the impression of a sound source placed at any location in space, regardless of the actual location of the loudspeakers.
Theory vs. Reality
It’s been said that the difference between theory and practice is that, in theory, there is no difference between theory and practice, whereas in practice, there is. This is certainly true both of binaural recordings (or processing) and crosstalk cancellation.
In the case of binaural processing, in order to produce a convincing simulation of a sound source in a position around the listener, the simulation of the acoustical characteristics of a particular listener’s head, torso, and (most importantly) pinnæ (a.k.a. “ears”) must be both accurate and precise. (see Footnote 3)
Similarly, a crosstalk cancellation system must also have accurate and precise “knowledge” of the listener’s physical characteristics in order to cancel the signals correctly; but this information also crucially includes the exact locations of the loudspeakers and the listener (we’ll conveniently pretend that the room you’re sitting in does not exist).
In the end, this means that a system with adequate processing power can use two loudspeakers to simulate a “virtual” loudspeaker in another location. However, the details of that spatial effect will be slightly different from person to person (because we’re all shaped differently). Also, more importantly, the effect will only be experienced by a listener who is positioned correctly in front of the loudspeakers. Slight movements (especially from side-to-side, which destroys the symmetrical time-of-arrival matching of the two incoming signals) will cause the illusion to collapse.
Beosound Theatre gives you the option to choose Virtual Loudspeakers that appear to be located in four different positions: Left and Right Wide, and Left and Right Elevated. These signals are actually produced using the Left and Right front-firing outputs of the device using this combination of binaural processing and crosstalk cancellation in the Dolby Atmos processing system. If you are a single listener in the correct position (with the Speaker Distances and Speaker Levels adjusted correctly) then the Virtual outputs come very close to producing the illusion of correctly-located Surround and Front Height loudspeakers.
However, in cases where there is more than one listener, or where a single listener may be incorrectly located, it may be preferable to use the “side-firing” and “up-firing” outputs instead.
Wrapping up
As I mentioned at the start, Beosound Theatre on its own has 11 outputs:
- Front-firing: Left, Centre, and Right
- Side-firing: Left and Right
- Up-firing: Left and Right
- Virtual Wide: Left and Right
- Virtual Elevated: Left and Right
In addition to these, there are 8 wired Power Link outputs and 8 Wireless Power Link outputs for connection to external loudspeakers, resulting in a total of 27 possible output paths. And, as is the case with all Beovision televisions since Beoplay V1, any input channel (or output channel from the True Image processor) can be directed to any output, giving you an enormous range of flexibility in configuring your system to your use cases and preferences.
1. In the case of many audio playback systems, the “sweet spot” is directly in front of the loudspeaker pair or at the centre of the surround configuration. In the case of a Bang & Olufsen system, the “sweet spot” is defined by the user with the help of the Speaker Distance and Speaker Level adjustments.
2. Of course, the cancelling signal of the right loudspeaker also bleeds to the left ear, so the left loudspeaker has to be used to cancel the cancellation signal of the right loudspeaker in the left ear, and so on…
3. For the same reason that someone else should not try to wear my glasses.