Clinging to long-standing fallacies

Stephen Colbert once said that George W. Bush was a man of conviction. He believed the same thing on Wednesday that he believed on Monday, no matter what happened on Tuesday….

Often people ask me questions about sound quality. In “the old days”, it was something like “which is better, analogue or digital?” Later, it became “which is better, MP3 or Ogg Vorbis (or something else)?”. These days, it’s something like “which streaming service has the best quality?” or “Is high-resolution audio really worth it?” Or, it’s a more general question like “what loudspeaker (or headphones) would you recommend?”

My answer to these questions is always the same. It’s a combination of “it depends….” and “whatever I tell you today, it might be different tomorrow…”Something that is true on Monday may not still be true on Wednesday…

Recently, during a discussion about something else, I told someone that many, if not most, mobile devices will clip (and therefore) distort a signal if you try to boost it (e.g. with a “bass boost” or a “pop” setting instead of playing the signals “flat”), but they won’t if you cut. Therefore, on a mobile device, it’s smarter to cut than to boost a signal.

I made that statement based on some past measurements that were done that showed that, if you put a 0 dB FS signal at a low frequency (say, 80 Hz) on different mobile devices, and turned the “bass boost” (or equivalent, such as a “pop” or “rock” setting) on, the signal would be often be clipped, and therefore it would increase the level of distortion – sometimes quite dramatically. The measurements also showed that this was independent of volume setting. So, turning down the level didn’t help – it just made things quieter, but maintained the same THD value. This is likely because the processing in such devices & software was done (a) before the volume control and (b) in a fixed-point system that does not have a carefully-managed headroom.

 

Fig 1. The minijack headphone output of a mobile device that was measured a long time (about 2 years) ago. This was done at a maximum volume setting  (notice the reading of 2.38 V peak-to-peak in the centre bottom of the display) playing a .wav file with an 80 Hz sine wave (displayed on the bottom left). The EQ in the device was set to “Flat”.

 

Fig 2. The output of the same device at the same maximum volume setting, The EQ was set to boost the bass signals. Notice that the signal is now clipped.

 

Fig 3. The output of the same device at a lower volume setting (notice that the Peak-to-Peak measurement shows 291 mV on the centre bottom of the display), The EQ was set to boost the bass signals. Notice that the signal is still clipped as much as it was at a high volume setting – only a little extra noise is visible, because we’re closer to the noise floor of the device.

 

So, just to be sure that this was still true on newer devices and software, I did a couple of quick measurements. I put a logartihmically-swept sine wave with a level of 0 dB FS and a frequency range of 20 Hz to 2 kHz on a current mobile audio device with a minijack headphone output. With the output volume at maximum and the EQ set to “OFF” or “FLAT”, I recorded the output and did a little analysis.

Fig 4. A plot of the absolute value of the signal as it is swept in frequency from 20 Hz to 2000 Hz. Notice that the y-axis is zoomed in to a total range of only 2 dB.

Although Figure 4 is a plot of the time-domain output of the system (in other words, I’m just plotting 20*log10(abs(signal)), it can be read as a frequency response plot (which is why I’ve labelled it that way on the x-axis). Actually, though, I have to be explicit and say that we’re actually looking at the absolute value of the signal itself, and the y-axis is labelled as the frequency at the time of the signal coming in.

If we zoom in on the signal, we get the plot shown in Figure 5.

Fig 5. Zooming in to the plot in figure 1, when the signal is sweeping between 40 Hz and 41 Hz. I’m looking for clipping, but I don’t see anything worth worrying about.

 

Then I turned the bass boost setting to “ON” and repeated the recording, without changing anything else. The result is shown in Figure 6.

Fig 6. the equivalent of Figure 1, but with bass boost set to ON on the mobile device. Again, this can be read as a frequency response plot – but notice that the vertical scale is now 20 dB.

Zooming in on the same 40 Hz region, we see the following:

Fig 7. An equivalent to Figure 2 – but with the bass boost set to ON on the mobile device. Again, it’s nice to see that there is no significant clipping…

So, as can be seen in those plots, the clipping problem that used to be quite obvious in some mobile devices has been corrected on this newer device due to a difference in the way the signal processing is implemented in the software.

However, as can be seen in Figure 6, this solution to the problem was to drop the midrange and treble by about 6 dB. (It’s also interesting that we lose almost 6  dB at 20 Hz when we think that we are boosting the “bass” – but that’s another discussion…) This might be considered to be a smart solution, since the listener can just turn up the level to compensate for the loss, if they wish. However, it does mean that, on this particular device, with this particular software version, if you do turn on the “bass boost” function, you’ll lose about 6 dB from your maximum level (which is equivalent to 4 times the sound power) in the midrange and high frequency regions (say, from about 600 Hz and up, give or take…). So, distortion has been traded for a lower maximum output level in the comparison between these two devices and software versions, two years apart.

So, generally speaking, it seems that I have fallen victim to exactly the problem that I often warn people to avoid. I believed the same thing on Wednesday that I believed on Monday… Then again, I know for certain that there are many people who are still walking around with the same distorting software / device that I complained in the “old days” (those first set of measurements are only 2 years old…). And, if you’re concerned about maximum output levels, the “new” solution might also not be optimal for your preferences.

So, it seems that “it depends” is still the safest answer…

  1. Millemissen says:

    ‘Bass boost’ – sure, a lot of people adore that.
    Reading your article, I thought of the well-known-switch on the BeoLabs.
    A lot of people (again) prefer to set it to free even with the speaker in a corner….to boost the bass.
    There must be something similar going on with the sound we/they hear then?
    Or am I on a wrong track?

    Greetings
    MM

  2. Hi MM,

    Although the Free/Wall/Corner switch will change the low frequency response of the BeoLAb loudspeakers, the difference between those and the devices shown in this posting is the dynamic protection – particularly ABL.

    So, at a low level, the F/W/C switch will result in a similar effect, however, at a high level, the ABL will dynamically change this response, depending on the frequency-dependent level of the signal.

    Cheers
    -geoff

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