Click here to purchase the entire book in PDF format. What a compressor does. So you're out for a drive in your car, listening to some classical music played by an orchestra on your car's CD player. The piece starts off very quietly, so you turn up the volume because you really love this part of the piece and you want to hear it over the noise of your engine. Then, as the music goes on, it gets louder and louder because that's what the composer wanted. The problem is that you've got the stereo turned up to hear the quiet sections, so these new loud sections are really loud - so you turn down your stereo. Then, the piece gets quiet again, so you turn up the stereo to compensate. What you are doing is to manipulate something called the ``dynamic range'' of the piece. In this case, the dynamic range is the difference in level between the softest and the loudest parts of the piece (assuming that you're not mucking about with the volume knob). By fiddling with your stereo, you're making the soft sounds louder and the loud sounds softer, and therefore compressing the dynamics. The music still appears to have quiet sections and loud sections, but they're not as different as they were without your fiddling. In essence, this is what a compressor does - at the most basic level, it makes loud sounds softer and soft sounds louder so that the music going through it has a smaller (or compressed) dynamic range. Of course, I'm oversimplifying, but we'll straighten that out. Let's look at the gain response of an ideal piece of wire. This can be shown as a transfer function as seen in Figure 6.21. Figure 6.21: The gain response or transfer function of a device with a gain of 1 for all input levels. Essentially, output = input. Now, let's look at the gain response for a simple device that behaves as an oversimplified compressor. Let's say that, for a sine wave coming in at 0 dBV (1 Vrms, remember?) the device has a gain of 1 (or output=input). Let's also say that, for every 2 dB increase in level at the input, the gain of this device is reduced by 1 dB - so, if the input level goes up by 2 dB, the output only goes up by 1 dB (because it's been reduced by 1 dB, right?) Also, if the level at the input goes down by 2 dB, the gain of the device comes up by 1 dB, so a 2 dB drop in level at the input only results in a 1 dB drop in level at the output. This generally makes the soft sounds louder than when they went in, the loud sounds softer than when they went in, and anything at 0 dBV come out at exactly the same level as it goes in. Figure 6.22: The gain response (or transfer function) of a device with a different gain for different input levels. Note that a 2 dB rise in level at the input results in a 1 dB rise in level at the output. If we compare the change in level at the input to the change in level at the output, we have a comparison bewteen the original dynamic range and the new one. This comparison is expressed as a ratio of the change in input level in decibels to change in output level in decibels. So, if the output goes up 2 dB for every 1 dB increase in level at the input, then we have a 2:1 compression ratio. The higher the compression ratio, the greater the effect on the dynamic range. Notice in Figure 6.22 that there is one input level (in this case, 0 dBV) that results in a gain of 1 - that is to say that the output is equal to the input. That input level is known as the rotation point of the compressor. The reason for this name isn't immediately obvious in Figure 6.22, but if we take a look at a number of different compression ratios plotted on the same graph as in Figure 3, then the reason becomes clear. Figure 6.23: The gain response of various compression ratios with the same rotation point (at 0 dBV). Blue = 2:1 compression ratio, red = 3:1, green = 5:1, black = 10:1. Normally, a compressor doesn't really behave in the way that's seen in any of the above diagrams. If we go back to thinking about listening to the stereo in the car, we actually leave the volume knob alone most of the time, and only turn it down during the really loud parts. This is the way we want the compressor to behave. We'd like to leave the gain at one level (let's say, at 1) for most of the program material, but if things get really loud, we'll start turning down the gain to avoid letting things get out of hand. The gain response of such a device is shown in Figure 6.24. Figure 6.24: A device which exhibits unity gain for input signals with a level of less than 0 dBV and a compression of 2:1 for input signals with a level of greater than 0 dBV. The level where we change from being a linear gain device (meaning that the gain of the device is the same for all input levels) to being a compressor is called the threshold. Below the threshold, the device applies the same gain to all signal levels. Above the threhold, the device changes its gain according to the input level. This sudden bend in the transfer function at the threshold is called the knee in the response. In the case of the plot shown in Figure 6.24, the rotation point of the compressor is the same as the threshold. This is not necessarily the case, however. If we look at Figure 6.25, we can see an example of a curve where this is illustrated. Figure 6.25: An example of a device where the threshold is not the rotation point. The threshold is 0 dBV and the rotation point is 10 dBV. This device applies a gain of 5 dB to all signals below the threshold, so an input level of -20 dBV results in an output of -15 dBV and an input at -10 dBV results in an output of -5 dBV. Notice that the threshold is still at 0 dBV (because it is the input level over which the device changes its behaviour). However, now the rotation point is at 10 dBV. Let's look at an example of a compressor with a gain of 1 below threshold, a threshold at 0 dBV and different compression ratios. The various curves for such a device are shown in Figure 6.26. Notice that, below the threshold, there is no difference in any of the curves. Above the threshold, however, the various compression ratios result in very different behaviours. Figure 6.26: A plot showing a number of curves representing various settings of the compression ratio with a unity gain below threshold and a threshold of 0 dBV. red = 1.25:1, blue = 2:1, green = 4:1, black = 10:1. There are two basic ``styles'' in compressor design when it comes to the threshold. Some manufacturers like to give the user control over the threshold level itself, allowing them to change the level at which the compressor ``kicks in.'' This type of compressor typically has a unity gain below threshold, although this isn't always the case. Take a look at Figure 6.27. This shows a number of curves for a device with a compression ratio of 2:1, unity gain below threshold and an adjustable threshold level. Figure 6.27: A plot showing a number of curves representing various settings of the threshold with a unity gain below threshold and a compression ratio of 2:1. red threshold = -10 dBV, blue threshold = -5 dBV, green threshold = 0 dBV, black theshold = 5 dBV. The advantage of this design is that the bulk of the signal, which is typically below the threshold, remains unchanged - by changing the threshold level, we're simply changing the level at which we start compressing. This makes the device fairly intuitive to use, but not necessarily a good design for the final sound quality. Let's think about the response of this device (with a 2:1 compression ratio). If the threshold is turned up to 12 dBV, then any signal coming in that's less than 12 dBV will go out unchanged. If the input signal has a level of 20 dBV, then the output will be 16 dBV, because the input went 8 dB above threshold and the compression ratio is 2:1, so the output goes up 4 dB. If the threshold is turned down to -12 dBV, then any signal coming in that's less than -12 dBV will go out unchanged. If the input signal has a level of 20 dBV, then the output will be 4 dBV, because the input went 32 dB above threshold and the compression ratio is 2:1, so the output goes up 16 dB. So what? Well, as you can see from Figure 6.27, changing the compression ratio will affect the output level of the loud stuff by an amount that's determined by the relationship bewteen the threshold and the compression ratio. Consider for a moment how a compressor will be used in a recording situation: we use the compressor to reduce the dynamic range of the louder parts of the signal. As a result, we can increase the overall level of the output of the compressor before going to tape. This is because the spikes in the signal are less scary and we can therefore get closer to the maximum input level of the recording device. As a result, when we compress, we typically have a tendancy to increase the input level of the device that follows the compressor. Don't forget, however, that the compressor itself is adding noise to the signal, so when we boost the input of the next device in the audio chain, we're increasing not only the noise of the signal itself, but the noise of the compressor as well. How can we reduce or eliminate this problem? Use compressor design philosophy number 2... Instead of giving the user control over the threshold, some compressor designers opt to have a fixed threshold and a variable gain before compression. This has a slightly different effect on the signal. Figure 6.28: A plot showing a number of curves representing various settings of the gain before compression with a fixed threshold. The compression ratio in this example is 2:1. The threshold is fixed at 0 dBV, however, this value does not directly correspond to the input signal level as in Figure 6.27. The red curve has a gain of 10 dB, blue = 5 dB, green = 0 dB, black = -5 dB. Let's look at the implications of this configuration using the response in Figure 6.28 which has a fixed threshold of 0 dBV. If we look at the green curve with a gain of 0 dB, then signals coming in are not amplified or attenuated before hitting the threshold detector. Therefore, signals lower than 0 dBV at the input will be unaffected by the device (because they aren't being compressed and the gain is 0 dB). Signals greater than 0 dBV will be compressed at a 2:1 compression ratio. Now, let's look at the blue curve. The low-level signals have a constant 5 dB gain applied to them - therefore a signal coming in a -20 dBV comes out at -15 dBV. An input level of -15 dBV results in an output of -10 dBV. If the input level is -5 dBV, a gain of 5 dB is applied and the result of the signal hitting the threshold detector is 0 dBV - the level of the threshold. Signals above this -5 dBV level (at the input) will be compressed. If we just consider things in the theoretical world, applying a 5 dB gain before compression (with a threshold fixed at 0 dBV) results in the same signal that we'd get if we didn't change the gain before compression, reduced the threshold to -5 dBV and then raised the output gain of the compressor by 5 dB. In the practical world, however, we are reducing our noise level by applying the gain before compression, since we aren't amplifying the noise of the compressor itself. There's at least one manufacturer that takes this idea one step further. Let's say that you have the output of a compressor being sent to the input of a recording device. If the compressor has a variable threshold and you're looking at the record levels, then the more you turn down the threshold, the lower the signal going into the recording device gets. This can be seen by looking at the graph in Figure 6.27 comparing the output levels of an input signal with a level of 20 dBV. Therefore, the more we turn down the threshold on the compressor, the more we're going to turn up the input level on the recorder. Take the same situation but use a compressor with a variable gain before compression. In this case, the more we turn up the gain before compression, the higher the output is going to get. Now, if we turn up the gain before compression, we are going to turn down the input level to the recorder to make sure that things don't get out of hand. What would be nice is to have a system where all this gain compensation is done for you. So, using the example of a compressor with gain before compression: we turn up the gain before compression by some amount, but at the same time, the compressor turns down its output to make sure that the compressed part of the signal doesn't get any louder. In the case where the compression ratio is 2:1, if we turn up the gain before compression by 10 dB, then the output has to be turned down by 5 dB to make this happen. The output attenuation in dB is equal to the gain before compression (in dB) divided by the compression ratio. What would this response look like? It's shown in Figure 6.29. As you can see, changes in the gain before compression are compensated so that the output for a signal above the threshold is always the same, so we don't have to fiddle with the input level of the next device in the chain. If we were to do the same thing using a compressor with a variable threshold, then we'd have to boost the signal at the output, thus increasing the apparent noise floor of the compressor and making it sound as bad as it is... Figure 6.29: The gain response curves for various settings on a compressor with a magic output gain stage that compensates for changes in either the threshold or the gain before compression stage so that you don't have to. As you can see from Figure 6.29, the advantage of this system is that adjustments in the gain before compression (or the threshold) don't have any affect on how the loud stuff behaves - if you're past the threshold, you get the same output for the same input.