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How compressors compress

On its simplest level, a compressor can be thought of as a device which controls its gain based on the incoming signal. In order to do this, it takes the incoming audio and sends it in two directions, along the audio path, which is where the signal goes in, gets modified and comes out the output; and the control path, (also known as a side chain) where the signal comes in, gets analysed and converted into a different signal which is used to control the gain of the audio path.

As a result, we can think of a basic compressor as is shown in the block diagram in Figure 6.45.

Figure 6.45: A very simple way of thinking of the internal operation of a compressor.

Notice that the input gets split in two directions right away, going to the two different paths.

At the heart of the audio path is a device we have't seen before - it's drawn in block diagrams (and sometimes in schematics) as a triangle (so we know right away it's an amplifier of some kind) attached to a box with an ``X'' through it on the left. This device is called a voltage controlled amplifier or VCA. It has one audio input on the left, one audio output on the right and a control voltage (or CV) input on the top. The amplifier has a gain which is determined by the level of the control voltage. This gain is typically applied to the current through the VCA, not the voltage - this is a new concept as well... but we'll get to that later.

If you go to the VCA store and buy a VCA, you'll find out that it has an interesting characteristic. Usually, it will have a logarithmic change in gain for a linear change in voltage at the control voltage input. For example, one particular VCA from THAT corporation has a gain of 0 dB (so input = output) if the CV is at 0 V. If you increase the CV by 6 mV, then the gain of the VCA goes down by 1 dB.

So, for that particular VCA, we could make Table 6.3 which we'll use later.

Table 6.3: The relationship between the control voltage and the gain applied to the audio signal for the hypothetical VCA shown in Figure 6.45

Control Voltage (mV)	Gain of Audio signal (dB)
-12	+2
-6	+1
0	0
+6	-1
+12	-2

The only problem with the schematic so far is that the VCA is a current amplifier not a voltage amplifer. Since we prefer to think in terms of voltage most of the time, we'll need to convert the voltage signal that we're feeding into the compressor into a current signal of the same shape. This is done by sticking the VCA in the middle of an inverting amplifier circuit as shown in Figure 6.46:

Figure 6.46: A slightly more detailed schematic of the inner workings of a compressor.

Here's an explanation of why we have to build the circuit like this. Remember back to the first stuff on op amps - one of the results of the feedback loop is that the voltage level at the negative input leg MUST be the same as the positive input leg. If this wasn't the case, then the huge gain of the op amp would result in a clipped output. So, we call the voltage level at the negative input ``virtual ground'' because it has the same voltage level as ground, but there's really no direct connection to ground. If we assume that the VCA has an impedance through it of 0$\Omega$ (a pretty safe assumption), then the voltage level at the signal input of the VCA is also the same as ground. Therefore the current through the resistor on the left in the above schematic is equal to the ``Audio in'' voltage divided by the resistor value. Now, if we assume that the VCA has a gain of 0 dB, then the current coming out of it equals the current going into it. We also happen to remember that the input impedance of the op amp is infinite, therefore all the current on the wire coming out of the VCA must go through the resistor in the feedback loop of the op amp. This results in a voltage drop across it equal to the current multiplied by the resistance.

Let's use an example. If the ``Audio in'' is 1 Vrms, then the current through the input resistor on the left is 0.1 mArms. That current goes through the VCA (which we'll assume for now has a gain of 0 dB) and continues on through the feedback resistor. Since the current is 0.1 mA rms and the resistor is 10 k$\Omega$, then the voltage drop across it is 1 V rms. Therefore, the ``Audio out'' is 1 V rms but opposite in polarity to the input. This is exactly the same as if the VCA was not there.

Now take a case where the VCA has a gain of +6 dB for some reason. The voltage level at its input is 0 V (virtual ground) so the current through the input resistor is still 0.1 mA rms (for a 1 V rms input signal). That current gets multiplied by 2 in the VCA (because it has a gain of +6 dB) making it 0.2 mA rms. This all goes through the feedback resistor which results in a voltage drop of 10k * 0.2 mArms = 2 V rms (and opposite in polarity). Ah hah! The gain applied to the current by the VCA now shows up as a gain on the voltage at the output. Now all we need to do is to build the control circuitry and we have a compressor...

The control circuitry needs a number of things: a level detector, compression ratio adjustment, threshold, threshold level adjustment and some other stuff that we'll get to. We'll take them one at a time. For now, let's assume that we have a compressor which is only looking at the RMS level to determine its compression characteristics.