Click here to purchase the entire book in PDF format. Threshold The first thing we'll need to make a threshold detection circuit is a way of looking at the signal and dividing it into a low voltage area (in which nothing gets out of the circuit) and a high area (in which the output = the input to the circuit). We already looked at how to do this in a rather crude fashion - it's called a half-wave rectifier. Since the voltage of the wiper on the pot is going positive and negative as the input signal goes up and down respectively, all we need to do is rectify the signal after the wiper so that none of the negative voltage gets through to the CV input of the VCA. That way, when the signal is low, the gain of the VCA will be 0 dB, leaving the signal unaffected. When the signal goes positive, the rectifer lets the signal through, the gain of the VCA goes down and the compressor compresses. One way to do this would simply be to put a diode in the circuit pointing away from the wiper. This wouldn't work very well because the diode would need the 0.7 V difference across it to turn on in the first place. Also, the turn-on voltage of the diode is a little sloppy, so we wouldn't know exactly what the threshold was (but we'll come back to this later). what we need then, is something called a precision rectifier - a circuit that looks like a perfect diode. This is pretty easy to build with a couple of diodes and an op amp as is shown in the circuit in Figure 6.50. Figure 6.49: A circuit diagram for a precision rectifier. Notice that the circuit has two effects - the first is that it is a half-wave rectifier, so only the positive half of the input gets through. The second is that it is an inverting amplifier, so the output is opposite in polarity to the input - therefore, in order to get things back in the right polarity, we'll have to flip the polarity once again with a second inverting amplifier with unity gain. If we add this circuit between the wiper and the VCA CV input like the diagram shown in Figure , what will happen? Figure 6.50: A slightly more detailed schematic of the inner workings of a compressor. Now, if the input level is -10 dBV or lower, the output of the RMS detector is 0 V or lower. This will result in the output of the half-wave rectifier being 0 V. This will be multiplied by -1 in the polarity inversion, resulting in a level of 0 V at the CV input of the VCA. This means that if the input signal is -10 dBV or lower, there is no gain change. If, however, the input level goes above -10 dBV, then the output of the RMS detector goes up. This gets through the rectifier and comes out multipled by -1, so for every increase in 1 dB above -10 dBV at the input, the output of the rectifier goes DOWN by an amount determined by the position of the wiper. This is multiplied by -1 again at the polarity inversion and sent to the CV input of the VCA causing a gain change. So, we have a threshold at -10 dBV at the input. But, what if we wanted to change the threshold level? In order to change the threshold level, we have to trick the trheshold detection circuit into thinking that the signal has reached the threshold before it really has. Remember that the output of the RMS detection circuit (and therefore the wiper on the pot) is DC (well, technically speaking, it varies if the input signal's RMS level varies, but we'll say it's DC for now). So, we need to mix some DC with this level to give the input to the rectification circuit an additional boost. For example, up until now, the threshold is -10 dBV at the input because that's where the output of the RMS detector crosses from negative to positive voltage. If we wanted to make the threshold -20 dBV, then we'd need to find out the output of the RMS detector if the signal was -20 dBV (that would be -60 mV because it's 6 mV per dB and 10 dB below -10 dBV) and add that much DC voltage to the signal before sending it into the rectification stage. There are a couple of ways to do this, but one efficient way is to combine an inverting mixer circuit with the half-wave rectifier that's already there. Figure 6.51: A slightly more detailed schematic of the inner workings of a compressor. The threshold level adjustment is just a controllable DC level which is mixed with the DC level coming out of the RMS detector. One important thing to note is that when you turn UP this level to the top of the pot, you are acutally getting a lower voltage (notice that the top of the pot is connected to a negative voltage supply). Why is this? Well, if the output of the threshold level adjustment wiper is 0 V, this gets added to the RMS detector output and the threshold stays at -10 dBV. If the output of the threshold level adjustment wiper goes positive, then the output of the RMS detector is increased and the rectifier opens up at a lower level, so by turning UP the voltage level of the threshold adjustment pot, you turn DOWN the threshold. Of course, the size of the change we're talking about on the threshold level adjustement is on the order of mV to match the level coming out of the RMS detector, so you might want to be sure to make the maximum and minimum values possible from the pot pretty small. See the THAT Corp .pdf file linked at the bottom of the page for more details on how to do this. So, now we have a compressor with a controllable compression ratio and a threshold with a controllable level. All we need to do is to add an output gain knob. This is pretty easy since all we're going to do is add a static gain value for the VCA. This can be done in a number of ways, but we'll just add another DC voltage to the control voltage after the threshold. That way, no matter what comes out of the threshold, we can alter the level. Figure 6.52: A slightly more detailed schematic of the inner workings of a compressor. The diagram in Figure 6.52 shows the whole circuit. Note that the output gain control has the + DC voltage at the top of the pot. This is because it will become negative after going through the polarity inversion stage, making the VCA go up in gain. Since this DC voltage level is added to the control voltage signal after the threshold detection circuit, it's always on - therefore it's basically the same as an output level knob. In fact, it is an output level knob. Everything I've said here is basically a lead-up to the pdf file below from THAT Corp. It's a good introduction to how a simple RMS-based compressor works. It includes all the response graphs that I left out here, and goes a little further to explain how to include a soft knee for your circuit. Definitely recommended reading if you're planning on learning more about these things...