Click here to purchase the entire book in PDF format. Time Response: Attack and Release Now that we're talking about the RMS and the smoothed peak of the signal, we have to start considering what time it is. Up to now, we've been only looking at the output level or the gain of the compressor based on a static input level. We have been assuming that the only thing we're sending through the unit is a steady-state sine tone. Of course, this is pretty boring to listen to, but if we're going to look at real-world signals, then the behaviour of the compressor gets pretty complicated. Let's start by considering a signal that's quiet to begin with and suddenly gets louder. For the purposes of this discussion, we'll simulate this with a pulse-modulated sine wave like the one shown in Figure 6.37. Figure 6.37: A sine wave that is suddenly increased in level from a peak value of 0.33 to a peak value of 1. Unfortunately, a real-world compressor cannot respond instantaneously to this sudden change in level. In order to be able to do this, the unit would have to be able to see into the future to know what the new peak value of the signal will be before we actually hit that peak. (In fact, some digital compressors can do this by delaying the signal and turning the present into the past and the future into the present, but we'll pretend that this isn't happening for now...). Let's say that we have a compressor with a gain before compression of 0 dB and a threshold that's set to a level that's higher than the lower-level signal in Figure 6.37, but lower than the higher-level signal. So, the first part of the signal, the quiet part, won't be compressed and the later, louder part will. Therefore the compressor will have to have a gain of 1 (or 0 dB) for the quiet signal and then a reduced gain for the louder signal. Since the compressor can't see into the future, it will respond somewhat slowly to the sudden change in level. In fact, most compressors allow you to control the speed with which the gain change happens. This is called the attack time of the compressor. Looking at Figure 6.38, we can see that the compressor has a sudden awareness of the new level (at Time = 500) but it then settles gradually to the new gain for the higher signal level. This raises a question - the gain starts changing at a known time, but, as you can see in Figure 6.38, it approaches the final gain forever without really reaching it. The question that's raised is ``what is the time of the attack time?'' In other words, if I say that the compressor has an attack time of 200 ms, then what is the relationship between that amount of time and the gain applied by the compressor. The answer to this question is found in the chapter on capacitors. Remember that, in a simple RC circuit, the capacitor charges to a new voltage level at a rate determined by the time constant which is the product of the resistance and the capacitance. After 1 time constant, the capacitor has charged to 63 $\%$ of the voltage being applied to the circuit. After 5 time constants, the capacitor has charged to over 99 $\%$ of the voltage, and we consider it to have reached its destination. The same numbers apply to compressors. In the case of an attack time of 200 ms, then after 200 ms has passed, the gain of the compressor will be at 63 $\%$ of the final gain level. After 5 times the attack time (in this case, 1 second) we can consider the device to have reached its final gain level. (In fact, it never reaches it, it just gets closer and closer and closer forever...) Figure 6.38: The change in gain over time for a sudden increase in signal level going from a signal that's lower than the threshold to one that's higher. This is called the attack time of the compressor. (Notice that this looks just like the response of a capacitor being charged to a new voltage level. This is not a coincidence.) What is the result of the attack time on the output of the compressor? This actually is pretty interesting. Take a look at Figure 6.39 showing the output of a compressor that has the signal in Figure 6.37 sent into it and responding with the gain in Figure 6.38. Notice that the lower-level signal goes out exactly as it went it. We would expect this because the gain of the compressor for that portion of the signal is 1. Then the signal suddenly increases to a new level. Since the compressor detection circuit take a little while to figure out that the signal has gotten louder, the initial new loud signal gets through, almost unchanged. As we get further and further into the new level in time, however, the gain settles to the new value and the signal is compressed as we would expect. The interesting thing to note here is that a portion of the high-level signal gets through the compressor. The result is that we've created a signal that sounds like more of a transient than the input. This is somewhat contrary to the way most people tend to think that a compressor behaves. The common belief is that a compressor will control all of your high-level signals, thus reducing your dynamic range - but this is not exactly the case as we can see in this example. In fact, it may be possible that the perceived dynamic range is greater than the original because of the accents on the transient material in the signal. Figure 6.39: The output of a compressor which is fed the signal shown in Figure 6.37 and responds with the gain shown in Figure 6.38. Similarly, what happens when the signals decreases in level from one that is being compressed to one that is lower than the threshold? Again, it takes some time for the compressor's detection circuit to realize that the level has changed and therefore responds slowly to fast changes. This response time is called the release time of the compressor. (Note that the release time is measured in the same way as the attack time - it's the amount of time it takes the compressor to get to 63$\%$ of its intended gain.) For example, we'll assume that the signal in Figure 6.40 is being fed into a compressor. We'll also assume that the higher-level signal is above the compression threshold and the lower-level signal is lower than the threshold. Figure 6.40: A sine wave that is suddenly decreased in level from a peak value of 1 to a peak value of 0.33. This signal will result in a gain reduction for the first part of the signal and no gain reduction for the latter part, however, the release time of the compressor results in a transition time from these two states as is shown in Figure 6.41. Figure 6.41: The change in gain over time for a sudden decrease in signal level going from a signal that's higher than the threshold to one that's lower. This is called the release time of the compressor. (Notice that this looks just like the response of a capacitor being charged to a new voltage level. This is not a coincidence.) Again, the result of this gain response curve is somewhat interesting. The output of the compressor will start with a gain-reduced version of the louder signal. When the signal drops to the lower level, however, the compressor is still reducing the gain for a while, therefore we wind up with a compressed signal that's below the threshold - a signal that normally wouldn't be compressed. As the compressor figures out that the signal has dropped, it releases its gain to return to a unity gain, resulting in an output signal shown in Figure 6.42. Figure 6.42: The output of a compressor which is fed the signal shown in Figure 6.40 and responds with the gain shown in Figure 6.41. Just for comparison purposes, Figures 6.43 and 6.44 show a number of different attack and release times. Figure 6.43: Four different attack times for compressors with the same thresholds and compression ratios. Figure 6.44: Four different release times for compressors with the same thresholds and compression ratios. One last thing to discuss is a small issue in low-cost RMS-based compressors. In these machines, the attack and release times of the compressor are determined by the time constant of the RMS detection circuit. Therefore, the attack and release times are identical (normally, we call them ``symmetrical'') and not adjustable. Check your manual to see if, by going into RMS mode, you're defeating the attack time and release time controls.