The character of the rectifier, which is actually an RMS converter that is combined with a very fast peak detection circuit can be modified by the 3-position CREST switch. In position 1 it works as a fast peak detector while position 4 selects a moderate RMS conversion. Position 2 selects a well-selected compromise between the two worlds and offers a characteristic of the rectifier that avoids the disadvantages of both pure principles. Actually, setting 2 selects a very fast RMS conversion that, on our opinion, is the best choice for many applications that require dynamic treatment. With appropriate settings of the time controls, threshold, ratio, and knee, it is possible to achieve more than 10 dB of loudness gain without an increase in level and without negative side effects. The compressed signal is just louder but sounds still natural.
The Envelope control is a useful helper to find a better solution for the distortion versus pumping problem. As soon as the release time is fast enough for pump free operation, low frequency distortion is possible due to the fact that the compressor modifies the wave form of the low frequency signal with such setting. Envelope does two things at a time. The release time is modified by the frequency of the input signal in a way, that a high level, low frequency signal increases the release time for a very short period of time. This part of the entire function works just as you would manually increase the release time a little as soon as a low frequency signal is treated. Of course, it is not possible to do this manually. You will always be too slow and too late. However, the envelope system does not have such problems. In addition there is a filter stage in the side chain that reduces the low frequency range in a special way that is not a simple high-pass filter but a frequency response that is optimized for this purpose. This combination results in a much better treatment of bass signals. The Toolkit compressor has a three-position Envelope switch. Position 1 and 2 of the switch determine different amounts of release modifications and low frequency reduction, while the off posisiton disables the circuit.
The standard controls, Threshold, Ratio, Attack, and Release have more than sufficient control ranges to make any setting possible. The regulation charateristics are optimized by the '12 o'clock' principle, which means that you have a good basic setting if you start with all pots set to the center position before you start optimizing the setting for the particular program material. The range of the threshold is from + 18 dB to -32 dB; the 0 dB posittion is internally calibrated. The ratio control covers the entire range from 1:1, which actually means that the compressor is off, to 20:1, which is limit operation. The setting of the Attack Time is very important, since it determines the sound performance of the unit more than anything else. Of course, we assume that you already have appropriate settings of the other controls before you start to figure out what you can do with the attack control. The range is from 0.1 ms, which is very fast, to 30 ms, which actually means that the reaction time of the compressor is so slow that there is no compression of transients anymore. The range from approximately 0.5 ms to 8 ms is mostly used with real world settings. Leaving the other controls unchanged, different attack times in that range will modify the tonal result significantly, if the release time is fast enough. If Crest is switched to RMS Mode, the minimum attack time is determined by the integration time of the RMS converter and not by the setting of the attack pot alone. With Crest 4, setting the attack time all to the left results in an attack time of approx. 2 ms.
The Release control determines the compromise between fast and pump-free recovery of the level and low frequency distortion that is the biggest problem of any analogue compression. The control range of the ToolKit compressor's relase time is from 50 ms to 3s. Of course, you will get a totally distorted signal if you set the release that fast, but if you compress a signal that does not contain high level low frequencies, you can really use such fast settings. This is the case with many vocals or instruments. However, if you want to be on the save side as far as the release time is concerned, start with a setting of 0.6 seconds.
The Soft Knee pot changes the ratio around the threshold point. Setting this control all to the left disables the soft knee function. With this setting, the transition from the unregulated state with levels below threshold to the regulated state above threshold is hard. Increasing the setting of the knee control makes this transition smoother. Actually, a setting all to the right expands the transition range to 6 dB. Within this range, there is a smooth transition from the linear part of the curve to the compression rate that is determined by the ratio control. Actually, the soft knee does not modify the sound performance of the compressor considerably but it makes possible to increase the compression by several dB's. There is no general rule for the use of this feature. It just depends on the program material, the setting of the other controls and your desired result if the knee function is more or less useful. In any case, ratio settings less than 2 reduce the effect of the soft knee control considerably.
In addition to the auto gain system there is a normal Gain pot with a range of +/- 20 dB. You will need this manual gain control as soon as you use an external fi lter in the side-chain insert, since the auto gain circuit cannot work precisely when the level at the rectifier input is different from the level at the input of the compressor. This is always the case with external treatments of the side-chain signal. In addition to this application you may want to modify the gain independently.
As soon as you change any setting of the compressor, the output level will be changed. More ratio or a lower threshold setting increases the gain reduction and reduces the output level. A longer attack time delays the gain reduction and results in more peaks and therefore in a higher output level. So the gain needs to be readjusted all the time you modify a setting. Apart from this tiresome work there is the problem that you cannot easily compare different settings, since the influence of the different levels is often higher than the result of the different settings of the compressor. Working like this is not very pleasant. A way out is the Autogain system. It is based on the idea, that a feed-forward controlled compressor system produces a gain reduction that can be calculated and therefore compensated.