Operating Manual -
signal before the gain control FET and then amplify it again. Of course this results in less than ideal noise per- formance and imposes a frustrating
A number of VCA’s based on the exponential volt-
The problem with simple transconductance am- plifiers is that, like FET VCA’s, they can handle only very small signals so the noise performance is poor. A num- ber of linearizing circuits have been devised to minimize this problem, but even the best transconductance amplifi- ers have an equivalent input noise of about
The best analog compromise to date is the “class AB current ratio multiplier.” Early implementation of this circuit used two matched pairs of transistors, one pair of NPN’s and one pair of PNP’s. The problem here is that excellent matched integrated NPN pairs were avail- able, but integrated PNP’s were not. The PNP’s had to be
The Ashly VCA
The Ashly VCA is an integrated current ratio mul- tiplier circuit. It has low noise
Detectors
It would seem that, of the two components in a compressor/limiter, the VCA is the more critical since the audio passes through it and the detector only provides it with a control voltage. Experience showed us that both are crucial to the overall sound and that, if anything, the detector’s performance is the harder to judge by conven-
tional measuring techniques. While the VCA is doing its job if it has low noise and distortion, the detector must con- stantly adjust the gain of the audio path in a manner which keeps the level under control while sounding acceptable to the listener. This constantly changing gain is a dynamic action, while conventional audio measurements like noise and distortion checks are Static (at a constant level). We became painfully aware of this problem with some of our earlier limiter prototypes which measured fine and sounded terrible. This led us to use a purely subjective approach in the design of the detector - we did a lot of listening to de- termine what sounded good and what didn’t.
Two important features emerged from this re-
search:
1.We designed the detector to let the attack and release times speed up as more and more limiting occurs. The compression ratio also increases. This lets us main- tain peaks fairly close to a constant ceiling level, but al- lows the illusion of increasing loudness as input level increases, thereby preventing complete loss of dynamics when limiting.
2.We incorporate a double release time constant. When release time was set slow with a single time con- stant, transients such as mic “pops” caused a quick re- duction in gain and a slow
When choosing a compressor/limiter, you can see that it is very important to listen to it in your particular application and see that it sounds the way you want. There are lots of these devices with seemingly excellent specs which sound very different with real program material applied to them.
Peak Or RMS
There are several ways of looking at a signal to determine its level. A peak detector looks at the maxi- mum voltage a signal reaches regardless of it’s waveform, while an RMS (root mean square) detector looks at the energy in a signal regardless of the short term voltage levels. This makes a peak detector the correct choice for preventing clipping, overmodulation, or tape saturation, while an RMS detector can be used to restrict material to a given loudness. When an RMS limiter is used to pre- vent clipping, the result is unpredictable. For instance, a flute and a snare drum which are limited to the same RMS level might have peak levels as much as 30dB apart! Use peak limiters to prevent clipping.
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