Today’s question, from inSync reader Daniel W, has to do with compressing signals coming in to digital mixers.
“We’ve got these great new digital mixers with all sorts of EQ and compression in them. On an analog board if you overload the inputs slightly, it’s nice to you. We all know what overloading digital does, so can you use the built-in compression to limit the incoming signal or must you still use an analog limiter/compressor to guarantee digital clipping won’t occur? It seems like even though I may get some great digital board, I still need analog limiters before every channel (at least the channels you suspect might have problems).”
This is a GREAT question and reveals a couple of underlying issues about digital mixers that not everyone has realized. The digital processing that is built in to these products can only be used once the signal has been converted to digital, which means after the A/D conversion process. So the answer to the question is, yes, you must use outboard compressors if you need to compress on the way into one of these mixers. But that is the real question: Do you really need to compress at this point? There are different schools of thought on compressing before or after recording in general. Most professionals would rather not apply any compression to a signal before recording, which would negate the need to deal with this question. Some people, however, like to compress when recording and, further, like to get as hot a signal as possible to their A/D converters because we all know digital is most linear in its upper range, near full code. So there are people who compress signals at this point in order to get the highest signal level possible into the converter without clipping it. If you need to do this you will need a compressor on the front end of one of these mixers. They generally have inserts on each channel to do this. On the other hand we must also consider that most modern systems are no longer 16 bit. 20 and 24 bit conversion is almost a minimum accepted standard these days. A 24 bit system has so much dynamic range that it is arguably no longer important to worry about extreme maximizing of signal levels to the top range. You can set levels up with enough headroom that there is little or no chance of clipping the converters (a very nasty sound indeed). Recording with 3 dB to 6 dB (or more) of headroom is acceptable on a 24 bit system because the audio will still be in great shape once converted. This was not true of 16 bit systems. They just didn’t have the resolution in their lower ranges. So while it is technically true that outboard compressors would have to be employed to maximize signal levels into a digital mixer, it is also true that this maximization probably isn’t necessary. In the end it is a matter of your personal taste and how you like to work.