Compression By Stealth

One of the best-known uses of compression is to increase the apparent loudness of a mix, or an individual voice or
instrument for that matter. Compression, as you know, works by reducing the high signal levels, bringing them
closer to the low-level passages, and then applying make-up gain. Thus the low-level signals are brought up and
the whole thing sounds louder. This is fine in theory, the trouble is that the effect of compressing the high-level
signals is very audible, necessitating great care in the set-up of the compressor and judicious compromise
between getting enough compression and not spoiling the overall sound. Ray Dolby told us this when, in the early
A-type noise reduction system, he left high levels completely alone and modified the gain only of signals below
-40dB. What we need is a compressor that only operates on low-level signals. Is there such a thing? Yes there is,
and it's in your rack already. You just have to use it in a different way. Since in this situation the object is to
bring up the lower levels of the track, what we need is a way of making the quiet sections louder without
affecting the loud sections.
The answer is to mix the uncompressed signal with a compressed version of the same (Figure 3). At levels below
the compressor's threshold the two signals will combine to produce a 6dB increase in level. Above the threshold
the compressed signal will be progressively reduced and add hardly any additional level to the mix. The result is a
form of compression where you can get more dynamic range reduction with fewer audible side-effects. I'm not
going so far as to say that it is always the best way, but it's certainly worth a try. Maybe some enterprising
company will bring out a gadget to do just this, in a convenient rack mounting package. By the way, if you try
this with a digital compressor you will get a lesson in the delay involved in digital processing. You will get comb
filtering and it will sound dreadful.