Most compressors have a gain reduction meter and the Warm Audio WA-2A Opto Compressor is no exception.
It takes the form of a large VU (Volume Unit) meter which is switchable to read the output signal level or the amount of gain reduction. (The two settings for output will be an explanation for another day.)
I have to quibble about the term 'gain reduction', which is unfortunately industry standard, but confusing. I'll come to that later.
The way a compressor works is to lower the level of loud signals while keeping the level of quiet signals the same. Thus it reduces the dynamic range of the signal. Because the peak level of the signal is now lower, most compressors have a control to bring the level back up again at the output. This may be called 'gain make-up', 'output gain', or simply 'output', or something that means the same thing.
The amount by which the level is lowered instantaneously is known as the gain reduction, and is measured in decibels. This is an important visual guide to help setting the controls of the compressor.
Too little gain reduction and you're not doing much. Too much gain reduction and the instrument or vocal will probably sound too compressed.
So 1 dB or 2 dB of gain reduction wouldn't be much, but might be enough in some situations. 20 dB of gain reduction would be a lot, and would only be used as a special effect.
Somewhere between around 5 dB and 10 dB would be useful in most contexts.
Something else the gain reduction meter will tell you is if you are compressing the signal at levels where it doesn't need to be compressed. This isn't something you hear about much, but it is important.
Suppose for instance that you set the controls of the compressor so that even when an instrument is playing at its quietest the gain reduction meter shows around 6 dB.
What this is telling you is that you are compressing signal levels that don't need to be compressed. That's not so much of a problem when the instrument is playing continuously. But if the instrument stops and starts, then each time it starts the compressor slams into 6 dB of compression before it actually gets to work. This can sound quite odd on the initial transient when the instrument starts playing.
The answer is to back off the amount of compression so that the needle of the meter barely flinches when the instrument is playing quietly. It won't sound any less compressed, but it will avoid that slamming effect each time the instrument starts up. Of course, you can slam the compressor as a special effect. It can work quite nicely on drums, with a fast release setting.
Now, my quibble about calling the control 'gain reduction'. Gain is any change in the level of a signal, upwards or downwards. So the meter could simply be called 'gain' and the scale calibrated in negative numbers of decibels. Or it could be called level reduction, in which case it would be calibrated as the meter is in the Warm Audio WA-2A Opto Compressor. It isn't a big deal really, but my feeling is that it is good to use these expressions correctly as they apply all the way through the audio chain, whatever equipment you use, whether analog or digital.
P.S. Because this is a VU meter, it will be slower to respond than an LED bargraph could be and not follow fast peaks correctly. In the context of a compressor however, it will correspond better to the subjective loudness of the signal. Whether you would like to know the amount of gain reduction on peaks or the RMS level of the signal would be a personal preference, but in practice it wouldn't be a hardship to adapt to either.
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