I’m new to compression for ACX compliance and other purposes and am trying to understand the basics so please help relieve my ignorance.
Compression based on RMS vs. on peaks:
The online manual says compression based on RMS is downward while compression based on peaks is upward. I started with a mono track with ACX check yielding overall peak -5.3 and overall RMS -31.0, duplicated it twice (within same project), and compressed both copies with threshold -12.0, compressing copy A based on RMS and copy B based on peaks. ACX checks on the copies after compression yielded: for A, peak -6.0, RMS -31.0; for B, peak -2.1, RMS -23.9.
– Questions:
- Why didn’t compressing copy A based on RMS change the RMS level? Is this because the starting RMS of -31.0 was below the -12.0 threshold so didn’t trigger compression? If so, why did the overall peak level decline from -5.3 to -6.0?
- Was the pre-to-post level increase in copy B (compressed based on peaks) attributable to the fact that the initial peak of -5.3 exceeded the -12.0 threshold?
Make-up gain:
The make-up gain option seems to allow gain only to 0db.
– Questions:
- Is any other target db level available, and if so how set it? (ACX wants peak and RMS levels below 0db so it’s hard to believe Audacity wouldn’t offer ACX-compliant make-up gain.)
- If using make-up gain, is the gain based on RMS or peak level? Or does this depend on which of these was used as the basis for compression? (Make-up gain based on RMS would seem to yield peaks exceeding 0db and thus clipping, which wouldn’t be good.)
Compression vs. Limiting:
The compression and limiting effects seem very similar. Is the key difference that the latter works only on maximum rather than on both max and min levels? If not so, please clarify.
Thanks in advance.