From: Fmiser <fm...@gm...> - 2009-08-29 06:17:03
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> Wesley Chow wrote: > compand 0.3,1 6:-70,-60,-20 -5 -90 0.2 > > And it seems to work reasonably well. Not being an audio > engineer myself, I don't really understand what any of those > numbers mean. Can someone suggest a better set of values to > try? It is highly subjective - so "better" is tricky to achieve. What is a good set of values can vary depending on what you like to hear, did it come from cassette tape or CD, type of music, instruments (snare drum solo will be different than a pan flute solo), etc. But I'll try to explain in non-engineer terms what those numbers mean. Hopefully then you can do some educated experimenting to figure out what works best for you. *smiles* * The first two (0.3,1) are "attack" and "release". The compander is effectively a robot controlling a volume control. These values determine how fast the robot is allowed to turn the knob. "Attack" is how fast it's allowed to be turned down. "Release" is how fast it can be turned back up. If the attack is set to fast (small number), then every sharp, snap-like sound will be "caught". That's good if the snare drum hits are what need to be control. But not so good if you just want the soft passages louder. If the release is set to short (small number), it's more likely there will be "pumping" - that is, it's audible the volume is being turned down and up. If your goal is to make the quiet passages louder, and keep the clapping _sections_ from overwhelming - try large-ish values. Experiment - but you probably want them to stay near each other. "0.02,5" will probably not be what you want. * The next one is "knee" This is how "soft" is the transition from "It's quiet enough - leave it alone." to "Too loud - better turn it down." This is rather helpful because music does not have a steady volume. Without a knee, as the music crosses back and forth across the "trigger volume" (threshold), the contrast between unchanged and compress audio can be rather audible. Bigger numbers are softer transitions. Probably want it between 3-12. 6 should work well most of the time. * Now a series of "compander transfer function" values. These must be in pairs -except if one is missing it's presumed to be 0. Before I go further, I think I have to fully explain decibels. Uhm, well - I don't have time for that. So here's the short, what-it-means-here version. A "bel" is a measure of the change in a signal level. It's a _big_ change - so practically everyone uses "one-tenth-of-a-bell" called a decibel (dB). (remember your SI/metric prefixes?) So by definition, 0 dB means "same as", -6 dB means "6 decibels less", and 6 dB means "6 decibels more". In most digital systems, we measure everything relative to the maximum signal that can be described, which we call "0 dB FS" (FS standing for "Full Scale"). Now, any signal level can be described by a single value - say "-30 dB". This means that particular signal is 30 decibels less than the absolute maximum this digital system can have. The transfer function is really just a map. Each pair is telling the robot "If the input signal level is __, I want you to change it so it's __ instead." Effectively then a line is drawn between the point mapped out to give a final instruction set. So "-70,-60,-20" mean "When the signal is -70 dB turn it up to -60 dB, and when it's -20 dB turn it up to (the presumed) 0 dB." Often I set first pair to tie down the recording's silence. Since it's not really silent and I down want the noise turned up really loud, I start with a pair like "-90,-90". If it's from a cassette or VHS tape, it could be "-50,-50" The next pair is the the main relationship "-80,-20" is pretty extreme. "-80,-40" is medium. "-80,-60" is mild. For normal music (not special effects), you want each pair closer to zero than the previous pair. * Next is the "gain" value Because of the attack value, it takes time for the robot to turn down strong signals. This gain value is so there is sure to be room for the short-term signal spikes to not cause clipping. The faster the attack time, the less spare room you need here. If you get clips make this value a lower level. (which means a larger negative number). I find "-5" to often not be enough room and there's clipping, so I find I often have it set to "-10" or "-12". * A single number for the initial level value. This makes no change to the audio signal. Because of the attack value, it takes time for the robot to figure out what the signal level is and therefore what changes need to be made. If this value is given, it tells the robot "Pretend the signal level is __ when we start." This is especially helpful if the beginning of the sound file is _not_ silence. * Delay Put this many seconds delay between the meter and the volume control. In effect, this makes the meter able to see into the future. Usually a value the same as the attack works well. If you have a large decay, it can still work okay set longer than the attack. Too big, and the robot will be changing the volume at the wrong time. -- Philip |