On Sun, Sep 26, 2010 at 08:52:30AM +0200, Philipp Überbacher wrote: > What's an ad-hoc algorithm? Why are general purpose audio > compressors/limiters insufficient? When you compress or limit a voice or instrument, the intention is to either *modify* its sound, or to reduce its dynamics so it becomes easier to integrate into the mix. The exact technical level at the output of the compressor is not really an issue in that case. The operation will be followed by EQ, effects and other tracks will be added, so the actual level doesn't matter that much. A compressor set for 'infinite' ratio (or a limiter) will output signals well above its threshold - it has finite attack time, and even if set for 'peak' detect it will still do the wrong thing. Peak limiting as the last step in mastering (to ensure that the signal stays within the [-1..+1] range uncon- ditionally is quite a different matter. You operate on a mix, the artistic decisions have been taken and you don't want to really modify the sound - just raise its level as much as possible while staying within the purely technical limits imposed by distribution in a digital format. And the limits in this case are *hard* - no single sample should go outside the allowed range. Single isolated samples exceeding the limits can in most cases just be clipped. Short overloads (< 1ms or so) can be handled by looking ahead and operating only on the high frequencies. Longer ones can be handled by more conventional limiting algorithms. There are good reasons to make attack and release times dependent on spectral features, amount of overload, recent history etc. So any limiter trying to do this job would need a mix of several special algorithms. You can go some way by making the operation multi-band. But a mult-band limiter put together from separate plugins can't really do this. First, none of the components has any idea of the final (summed) output. But that is the signal we want to control quite accurately - the separate outputs really don't matter here. Second, the components don't know what the others are doing - they can't coordinate in any way. There is nothing that prevents e.g. the LF limiter to reduce gain by 10 dB while the rest of the frequency range is left untouched. What you want in such cases is that the MF and HF limiters at least apply part of that reduction as well. Doing all this right requires a limiter designed for the job. Ciao, -- FA There are three of them, and Alleline. _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
