Fons Adriaensen wrote: > On Sun, Aug 23, 2009 at 07:27:18PM +0100, Dan Mills wrote: > > >> Lets say your card is aligned so that 0dbFS = +18dbu (EBU standard), >> then 0Vu = +4dbu = - 14dbFS, so a software VU calibrated for 0Vu = >> -14dbFs should read the same as an external Vu calibrated for +4dbu = >> 0Vu. If it does not then either a calibration setting is off somewhere >> or one of the meters is faulty. >> > > True. > > But even a definition such as 'dB FS' is ambiguous, and > it's easy to make mistakes as a result of that. > > Consider a sine wave that is just below digital clipping. > This would be called '0 dB FS', but the actual RMS level > is 3 db lower. I've seen at least one context where this > same signal would be called -3dB FS. Which somehow makes > sense as well. > > AFAIK the first interpretation is the more common one. >
'Peak level' vs 'intrinsic level': * 'dbFS' refers to 'peak level', 'full-scale square wave' * 'dbFS RMS' refers to 'intrinsic level, 'full-scale sine wave'. This is how is in Germany distinguished and because of the words on English, it might be an international way to distinguish it. >> actually fairly common with professional cards. >> > > And it avoids a lot of problems. Semi-pro cards will not > have the correct levels, but some can be quite consistent > between channels. For example my Terratec EWS88MT has less > than +/- 0.1 dB variation between its 8 channels, both > for input and output. But the actual level is just +3.5dBu > for a FS sine wave. > > The real misery starts when (as reported in a recent post > on LAU or LAD), a user finds four volume controls between > his audio file and the physical output (plus of course > a fifth one on his amplifier), and then of course gets > confused on how to set all of them. I guess this is the leading point. Even professional IO amps at times need calibration. We should imagine a setup like 'sound card --> tube pre-amps --> mastering recorder'. For this scenario a VU meter needs to be an external one. Ralf _______________________________________________ Linux-audio-dev mailing list Linux-audio-dev@lists.linuxaudio.org http://lists.linuxaudio.org/mailman/listinfo/linux-audio-dev