[linux-audio-user] (no subject)

Mark Knecht markknecht at comcast.net
Mon Apr 12 15:20:19 EDT 2004


On Mon, 2004-04-12 at 11:27, Anahata wrote:
> On Mon, Apr 12, 2004 at 09:53:06AM -0700, Mark Knecht wrote:
> >    I wonder if you have any info on something I was reading in Bob
> > Katz's book recently, that many A/D converters actually have compressors
> > or other sorts of non-linear circuits built into them which effect the
> > use of the converter when it gets anywhere near maximum value anyway. 
> 
> It's *extremely* desirable for any clipping do be done in the analogue
> domain before it reaches the A-D converter.

Certainly. Any clipping is bad, whether digital or analog, but given the
choice I'd far rather do it in the analog domain.

One problem that I'm faced with on the Pro Tools 002 R is that it has a
nice preamp, and a nice converter, but no way to patch a compressor in
the middle. For this reason I decided to record at pretty low levels to
give me enough headroom to not clip when the drummer laid into his snare
or kick.

You could correctly argue that recording at low volumes reduces your
SNR, which is true, but unless you're recording in a professionally
built studio my experience is that normal room noise from AC, fans,
lights, etc. is way above the noise floor anyway. 

<SNIP>
> 
> Adding a compressor or fast-acting limiter is another matter. Again I
> don't know if any do this. I would have thought it was compromising the
> signal quality, but you could argue that the user is asking for trouble
> if he sets the levels too high anyway, and that the limiter has no
> effect on lower level signals.

One of Katz's points, and I'm sure this is based on his experience and
on specific A/D's, is that a full level input signal may actually be
coming close to the power supply level that the A/D is operating at.
I.e. - if I'm accepting +/-5v inputs to get full scale, but the chip is
operating on a 5V power supply, then the converter circuits will most
certainly go non-linear. However, if full scale input is +/-5 volts but
the A/D is operating on a 12V supply then the circuits can remain linear
even at full scale.

- Mark



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