Subject: Re: [linux-audio-user] (no subject)
From: Anahata (anahata_AT_treewind.co.uk)
Date: Mon Apr 12 2004 - 21:27:50 EEST
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.
Clipping produces rich out-of-band harmonic content which gets aliased
to in-band content if the clipping is done by the A-D converter
itself. The analogue signal should get clipped before it enters the
anti-aliasing fiter, so those harmonics are removed. Any in-band
harmonics remain, of course.
This may explain why overloading an A-D can sound so bad - it's worse
than razor sharp analogue signal clipping.
Incidentally, the analogue clipping level should be a little below
should be below full scale, to allow for the anti-aliasing filter to
overshoot. Ideally the maximum overshoot level should remain within the
range of the A-D.
I have no idea how many of the commercially available audio A-D
converters do this properly. I suspect that low-cost sound cards don't
even have an anti-aliasing filter at all.
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.
-- Anahata anahata_AT_treewind.co.uk -+- http://www.treewind.co.uk Home: 01638 720444 Mob: 07976 263827
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