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Re: Digital Distortion

Subject: Re: Digital Distortion
From: "Raimund Specht" <>
Date: Sat, 21 Jun 2003 09:28:26 -0000
Walter Knapp wrote:
> The real culprit, and I think what Rich was getting at is 
overdriving in 
> pre and A/D. He is right, this is common. It will be just as 
common in 
> the new solid state digitals. It's a matter of teaching folks to 
> digital recording. There's still a lot of recording advice 
> around that's straight from analog recording and ruins digital 
> recordings. We need to root that out.

Yes, overdriving in pre and A/D is much more critical in digital 
equipment than in analog tape recordings. Overdriving an analog tape 
recorder may add a few regular harmonics only. Instead, overdriving 
digital equipment will lead to additional irregular (mirrored) 
harmonics. This kind of distortion results from alias effects in the 
digital sampling process. Analog clipping within the pre-amplifier 
and the A/D input stage may produce very high-frequency harmonics 
reaching the ultrasonic domain. During normal operation, high-
frequency components would be removed by the integrated anti-alias 
filter of the recorder. However, when this anti-alias circuit is 
overdriven, that low-pass filter will fail. As a result, all the 
requency components higher than one half of the sampling rate will 
be folded down into the audible range. These "mirrored" harmonics 
are responsible for the extremely unpleasant distortions.



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