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Author Topic: Dither - And I Guess I Should Know The Answer To This  (Read 2016 times)

Barry Hufker

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Dither - And I Guess I Should Know The Answer To This
« on: April 15, 2008, 12:07:26 AM »

Today I've been mulling over the notion of digital dither and pre-emphasis.  So I need someone with a logical mind to tell my illogical mind the answer.

The question is: Would there be a sonic benefit (according to theory anyway)if one were to:
1. Record digital audio in 24 bits
2. Create a "pre-emphasis" by boosting the mid and high frequencies at the time of recording or in "post" by EQ'ing the file after the recording.
3. Dither the recording and noise-shape it to 16 bits.
4. De-emphasize the mids and highs back to their "natural" or "original" balance before pre-emphasis.

It seems to me there ought to be a benefit, which should be less coloration of the audio from noise-shaping.  Often, when noise-shaping, there is a boost in the high frequencies which has a poor effect on "S" sounds and makes some voices/instruments sound metallic.  The method above is an attempt to reduce that effect.

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bruno putzeys

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Re: Dither - And I Guess I Should Know The Answer To This
« Reply #1 on: April 15, 2008, 05:33:07 AM »

The idea of matching pre-emphasis curves and noise shaped dither has been proposed earlier by Peter Craven, in the context of slow-rolloff reconstruction filters.

It doesn't work exactly like you were hoping though. Step four produces output words longer than 16 bits, making it necessary to dither/truncate again. So, you can only use it if there is no need to return to 16 bits after the de-emphasis. This is the case when de-emphasis is done on the replay side.

What would do then is apply standard 15us/50us emphasis in step 2 and use a noise shaped dither with the exact same shape for step 3. Then you can move step 4 over to the replay side by setting the emphasis bit on the CD. Older CD players did their deemphasis in the analogue domain, newer ones use a 24-bit digital filter for this. Either way, this will do the trick. I haven't done any calculations but off the top of my head I think you'd gain about 7dB in noise performance.
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Barry Hufker

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Re: Dither - And I Guess I Should Know The Answer To This
« Reply #2 on: April 15, 2008, 10:28:02 AM »

Thanks Bruno for taking time to offer this great answer.
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