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Accumulating noise

Posted by Homer (A) on May 5, 2002 at 05:57:32

In Reply to: Re: Thanks, but... posted by Charles Hansen on May 3, 2002 at 08:11:30:

First, I really appreciate your replies. I hope I don't seem too argumentative, but there are just a couple more things I don't quite understand.

The way you've characterized noise performance of the DAC in bits is a nice conceptual model, but lacking in specific details. Is this absolute performance, or what percentile of noise falls within the ranges your specifying? As I've said, I don't know much about DACs, so perhaps this noise is really fairly addative. But, as it relates to quantization noise from digital processing, I assert that the DAC noise you've stated will *hide* nearly all of it. In terms of absolute ranges - yes, the most-significant bit in which you'll find noise will now add to whatever you previously had, but statistically, I believe you'll start to approach a gaussian distribution with a sigma that increases slightly (much more slowly than addative), each time you add in a new noise source. This means that noise won't creep into the signal nearly as fast as you suggest.

Furthermore, I was assuming that the dither was added after oversampling, when I was talking about processing. The analog filtering will turn the dither into slightly more precision, in the signal band.

Another thing to keep in mind is that the signal you're converting is COMPRESSED!! The level of performance you're talking about is really better than the DVD medium is designed to support. Except in some carefully constructed test-cases, DVDs just can't supply 8 full bits of precision. Personally, I like my source material to be the limiting factor of my A/V systems, but beyond a certain point, you're just helping the viewer see how bad the compression artifacts of DVDs really are. I actually want to try adding dither in the DCT-domain, based on the quantization matrix. I predict that the image will look more noisy, but you won't see any ringing artifacts. Unfortunately, the additional digital noise may not please some cadence-based line doublers, (though that could be remedied, if they determine cadence from the non-dithered stream, and then line-double the dithered one).

Finally, I agree that it doesn't make a whole lot of sense to put this digital processing in the player, but then it doesn't make a whole lot of sense to put the DAC in the player, either. Ideally, you wouldn't need any of it. However, the nice thing about digital processing is that, unless it's designed by a moron, you can turn it off without incurring any performance penalties for it being there. Also, putting gamma-correction in the player will result in better system-wide noise performance, if the gamma needs to be increased. Same thing with black-level control.

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The Cable Cooker

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