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Re: A lot of people don't understand dither

Posted by Werner (A) on March 4, 2007 at 23:59:30

In Reply to: Re: A lot of people don't understand dither posted by Christine Tham on March 2, 2007 at 14:30:28:

"Typical ambient noise levels captured by the mic is not sufficiently random"

I didn't say 'ambient', I said 'wideband noise, with a suitable distribution'. How about the thermal noise generated in all (equivalent) circuit resistances and capacitances right from your nice tubed large-diaphragm Neumann through the preamps, mixer, ADC front-end and ADC comparators. And this for 24 or more tracks summed. It is wideband, it is near-Gaussian, and it is present in almost all commercial recordings. And it dithers very nicely, thank you. (Try it!)

Have a look at the self-noise of your own minimalist recording setup. Ask yourself why it is not at -144dBFS, even with the microphone replaced with a short and all 60Hz harmonics discarded.

"However, the self dithering comes from the decimation process in a sigma delta ADC. "

My assertions are entirely independent of the architecture of the quantiser.

"Again, ambient noise levels are not necessarily random"

Again, I was not referring to ambient noise.

As for hearing below the noise floor, this is a sad mis-nomer. Yes, our ear can hear below the *noise level* (insofar no maskers are present), but no, we cannot hear below the *noise density floor*.
Try it: take a white noise density floor and then track a 3kHz fade-out into it while keeping an eye on the signal spectrum AND on the summed level (integral of noise density).
You can do this at any level. You'll find you can resolve the sine down to 30dB or so below level, but you'll also find you'll lose it right when hitting the density floor.

(However, it would be interesting to try this with stereo noise and the sine panned dead-center. I imagine one can get a couple dBs lower then.)

"reduce the color depth from 24-bits to 16-bits with and without dither and then compare the results. Your eyes will notice banding if dither is not used."

Well, that's what said. Your analogy is wrong because in the case of truncating an image from 8/8/8 to 5/6/5 the target quantisation noise floor is so high that there is no dither-able noise present in the image at that level. Most of the noise visibly present in the source image is correlated and/or of insufficient bandwidth (shot-noise, algorithm quantisation and errors, dark current, and the shot-noise of the dark current ... none of them remotely gaussian). That's why you need to add external dither.

But not necessarily so in audio where we know that the presence of thermal noise at about -120dBFS (or higher) *per track* is a given. Whether a fiinished recording contains enough such noise to be effective as dither for 16 bit output depends on the structure of that recording. But many real, commercial, non-minimalist, recordings seem to qualify for this.

Not that this matters a lot.

bring back dynamic range

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