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Re: A lot of people don't understand dither

Posted by Christine Tham (R) on March 6, 2007 at 16:55:21

In Reply to: Re: A lot of people don't understand dither posted by Werner on March 6, 2007 at 00:00:26:

I'm sorry, but you are trying to change the subject.

To quote the original statement made by Todd:

"This is also why applying dither noise isn't necessary when truncating data of 24 bits or more down to 20 bits or more. The ambient noise alone would be adequate for such application."

As I've shown, this statement is NOT true (at least not generally). Ambient noise alone is not adequate (and I think we both agree on this).

You seem to be arbitrarily trying to limit the scope of the discussion to specific situations such as poor quality DACs or noisy preamps or mixing a LOT of tracks. Whilst we can argue on and on about specific cases (and whether they represent a "norm" or not), the original statement without qualifications is not factual. Remember, it only takes one counter example to disprove a generalisation.

Even if we wanted to discuss your specific cases, your examples are not valid.

Let's consider your example of mixing 24 or more tracks. You said:

*** Even when all individual tracks are at around 20b dynamic range, their sum will have a higher noise floor unless massive-scale noise gating is used during mixdown (as in the old days). ***

I suspect you don't have a lot of experience mixing. If you mix a lot of tracks, the final mix will exceed 0dBFS unless you attenuate the levels of individual tracks. Therefore the noise floor does not necessarily increase. For example, let's say I have 2 tracks (both with peaks just under 0dBFS). The overall mix will have peaks potentially at +3dBFS unless I attenuate each track by -3dB. Doing so reduces the noise floor of each by -3dB, so the noise floor of the mix is actually at the same level as for the individual tracks.

In fact what typically happens in a mixer is by the time you attenuate the levels of individual tracks, noise levels below 20 bit of the individual tracks are in danger of being "truncated" out of the mix. For example, if we assume noise is at the 20 bit level, attenuating the track level by say -20dB (not atypical for a multi-track mix) causes the noise level to be now at 24 bit level where it is in danger of being truncated out of the final mix altogether (assuming that the mixer operates at internal resolution of 32-bit floating point).

Dithering is advisable to reduce the quantisation noise generated by this truncation, even if the final mix is at 24-bit depth. So mixing actually makes it even more neccesary to apply dithering.

*** Many use ADCs limited at 17-18b performance. ***

No. You are confusing between the measured dynamic range of an ADC and the actual level at which the noise is sufficiently "dense" or "wideband" (your terminology) to act as an effective ditherer. The laws of physics does not change depending on the quality of the ADC. An ADC with a dynamic range of say 102dB (corresponding to your "17 bit" of performance) will have a "thermal noise" envelope far lower than -102dB. Measure it yourself. What you see around -102dB are harmonic spikes that are not sufficient to cause dithering.

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