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Old 10-04-1999, 01:43 AM
Monte McGuire Monte McGuire is offline
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Join Date: Dec 1969
Location: Malden, MA USA
Posts: 292
Default Re: Pro Tools Sound Quality???

Rail,

I don't think a super wide TDM bus is needed if every processing step dithers the accumulator before going to the TDM bus. In that case, the only difference would be the small resolution loss from adding dither at the 24 bit level, and that is a very small increase in noise. Remember, true uncorrelated noise is relatively benign, but the cross modulation distortion products caused by truncation are not.

To put some numbers on it, assume you have three inserts on each track of a 32 channel mix. The added noise of the dither between each plug in amounts to a noise floor of around -140dBFS per track. Summing 32 of these tracks at unity gain yields a noise floor of -125dBFS.

Currently, that's about as quiet as a single channel of conversion, so I don't think it's really going to have a huge impact. Plus, if the mixer worked properly and you could use the faders to attenuate individual tracks, then the resulting noise floor could be a good bit lower - there is no need to run all the faders at unity gain and the noise floor could drop quite a bit.

However, if dither is not going to be used, then yes, a wide TDM bus would help a lot. It seems to me that this is a great argument for dither in that we could get the same sound quality in a 24 bit chain as we could from a true 48 bit path, plus a little extra noise.

One final comment about a wide TDM bus. Native floating point based systems are usually undithered between processing steps and they have an internal chain that is essentially the same width as their 24+8 bit accumulator. Given that the new Digi 001 system uses only floating point processing and has session file and plug in compatibility with PT TDM, it would be a great test to run the same mix on an 001 system as well as the current TDM mixer and a new dithered TDM mixer.

Since floating point processing will clean up the plug ins as well as the mixer, I wouldn't be surprised if a mix sounds best on the 001.
I know I have been pleasantly surprised by the quality of work some of my friends are doing on low end floating point systems and frankly, if the editing and latency on those systems were better, I'd have jumped ship!


Monte McGuire
mcguire@world.std.com