kent walker wrote on Sat, 12 February 2005 21:52 |
Well, this is what Bob Katz writes in his mastering book anyway. I've never read the claim that a 16 bit system can give a perceived dynamic range of 115 dB anywhere else, but I bet he has a source or a reason for saying that, maybe it is the noise shaping thing. Thought I would check it against your findings.
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Hi, guys. Didn't I write something like "it is possible to hear a test tone of as low as -115 dBFS in a properly-dithered 16 bit system". That's the ABSOLUTE limit of conceivable audibility in a dithered 16 bit system. You can prove it yourself with a pair of headphones, an excellent, low-noise and linear D/A converter.
Whether this translates to 115 dB of true dynamic range is another question. It's probably more like 105 or so before the dither noise overcomes the signal. We're talking "very low" here... ironically, though, I do believe that 24 bit masks the low level signal less than 16 bit, though it is an EXTREMELY subtle difference, and mostly makes a difference in cumulative processing. In other words, "start at 16 bit, process (at 24 to 48 bits), and end at 16 bit" sounds more veiled than "start at 24 bit, process (at 24 to 48 bits) and end up at 16 bit".
I've mastered some pretty transparent-sounding masters that originated in 16 bit by using noise shaped dither, but the number of "A pluses" I give to some of my "originally 16 bit sources" is far less than the number that my 24 bit sources have received. So, on the average, sounds better.
The irony is that 1/2" tape sounds great, and has a noise floor 20-30 dB noisier than 16 bit digital. I think I have some grasp on an explanation for this apparent contradiction, but would like to leave that for another time.
Regardless, to my ears, it's really a matter of noise accumulation, as we have seen that a properly dithered processing chain will not add quantization distortion.
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Are any 24 bit converters using dither? I know many don't. I am familliar with the limitations of dynamic range being 120 -something dB. Just curious if any designers have found it worthwhile to add a little dither anyway.
-Kent
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On the A to D converter side, I'm pretty certain that the laws of physics will take over and adequately self dither nearly every source we can conceive of to 21 or 22 bits, I'll bet my life. But if you were going for 20, 19, 18 or below, I'd recommend using proper dither in the converter, since, as Jay Frigoletto mentioned, it is highly unlikely that the "natural" noise sources will be random enough or have enough high frequency energy to totally and properly dither all the source to less than, say, 21 bits for the sake of argument.
But once you get past that and into processing, yup, it's desirable to dither every 48 bit (or 32 bit, or whatever) calculation upon reduction to 24. Is it REALLY REALLY important? Well, cumulatively it is. But the audibility of the quantization distortion from a single conversion from 48 to 24 is VERY VERY slight, and I would fail a blind test on that one.]