sjc@angband.UUCP (Steve Correll) (10/11/84)
> Some people have been complaining about the sound of Sheffield Lab's CD > releases. One of the problems with digital recording as attempted in > the manner that Sheffield does is that the digital recorder does not > have enough dynamic range. We have people in our physics audio lab > designing and building a dynamic range compression device for digital > recorders because of that. One theory has it that a digital recorder > with at least 120 dB SNR is required to provide adequate dynamic range > for live digital recording. If you record to capture the peaks of > something like LAB-2 (I've Got the Music in Me by Thelma Houston), the > lowest levels will be about 40 dB below that. This leaves you with 50 > dB SNR, which is fine for analog systems, though not too listenable, > but not for digital systems. This ratios is a voltage difference of > only about 300 times. This means that the lowest range is quantized > with only about 8 bits (2^8 = 256). What does 8 bit quantization sound > like? I've heard demos at 10, 8, and 4 bits quantization. Something > sounds wrong at 10 bits; at 8 bits, everything sounds harsh; at 4 bits, > the sound is barely recognizable. A compressor can be used to place > the signal up where more bits are being used and quantization error is > not as noticeable. This is a problem with all digital recording > systems handling a high dynamic range signal. More bits are required, > or a signal compression system, to get around this. Of course, 120 dB > SNR is hard to maintain in analog equipment, and a digital system using > linear encoding would require 20 bits to get that much SNR, and > finally, the number of bits generated would be 1.5 times as great. Any > comments? A 16-bit linear quantization system gives 65536 different levels, so if Thelma Houston's dynamic range is 40dB, she can be represented by levels 655..65535. It takes between 9 and 10 bits, not 8 bits, to encode 655; but even so, I don't see that the poor sound you ascribe to 8-bit (or 10-bit) encoding, which provides only 256 (or 1024) different levels, has anything to do with Thelma, who will be represented by 65535-655=64980 different levels. Thelma's softest utterance will be represented with greater resolution than the loudest signal in an 8-bit system. It's a little misleading to start with the 90dB dynamic range of the CD representation (actually 9XdB), subtract 40dB for the program material, and proclaim a 50dB S/N ratio. The newest $1000 Nakamichi cassette deck has a noise level 75dB below the DIN 0dB point (the latter giving about 0.5% THD) according to High Fidelity magazine, so a similar calculation would give it a 35dB signal-to-noise ratio. (Even if you allow 3.0% THD, that adds only 7.5dB). True, digital representation does not degrade gradually as do analog representation when the signal approaches the upper amplitude limit. But I question the widespread assumption that digital does not degrade as gradually as analog does when the signal approaches the lower amplitude limit: see the paper on "dithering" in the March 1984 Journal of the Audio Engineering Society. It is a pity that the CD medium didn't reserve space for a few extra bits of precision so that someday, when 20-bit DACs are reasonable, manufacturers could offer them in players at extra cost to those who want them. -- --Steve Correll sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc