When is 24 bits not 24 bits?

This is nerd stuff, for those who get into the technical side of audio. For non-nerds, the takeaway is simply the warning that “24-bit” digital doesn’t mean what you think it means; that “high resolution digital” is often not a whit better than regular CD quality—not because it shouldn’t be, but because it may not in fact be high-resolution; that the domination of audio by baloney did not end when digital audio began, and will never end, for baloney is eternal. And that the one way to judge audio gear and processes is still the same as always: listen listen LISTEN!.

If my car’s engine has eight cylinders, but only five of them have pistons, I will be disappointed in its performance.

If a digital converter is in 24-bit format, but its distortion and signal-to-noise ratio (SNR) correspond to 15-bit performance, I will likewise be disappointed.

Some 24-bit converters indeed do not reach 15-bit performance. Some reach 15 but not 16. Full 17-bit SNR and distortion are uncommon, and 18-bit performance is rare. Better performance is rarer still, which matters because people are discussing “24-bit sound” when they haven’t even heard 18 bits. This is like discussing V-8 performance based on the engine described above.

It also matters because more bits are indeed required for noise-free reproduction at realistic levels. See Louis D. Fielder, “Dynamic-Range Issues in the Modern Digital Environment” J. Audio Engineering Society, Vol. 43, No. 5, May 1995. “A dynamic range of over 120 dB is found to be necessary in the most demanding of circumstances….” (In a personal communication, Mr. Fielder stated that up to 124 dB may be needed.)

Using the formula “dB of dynamic range = 6.02n + 1.76,” where n is the number of bits; and bearing in mind that you can’t have a fractional bit, a range of 120 dB requires 20 bits; 124 dB, 21 bits. More bits yet are required for the analog-digital converter used for master recording, to allow for uncertainty in setting level. Twelve dB, or 2 more bits, is reasonable, which is not to say it will always be adequate!

When I choose a converter for my next recording, I will listen to the candidates, of course. There’s no other way to judge! But before I go to that trouble, I want to know I’m getting enough bits of performance, not just format. The pickings are thin.

Originally published Oct. 12, 2007 at hificritic.com

Copyright © 2007, 2013 James Boyk. All rights reserved.

Copyright © James Boyk 2013. All rights reserved.
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