Analog versus Digital. Observations from an ex-Dolby engineer on the Debate

Robert Katz 1 Comment

Subject: RE: Back to Analog
Bob:Great article. If I may, I’d like to add my understanding of the problem. Linear PCM is at it’s best with full-scale signals. PCM achieves its maximum linearity, and thus sonic resolution, at high levels. At the same time, linear PCM has low resolution, and thus has its highest non-linearity, at low levels. Analog tape compresses, and thus distorts, at high levels, but is linear and has low distortion at low levels. If we can lower the noise floor of analog tape, and avoid the overload characteristics in our recordings, we achieve great performance. That’s where 1/2″ 30ips comes in, and even technology such as SR noise reduction.

These methods increase the dynamic range sufficiently so that the noise floor is low and the overload characteristics are high enough to be out of the way. 1/2″ 30ips also provides a large recording area which gives good linearity over a wide dynamic range. Thus, I argue that the reason why 1/2″ 30ips sounds better than 16bit PCM has less to do with dynamic range and more to do with superior low level resolution.

Another way of looking at it is in terms of bits of resolution. 16 bit PCM, at an operating level of 20 dB below full-scale, is really around 12 bits of audio. At 40 dB below operating level, there are only 6 bits describing the audio signal. To grasp what this means, a 1-bit error in the digital description of the signal gives a distortion of 1.5 percent at 16 bit resolution.

(Note, part of his argument is irrelevant, since dither linearizes the PCM system at low levels and gets rid of this type of distortion. But I have no argument that 16 bits are not enough, for many other reasons)

Even if the digital description of amplitude is dead-on, there is significant harmonic distortion in the audible frequency range going on at 6-bit resolution. And it just gets worse as thelevel drops on digital recordings. How can we possibly experience good sonic detail at low levels with 16 bit PCM? At 40dB below operating level, analog tape maintains its linearity, and thus reproduces with better clarity. I don’t have figures at hand, but I’m sure the harmonic distortion is very low.

The point is that what we’re hearing is understandable in simple terms, and can be measured with common instruments. It’s not rocket science. All of this just supports your point even more, that analog recording has not yet lost it’s edge… -Michael Karagosian President, Cinema Group, Ltd.
(once upon a time, an engineer with Dolby Laboratories)

Of course, it can be argued that with dither, a 16-bit system can perform as well as that analog tape. And more and more linear A/D converters are being developed with excellent linearity at low levels. But the ears seem to indicate that despite the fact that analog tape’s noise is higher than the dither noise of 16-bit, the apparent low level resolution of the analog tape is superior to 16-bit digital. Once you get to 24 bits, though, low resolution of the digital improves, and the subjective judgments get closer.

I think it’s because the “effective sample rate” of analog tape is higher… Or, to look at it another way: the filtering in the analog tape is gentle, with response to about 30 kHz, but severe filtering in the 44.1 ks/s digital system is required, and I think I hear the artifacts of that filtering… Or because of analog tape’s slightly greater 2nd and 3rd harmonic distortion, even at low levels, than digital, which warms up the sound without fuzzing it. As opposed to “naked digital” which can be extremely revealing of any of the harshness in a source. When you move to 24/96 rates and higher, many people find the digital sound to be as pure or purer than analog tape.

I replied to Michael:

Thanks very much for the comments and your perspective, Michael!

Of course, one thing neither you nor I discuss directly is why analog seems to have more space. However, the relationship between “more bits” and increased space (e.g., 24-bit has more space and depth than 16) seems to point out that the spatial and depth information in a recording are directly related to low level resolution as well.

By the way, I was the first official “user” of Dolby SR in the New York area and on the “first users” list!

Best wishes,

Bob

And Michael has the best last word:

Bob, glad you liked it. I appreciate that you’re keeping the topic of quality alive. I feel like the music industry has been overridden by the surge of technology, a lot of which is not music-friendly, and most of which is poorly understood by users.

Michael

Comments 1

  1. Definitely. I have been transferring some favorite selections to digital recently. 16-bit is a significant upgrade to most consumer analog, but you really need to dial in the recording levels perfectly to get optimal resolution captured. When I use 24-bit, I feel like I have some latitude to play with later on. I nudge the levels down to about half and normalize later. The final product sounds just as good as 16-bit, possibly better because I know there is no clipping.

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