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Phase Distortion, dv/dt, and Slew Rate

This has been bothering me for a long time. It's one of those things I don't understand that haunts me.

It is generally accepted that we can't hear even surprising amounts of phase distortion. If you look at the phonograph record processing and reproduction chain (bunches of filters, and each RIAA filter has its own design and phase distortions in recording and playback) there is an absolute boatload of phase distortion between original performance and the signal hitting your speakers. And yet records can sound awfully good.

So what? Just accept it and move on. I can't. Here's the pickle:

I don't think it's just a matter of phase distortion. I think that phase distortion of real-world signals MUST affect dv/dt and slew rate requirements in unpredictable ways.

A square wave can be thought of as being the sum of an infinite number of odd harmonics at specific amplitude ratios, all in phase. Passing a perfect square wave requires infinite frequency response and slew rate, so square waves can only be approximated in the real world.

Now picture two sine waves or other signals of fixed "shape" but shifting frequency. I'm thinking here of two (or more) voices singing in unison but with vibrato. As they independently slide around the central frequency the phase relationship varies unpredictably. So the needed slew rate varies too, but the peak slew rate could be very high. Now add a chorus of voices or instruments all doing vibrato and you have a Fourier function that has a whole mess of high-frequency components. Add the phase-vagueries of the recording/playback chain, and the problems compound. I think that may be why recording choruses is especially difficult-the slew rate/frequency requirement may be just awful, far in excess of what's apparent by the natural frequency content of the human voice. And there are woefully few good chorus recordings.

Walt Jung and others have written extensively about the need for high slew rate. I think I've become a believer. But that's not going to fix recordings already made-you can't unclip peaks.

I tried to do the modelling of sine waves with variable phase relations (d^2 (sin x + sin y)/dt) to find the maximum rate of change) but no longer have the math skills.

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Topic - Phase Distortion, dv/dt, and Slew Rate - Lee of Omaha 11:36:19 12/15/15 (10)


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