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A New Method for Measuring Distortion Using a Multitone Stimulus and Noncoherence

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A new approach for measuring distortion provides a continuous distortion curve versus frequency, and the method is suitable for use with noise, music, and multitone stimuli. Distortion measures are derived from dual-channel analysis of the noncoherence between the stimulus and the response. The mathematics is based on Volterra theory, which is an extension of linear system theory but applied to nonlinear systems. Because the technique uses standard signal processing, the approach is simple, accurate, and repeatable.

JAES Volume 56 Issue 3 pp. 176-188; March 2008
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R. Belcher
Comment posted August 27, 2008 @ 11:23:08 UTC (Comment permalink)

I was very interested to see that this paper promotes the idea that non-linearity measurements based on multi-tone test signals might be more appropriate for audio. The British Broadcasting Corporation came to this conclusion in the 70's and actually produced a multi-tone test that gave good agreement between double-blind subjective assessments and objective measurements of audio non-linear distortion. The method had the disadvantage that the digital circuitry caused the test set to be more expensive than a conventional one. The patents have long since expired and the test method is now part of IEC standard 60748-4-3 for dynamic testing of ADCs. As it is now easy to implement using software on a PC and an audio interface, perhaps this comment will encourage others to investigate what this technique might now offer when they are making audio measurements.

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Author Response
Steve Temme
Comment posted September 3, 2008 @ 16:16:13 UTC (Comment permalink)

Dear Raymond,

Thanks for your comments. Have you ever tried using Pink Noise as the stimulus for non-linear measurements?

Best regards,
Steve Temme

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R. Belcher
Comment posted October 16, 2008 @ 16:08:10 UTC (Comment permalink)

Dear Steve,

Yes, I am aware that filtered and shaped random and pseudo random noise can be used to measure non-linearity as the BBC work I mentioned stated with that approach.

Best regards

Allan Belcher

William E. Whitlock
William E. Whitlock
Comment posted October 20, 2008 @ 10:41:57 UTC (Comment permalink)

A variation of your method was presented in a paper by the late Deane Jensen and Gary Sokolich, "Spectral Contamination Measurement", at the AES 85th Convention, 1988 (preprint #2725). In their (and my) opinion, much of what audiophiles describe as "the veil" consists of low-level, non-harmonic distortion products. I believe a major source of these is due to widespread use of op-amps under heavy negative feedback. As frequency increases (at ultrasonic frequencies, for example), the op-amps essentially run out of open-loop gain and start approaching open-loop conditions. This allows their inherent non-linearity to intermodulate distortion products from previous stages and the result is non-harmonic distortion products. It is one very good reason for passive bandlimiting at the input of every active amplifier stage. Otherwise, the effect is cumulative - and demonstrably ugly sounding. The test Jensen and Sokolich proposed is very similar to the AP (Cabot) multi-tone test.

Bill Whitlock
President & Chief Engineer
Jensen Transformers, Inc.
(and recent AES Fellow)

Author Response
Comment posted October 21, 2008 @ 16:01:12 UTC (Comment permalink)

Yes, the multitone test reveals cross-modulation products that are invisible with a single sine.
But I want to stress the fact that our method generalizes the measurement of distortion on arbitrary signals: noise, speech, music...
In our opinion, it is a much more powerful approach because it can be used, and give sensible results, on real world signals.

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Harry Brown
Comment posted November 30, 2008 @ 20:33:52 UTC (Comment permalink)

Dear Mr. Whitlock,
I had wondered whether the 1988 article by Jensen and Sokolich was available on this site along with the more recently published papers? I am conducting a research project for my Degree and would love to view the article. I am a fan of Jensen Transformers!
Harry Andrew-Brown

William E. Whitlock
William E. Whitlock
Comment posted December 2, 2008 @ 17:09:42 UTC (Comment permalink)

Harry Brown, thanks for your kind words. Click here to view the 1988 Jensen/Sokolich paper. It also anticipates using real music as a test signal. Edited by moderator

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