Multitone testing [Archive] - Audio & Video Forums


View Full Version : Multitone testing

Richard Black
11-15-2004, 05:30 AM
Hello Audioreview forum members, glad to be joining your number.

Got here via a protracted links route including Jon Risch's site, which seems to arouse passions. It mirrors, in some ways, work I've been doing for some years, and I'd just like to make the following observations:

If a system is linear, a single test will prove the point. In theory! Obviously you're limited by noise and other extraneous problems of resolution, but if a sine wave goes in and comes out _completely_ indistinguishable (apart from amplitude and phase) the system is linear.

In practice, because of the 3-decade bandwidth of audio, and given limits on measurement resolution, you need to try a few different frequencies to prove the point. For example, crossover distortion may be buried in noise at 200Hz but very clearly visible at 20kHz. Distortion due to stressed capacitors may be buried in noise above 1kHz but detectable at 20Hz. And so on. But if sinewave testing from 20-20k shows no discernible distortion, intermod testing ain't going to pick up anything. If this statement is untrue, a lot of people would be very interested to hear the exact conditions under which it breaks down!

However, if a system is nonlinear - and in the end of course they all are - any sinewave will be afflicted by harmonic distortion, and any combination of them will exhibit intermod. The advantage of intermod is that it can show up, in a single measurement and single graph, more detail about the nature of the distortion than any small number of HD tests. However, they all need analysing! 19/20k intermod, for example, is great for testing digital recorders because at that sort of frequency any harmonics get ruthlessly filtered off, while audio-band IM tones survive (and of course are directly relevant to what we hear, unlike the 17th harmonic of 20kHz :) ) I don't really see anything wrong with Risch's test but in the end it's entirely horses for courses: some will find it simpler to apply and interpret than a variety of single-tone signals, others won't.

By the way, UK hi-fi reviewer Paul Miller has been performing, and publishing the results of, 3- and 4-tone IM tests for several years. His are really quite funky, using typically a couple of steady tones plus one swept. His rainbow-coloured pseudo-3D graphs are, I suggest, a model of intelligent data presentation. He's at, though having just looked I can't see any of the graphs in question! Old back issues of Hi-Fi Choice used to have lots of them.


11-15-2004, 05:59 AM
The first problem I have with Mr. Risch's test theories is that they are unnecessary. The existing measurement systems for non linear distortion in electronic circuits are based on mathematical theory going back hundreds of years to Laplace and Fourier and the practice based on them has been universally accepted by electronics engineers and mathematicians alike over the last eighty years. To the degree that his methods get the same results, they are correct. To the degree that the results diverge, IMO they are wrong. There are many areas for improvement between the correlation of electrical measurements and actual performace of audio equipment but this in not one of them.

The second problem is that his new method is arbitrary and unnecessarily complicated. There is no valid reason I can see for scientists and engineers to throw away their current test equipment. The measurement procedure is well established, well understood, and the results easily interpreted. We know how to get them and we know what they mean.

Mr. Risch seems to invent solutions to problems which don't exist, tries to upset entire ways of thinking which have evolved over decades with flimsy and unconvincing evidence and little or no underlying hard mathematical reasoning showing where the prior theories are wrong and his theories give the correct answers, and will fight to the death with anyone who disagrees with him. Have you read any of his theories about strand jumping to justify his position about loudspeaker wires? I am not going to engage in another long drawn out pointless battle over his "novel" ideas but when I see that they are accepted by the mainstream of electrical engineers, I'll take another serious look at them. So far not one electrical theory he ever posted made any sense to me and I am much too busy to spend more time rehashing them. Apparantly the people who design, build, and sell test equipment aren't convinced either. I haven't seen any new test gear appear which is specifically intended to apply his methods.

Richard Black
11-15-2004, 06:59 AM
"The first problem I have with Mr. Risch's test theories is that they are unnecessary."

Well, I guess I can't argue very strongly against that. By the way, he cites the original 'spectral contamination' test of Jensen or someone - wasn't there some justification of that at the time? Something to do with showing up quite subtle modulation noise or something like that, mostly in transducers as I recall. Sometimes new tests come in handy - though I'd need some convincing that amps and cables are the most urgent cases in point.

"Have you read any of his theories about strand jumping to justify his position about loudspeaker wires?"

Oh my life, if I come across one more argument about that, in the absence of some really quite convincing proof at any rate, I'll scream. It's well known that strand jumping is entirely due to the fairies at the bottom of the garden.


11-29-2004, 12:43 PM
If you do a search here on my moniker, or on Jon Risch, you will find a long thread on this test methodology and the problems with it. There are also links to people such as Steve Eddy, who also found problems with it.