Hello, all--

I hesitate to bring up this topic while the Doppler wars are still in an
unsettled state (please, guys, don't bring it here), but I've spent much
of this past week trying to learn about and measure the "transient
intermodulation distortion" and/or "slewing-induced distortion" of some
recording-related equipment, and would like to report in.

This is something I've been wanting to come to terms with for years. I
began by going to the AES Web site and downloading and reading the papers
of Prof. Matti Otala from the 1970s and early 1980s.

More up-to-date analyses of Otala's work have shown that what his tests
show is not limited only to dynamic distortion. And with the equipment I
have available, the best signal source I could create doesn't have nearly
as high a slewing rate as either of Otala's two main recommended test
signals. So my results are probably more like those of ordinary IM tests,
only weighted somewhat more toward revealing dynamic distortions.

I'm virtually certain that better-controlled test signals could provoke
worse behavior from some circuits, and I'd really like to see that, since
it could well reveal even further differences among different circuits.
Still, even with what I have, I was able to find some clear distinctions
among the distortion performance of different pieces of equipment.

That's about all I can say because of the limits of my test signals. OK,
I can say a few things in the more clear-cut cases:

- The ART "Tube MP" is grotesquely bad. By a considerable margin it had
the highest distortion, in its normal operating range, of any equipment
I tested. And the Aphex "Tubessence" preamp (model 107, not the 1100)
had second highest.

- My better preamps (Millenia Media HV-3B, FMR "Real Nice Preamp", Grace
Lunatec V2 and V3, M Audio DMP3, Sonosax SX-M2, Symetrix SX-202, and a
dbx 760X modified by Jim Williams) all had low noise and distortion when
stimulated by this test signal, with small differences in favor of the
Millenia Media preamp. They had distinctly different distortion spectra,
but were more or less similar in the total amount of their distortion.

- Despite its somewhat higher EIN specification, the noise/distortion
floor of the RNP in the presence of the test signal really wasn't
noticeably higher than that of most of the other good preamps.

- The M Audio "Flying Calf" A/D converter, surprisingly, held its own
against far more expensive converters, while the D/A section of the
"Flying Cow" converter was not nearly as good--though my "Cow" is an
older model, and the chips (cow chips?) in their newer models have a
higher specification.