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I think you really do need to look at the display in this case

Posted by Dave_K on August 17, 2015 at 06:22:50

In Reply to: RE: Wow, a 7KHz + 60 Hz IMD test? posted by Tony Lauck on August 16, 2015 at 12:42:05:

Sorry, I should have explained what I was getting at yesterday but was running out the door.

The test software is reporting what it calls "intermodulation distortion + noise" using the SMPTE frequencies but if you look at the IMD+N level reported in the summary (0.0019%) it seems obvious that the software is neither using the SMPTE analysis method nor some other method of quantifying the IM sidebands. Or at least it's not doing it correctly. If it were, the result would be more like an order of magnitude lower.

Given the reported level of IMD+N (0.0019% = -94 dB), I suspect the software is including THD + IMD + noise + anything else not the signal. If I'm right, it's a useless test because all it's really measuring is the level of power supply hum in Archimago's test rig. In fact, I think 5 of the 8 metrics reported in the RightMark summary appear to be just measuring power supply hum: Noise level, Dynamic range, THD, IMD + N, IMD + N (swept).

But in order to know for sure whether I'm right or not, you would need to blow up the display to see what's going on near 7 KHz. Maybe you would see +/- 60 Hz sidebands at the -100dB level, but I doubt it. And I suspect the spectral leakage around the 7k peak in an FFT plot would obscure any lower level sidebands.

I think Archimago has a problem with power supply hum in his test setup which is above the level of any distortion products and it results in inaccurate metrics being reported by his software. And the only way to really separate the power supply related components from distortion products is via FFT analysis, in which case choosing a very low f1 for the IMD test isn't helpful.

Also, I really do think the choice of 7KHz and 60Hz frequencies is dumb. I honestly thought that the SMPTE IMD test was a legacy test that nobody used anymore, but after searching around I see that some people are still using it. The problem I see with it is twofold: First, the choice of a very low f1 will make it hard to interpret the result via FFT because low level sidebands could be buried in the spreading of the peak at f2. Second, choosing an f1 that coincides with the power supply frequency could potentially lead to confusing results in systems with a power supply hum problem. I think the DIN method with 8KHz + 250Hz is more useful, as is the HF intermodulation test.

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