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Critic's Corner Discuss a review. Provide constructive feedback. Talk to the industry. |
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Critic's Corner Discuss a review. Provide constructive feedback. Talk to the industry. |
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In Reply to: RE: Audio Science Review posted by E-Stat on October 19, 2019 at 06:29:16:
Yeah, but...
In a simple harmonic distortion test, there is one tone and the harmonics are by nature correlated. For a bunch of other reasons, a simple harmonic test doesn't tell you much.
In an IMD test, it is possible to correlate the tones, especially if you have enough of them.
Whether the tones are correlated to each other or not is only a detail that isn't the crux of the situation.
The critical point, I think anyway, is that in actual systems that try to reproduce many tones, the power in all the intermodulation distortion products just completely swamps any simple harmonics. That is true in a home audio system as much as it is in a modern CATV or wireless transmission system. This was also described in great detail in the middle of the 20th century or thereabouts. Guys at Bell Labs and the BBC churned out papers on the subject.
Pretending that this doesn't matter is only fooling oneself.
In addition, the audio world tends to be a too blasé about interfering signals that are running around the system through the power line connections and so on. The general idea is that "if it's out of the audio band, you can't hear it", etc. Whether you can hear the stuff outside the band isn't the important part. These signals can and will mix (mix in the mathematical sense, not the usual audio engineering term) with other signals to create yet more distortion products within the audio band.
This problem has gotten worse over time with digital audio becoming a big part of home systems plus the garbage spread everywhere by various microprocessor based computing systems like you might find in your alarm clock and by poorly filtered switching power supplies.
The usual response to a comment like this is that the feedback system found in most amplifying devices cleans that right up. Not so! In order for the feedback to work you need adequate open loop gain at the frequencies you're trying to reject for the feedback to work. Guess what! The open loop gain drops dramatically in most amplifying devices aimed for audio use outside the audio band. So, this rejection mechanism doesn't work. This isn't some secret mystery either - people just choose to ignore it.
Anyway, I've gotten way off topic and have probably pissed off most people who had the bad fortune to stumble into this thread. So, fini.
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Follow Ups
- RE: Audio Science Review - CG 06:59:56 10/19/19 (3)
- RE: Audio Science Review - E-Stat 07:06:14 10/19/19 (2)
- RE: Audio Science Review - CG 08:00:48 10/19/19 (1)
- It appears we agree on the topic - E-Stat 08:09:41 10/19/19 (0)
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