[quote title=Terry Demol wrote on Sun, 26 June 2005 13:31
I see your point here, but I'm not sure I agree %100.
It would depend on the ADC's inherent IMD at higher
frequencies. It should be very low for a well designed unit
and as such should not be a significant issue. Possibly some
opamp based IP circuits are more susceptible to higher freq IM
due to their rising distortion Vs rising freq characteristics
but there are others that are very good HF performers.
Also IME low gain apps such as ADC front ends are the easiest
to keep linear at higher frequencies.
Regards,
Terry
[/quote]Terry,
I believe you were talking about some ultrasonic impact that happens in the air. David Satz. pointed out that such interaction only takes place at very high pressure levels, and to the best of my knowledge he is correct. But I decided to go with the assumption that some degree of what you said may possibly be correct, and the simple logic I used suggests that it is UNDESIRABLE to include the ultrasonic.
Now you are talking about AD non linearity at ultrasonic frequencies, which could completely changes the conversation. But "surprisingly", my answer is the same:
Any non linearity at high frequency, be it a converter, the air itself, the speakers, is undesirable. The presence of such non linearity will ADD signals that were not present prior to recording. The signals we want are already picked by the mic.
So anyone that wishes to have their gear extended to higher frequencies, better be sure that the linearity holds up over the range of operation. The arguments are often: "well, the device is less linear way up there but we do not hear that high", and of course such arguments are flawed, because non linearity may make signals we do hear at lower frequencies.
I read the Boyk paper a number of times with great interest, and I appreciate his work and his contributions. He measured all sorts of musical instruments in very high frequencies, and I do not dispute his findings at all. What I disagree with are the conclusions regarding what needs to be done in view of his findings. Some people, including some well known ear people understandably came to the very simplistic conclusion that we need to record it all, and that there is no harm in doing so. I disagree with that conclusion:
We want to record what we hear at the performance place, and nothing more. Any mechanism AT THE PERFORMANCE, by which high frequency energy will "fold back" to the audible range will be picked up and recorded. We need to stay true to that recorded material. We can do so by LIMITING the mic to the bandwidth of the ear, and by NOT ALLOWING the material we do not hear into the electronics. If we allow high frequencies that we did not hear at the performance into the electronics audio chain, we in fact become "sitting ducks" to any non linearity we may encounter.
Of course, I am not suggesting that all audio gear be limited to 20KHz. The frequency cutoff is a COMPROMISE between various factors (including "safety margins"). I am suggesting that it is good to eliminate signals above what we can hear, when all considerations allow so. I am saying that the arguments suggesting an advantage in recording higher frequencies than we can hear are "180 degrees out of phase". ..Or put another way the opposite of what they should be.
We are lucky that mic and speaker makers did not push the bandwidth in the manner that the converter makers did. With 20KHz mic, a high frequency non linearity is not a problem because there are no high frequency "tones" (energy) to "fold back".
Again, the high frequency (beyond our hearing) is either "trouble" or "potential trouble", making one more argument against 192KHz sampling rate. I have a few more up my sleeve, for another time.
A little off subject:
The MP3's are getting better, because they are trying to be "almost as good as possible". To do so, one needs to have a relatively clear grasp of what is the maximum possible required bandwidth and dynamic range. Only then does one start applying principles of psychoacoustics for data compression.
Meanwhile, the "leadership" of pro audio (many of whom are very lacking in technical knowledge) have pushed the industry into a far from optimal place! Stay tuned for the next installment of this insanity - people with lesser technical leading the industry towards 384KHz. Are the blind leading the ignorant, or are the ignorant leading the blind?
Regards
Dan Lavry
www.lavryengineering.com