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Re: [microsound] talk about supersonic signals and distortion



macrosound:
>Anything significantly higher than 20,000 Hz is cut completely. But what
is the point of using frequencies that high.

There are several reasons for using frequencies over 20 kHz, and for using 
sample rates over 44.1:

* at -audible- frequencies, e.g. 12 kHz, you can only differentiate a square 
wave from a sine by encoding overtones above 20 kHz. There is almost no 
resolution in high frequencies at CD sampling rate;
* for an 11 kHz tone, there are only four data points available per cycle, 
and thus only 3 possible phase displacements. this means encoding the 
'space' of high frequencies is virtually impossible;
* even if we may not 'hear' frequencies over 20 kHz, parts of the bone 
structure respond to them;
* further, many studies indicate that listeners are able to distinguish 
between recordings containing 'ultra-high' frequencies (over 20 kHz) and 
recordings which do not ;
* 20 kHz is only a median value for the upper limit of human hearing- I've 
seen speculation that some listeners may be able to hear up to 25 kHz;
* there are countless other problems with encoding high frequencies at 44.1- 
corrective circuits that smooth out high frequency waves because the low 
resolution causes distortion, and anti-aliasing filters that preclude 
reliable frequency data even in the high end of the -audible- range.

some of the above information is from Dominique Bassal's paper on mastering 
and electroacoustic music, which will soon be available from the CEC 
webpage.

best,
Ian

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