[sdiy] samp. freq for ADSR ?

[ASSI]

On Friday 02 April 2004 22:13, mark verbos wrote:
> Creating a 500Hz sampling frequency. Nyquist's theory is that the
> maximum sampled frequency is half the sample rate, leaving the Buchla
> MARF with a maximum of 250Hz (real low for audio).

This is a common misinterpretation of the Nyquist Theorem. The ideal 
reconstruction filter that would be necessary to reproduce a signal at 
fs/2 is not realizable (provided that the exact bandlimiting to fs/2 
was possible in the first place). If the reconstruction filter is not 
perfect, frequencies well above fs/2 and of course aliasing are the 
result. The reconstruction filter is also often used to attenuate 
quantization noise. Even though the latter does not really have to do 
with the Nyquist theorem, all this means that you will often want to 
use oversampling of at least eight. So for ADSR with 1ms ramp times you 
might use 8kHz sampling clock, a resolution of at least 10bit, a ~1kHz 
filter and perhaps some pre-equalization of the digital envelope data 
to counter the characteristic of the analog reconstruction filter.


Achim.
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