[sdiy] BLIT/BLEP virtual analogue synthesis

[ASSI]

On Wednesday 04 August 2010, Olivier Gillet wrote:
> Now I understand the flaw in my "brute-force -> sigma-delta" idea.

I don't think it really has a flaw, there are just different options on 
where to place the tradeoffs.

> In a typical use scenario, the oversampled signal that enters the
> sigma-delta loop doesn't contain any frequencies above the nyquist
> frequency of the input signal (say 24kHz for a 48kHz).

I'm not sure I understand what you are trying to get at.  A sigma-delta 
modulator, much simplified, takes a slow signal of high resolution and 
converts it into a fast signal of low resolution.  The frequency content of 
the modulated signal is massaged so that the much increased quantization 
noise ends up at high frequencies with lots of space to the frequency band 
of interest, so it can be easily filtered during signal reconstruction.  The 
other virtue of the low-resolution signal is that it is easier to have a 
linear conversion to analog.

> But what
> happens when sending into a sigma-delta loop a signal with frequencies
> filling up the full band (as in the "brute force oversampling
> oscillator" case)?

You normally wouldn't do that, but as long as the high-frequency content is 
noise-like (not overtly tonal and of much less amplitude than the signal of 
interest) it will probably just raise the noise floor.  Higher-order loop 
topologies on the other hand might not be stable in that case.

> Is there some kind of aliasing taking place?

Aliasing is always taking place in a discrete-time system.



Achim.
-- 
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