Filter problems

[Timo Tossavainen]

On Thu, 11 Nov 1999, Joe Wright wrote:

> With a little bit more analysis I found the hyper resonances are happening
> when the cutoff frequency is roughly a multiple of the frequency of the
> note.  I suppose this makes a lot a sense so how come this doesn't happen
> with other synths (software or hardware)?
> 
> What am I doing wrong?  Help?

One possibility is that you're not clipping the output properly.
The filtering amplifies frequencies around the resonant frequency so
the output of the filter may contain larger values than the input.
When you convert from float to int you need to check for this. 

But then again you said that lowering the volume didn't help...
At which stage of the system did you do that ? It wouldn't help
if you did it after the conversion to int, for instance if you do
(short)x and |x| > 32768 (assuming short is 16 bit). It should help 
if you lower the volume before the conversion to int, but then your output
has significantly lower volume than the input for high resonance settings
(in this case you normalize the filter at the resonant peak so the maximum
gain of the filter is 1.0)

If it isn't this then I don't have a clue, seeing the code might help...

Timo




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