Re: [Linuxsampler-devel] Envelopes , filtering and zipper noise

[Steve H. <S.W...@ec...>]

On Fri, Nov 15, 2002 at 01:12:42 +0100, Benno Senoner wrote:
> Hi,
> the shapes do look nice, I was wondering about two things:
> What the ideal coefficients would be so that very fast envelopes can
> be achieved while slower ones are smoothed out so that no zipper noise 
> is audibile.

I dont understand the question, the envelope shape is dynamic depending on
the length of the appropriate section (attack, decay, etc). The only thing
that needsa to be selected is the fudge factor (4.0 ish IIRC) which
determines epsilon for the envelope.

> BTW since in most of cases (in absence of envelopes) the LP smoothing is
> not needed, it would be wise to skip that code (to save cycles) when the
> envelope reached the final point (up to a very small epsilon since we
> are talking of exponentials). What do you suggest here ?

Well, the code for this is created on demand right? (a JIT sampler ;)
So if there is no envelope there is no envelope code. If there is an
envelope, then when we have reached epsilon, the envelope has (in theory)
come to an end, we can fade to 0.0 to make sure, and then the voice has
finished.

BTW do we know for sure than samplers use exponential envelopes? I guess we
need linear ones too, but they are easy to implement. We should probably
get some recordings from samples of high freq sinewaves with envelopes.
I think Frank N. has done this allready for the S2000, Frank are you on
the list?

> Regarding adding a let's say resonant LP filter to the sample output:
> I guess the coefficients need to be smoothed out in some way too,
> otherwise zipper noise will probably show up again.

Yes. Though you can get away without it if you know the cutoff isn;t
chaging too rapidly (eg. a slow LFO).

> What would a good tradeoff for achieving low-CPU usage zippernoise-free
> filter modulation would be ? (some kind of interpolation of precomputed
> coefficients ?)

I think so. Not all filters like you doing this, but most are OK IIRC.
I have some SVF and biquad test code around, so I can test this if you
like. I think my LADSPA SVF uses linear interp. to smooth the coefficients.

- Steve