[Fpga-synth] Signals and Sample Rates

[Magnus Danielson]

From: Scott Gravenhorst <music.maker at gte.net>
Subject: [Fpga-synth] Signals and Sample Rates
Date: Sat, 17 May 2008 07:14:09 -0700
Message-ID: <200805171414.m4HEE750002580 at linux7.lan>

> I am trying to understand how to deal with signals and sample rates.
> 
> For example, when a limit is given, such as for a digital state variable filter being 1/6 of the sample rate, higher frequencies causing instability, I think that this means a sine wave at 1/6 the sample rate.  But most of our synth signals are not sinewaves and thus can have frequency components much higher than the fundamental.  
> 
> How does one deal with the transients in sawtooth or rectangular waves?  
> 
> Is it "OK" to use such a filter because the energy in the highest harmonics tends to be diminished?  
> 
> I like the sound I get out of a digital SVF, but that's in a synth with a sample rate of 1 MHz.  Seems to be enough headroom there.  However, I'm working on a synth that currently has a sample rate of 125KHz.  That sets the limit much lower at 20.833 KHz.  Is that my magic number (being just above what most of us can hear) ?  I.e., is 125 KHz too low?
> 
> What sorts of things will I need to look for to judge the quality of my SVF at that sample rate?

Listen to it!

There are no "real" technical merits to what "sounds" good. Only a number of
technical merits that experience have shown to be in the right direction.

I think your reasoning about headroom sampling frequency wise is reasnoble.
Weither it sounds good at 10 kHz or 20 kHz is of marginal issue, but 100 Hz
to 5 kHz is where it needs to work. But yeah, the filter should not self-
oscillate in an annoying fashion.

There is much sine-ness left in a sine-wave of 6 or fewer samples.

Consider incorperating a natural LF roll-off at about 20 kHz in your SVF.
It might help to bring smoother responses. Just a thought, so bounce it around
and try it for yourself.

Cheers,
Magnus