understanding some terms and theory

[John Speth]

> >> A wavetable just holds a certain ammount of samples for a given
> >> waveform, right?  So no matter what the playback frequency you're
> >> getting the same number of samples per period?
> >
> >   I'd imagine that it'd depend on the design.  For example there are a
> >few designs that use a VCO to clock the wavetable.
>
> This method would be the simplest and cheapest to implement, but
> wouldn't it
> be more common for the clock frequency to remain constant (44.1kHz or
> whatever) and use some scaling algorithm to derive the output
> waveform from
> the wavetable data? That way the output from the digital oscillator could
> remain digital for effects processing and S/PDIF outputs etc.

The old VCO->counter->ROM->DAC chain works great for mono-voice apps but...

I've wondered about how the "pros" really do multitimbral sample playback.
I guess it just isn't practical to have many fast clocks stepping through
wavetables at asynchronous rates, digitally mixing, and sending the digital
mix to a DAC.  My guess would be that, at a constant sampling rate (like
44KHz), there's some fast, special purpose hardware that continually
calculates what sample value to send to the DAC thus making the post
filtering easier.  I'm only guessing at this so my question is this: Just
how DO the pros do it?  Does anybody know?

John Speth
Object Engineering, Inc
mailto:johns at oei.com