DIY samplers

[julian higginson]

On 4 Jun 1996, Christopher List wrote:

> Hi folks - 
> 
> I was wondering if anyone could point me in the right direction for some good 
> books that would touch on the subject of sampler design. I'm really curious 
> about how samplers control the pitch (playback rate, I know). Does the 
> processor just send out clock messages at different rates - i.e., is it a 
> matter of the firmware "waiting" between clocks, or is there usually some kind 
> of dedicated hardware that the processor can just tell what playback speed to 
> use? 
> 
> - Thanks,
> Chris
> 
I think you`ll find that samplers use interpolation to play back samples.
playing back samples at a different clock rate is OK if you only want
1 note polyphony but for a more useful sampler you need something else. 
Imagine 32 DA converters being controlled by 32 different clock signals 
<<shudder>> and addressing (without conflicts) different locations in RAM 
<<AAARGH!!!!>>.
I was thinking about making a sampler for myself a while ago, but realised
it would take up a little too much of my time.

here is a short description of what I nutted out:
what I would have done was have the processor doing all the work.
each note on would result in a record in a linked list being produced. this 
record would contain information such as the address of the sample, playback
speed, origional sample rate, etc
there would be a hardware interrupt at some rate, say, 44.1khz.
the processor would calculate the instantaneous level for each "instrument"
in the list of records, scale with envelope and then add them all up, and on
interupt, send the number to a DA converter. this must be done for each
independant output wanted.

hmmmmmm..... that didn`t come out quite as well as I wanted, hope you
get the basic idea of what I said. I have some paper where I wrote down the 
actual details (SW algorithm, HW block diagram) at home, if I can find it.

BTW: if anyone  knows of a better way to do this, feel free to correct me.

julian


This endless circulation of the electric fluid may appear
paradoxical, but it is no less true and real, and you may
feel it with your hands.                -Allesandro Volta