Tempco

[Fraser, Colin J]

> -----Original Message-----
> From: Tim Cockram [mailto:tim at redragon.demon.co.uk]
> Sent: Wednesday, May 13, 1998 10:45 AM
> To: Fraser, Colin J; synth-diy at mailhost.bpa.nl
> Subject: Tempco
> 
> This would work but you would need to use a high resoluton A/D , D/A
> process to get the pitch resolution required. e.g. an 18bit convertor
> would give a 0.038Hz resolution for a 10Hz to 10kHz 
> oscillator (16 bits
> would only give 0.15Hz resolution).  Could be expensive.  If you could
> run the convertors fast enough you could multiplex the process though.

Multiplexing was what I had in mind...
Would there be an audible side effect of the quantisation of frequency ?
Would this only show up if the frequency was swept ?
How small an error in frequency can we hear ?
Maplin have started stocking 24 bit dacs...

> On the other hand you could make the whole thing linear like Yamaha or
> Korg, but.........

The Poly Six (IIRC) has linear oscillators driven by a dac with an
exponential convertor on it's output that's multiplexed between all the
oscillators. Maybe that's a better solution for a multi-osc synth than
doing the exponential conversion digitally, then introducing an error at
the dac.
 
> So I guess the Tempco still rules (especially if Paul S. stocks them).

I'd certainly be buying some if he did (are you listening, Paul?)
 
> Heated or self compensating (Rene's or the minisonic) circuits still
> have their attractions.

I have used heated arrays (a la AN299) for single oscillators. I'm
thinking about building a poly synth, so I need to minimize the
component count per oscillator to have any chance of ever realising it.
I'll try out Rene's design sometime though - it could be the solution
for single oscillator modules. At the moment my modular has 4 CEM3340s,
but I'm not totally happy with the sound of them, and I don't think I be
getting hold of any more.

Colin f