Bill's Bits

[Fraser, Colin J]

> -----Original Message-----
> From: Dr S Grainger [mailto:steveg at bss10a.staffs.ac.uk]
> Sent: 30 October 1998 10:02
> To: synth-diy at mailhost.bpa.nl
> Subject: Bill's Bits
> 
> 	With the wavetable ideas I'm mainly interested in the phase-
> 	accumulating oscillator idea whereby the sampling replay
> 	rate is constant. This is usually achieved with a pitch 
> register/
> 	adder/accumulator arrangement which is clocked at a constant
> 	rate. This system can be easily multiplexed so that a polyphonic
> 	generator csn be produced.
> 
> 	Any suggestions on small dual-port RAMs and data concurrency
> 	gratefully accepted !!!
> 

Just this week, I picked up a Music 500 synthesiser (thanks Chris !). It's a
digital synth add on for the BBC micro.
It has 16 phase accumulator oscillators with 24 bit phase and frequency
registers.
Each oscillator output selects a sample from a wavteable held in RAM.
Frequency resolution is 0.0056 Hz (IIRC - maybe +/- a zero) with a constant
resolution over the whole range - a big plus over divide down oscillators.

It's a very neat design - I have a magazine article from ETI (Dec 1987) that
describes how it works.
It's described as a fixed-program, pipelined DSP, implemented mainly in
discrete 7400 series logic.

The unit is accessed from the host computer by writes to an area of RAM that
holds the wavetables and frequency and amplitude registers.
If I can ever deduce where the data gets written to I may use it as the
basis of a Microwave-esque synth with analogue filters...
There'a bit of hardware hacking to go tho' - I'd need to de-mux the the
output from the DAC to separate outputs for each channel.

Colin f