More stuff on the 303...

[Fraser, Colin J]

> -----Original Message-----
> From: pharding at mail.ndirect.co.uk [mailto:pharding at mail.ndirect.co.uk]
> Sent: 16 December 1998 10:23
> To: oakley at techrepairs.freeserve.co.uk
> Cc: synth-diy at mailhost.bpa.nl
> Subject: Re: More stuff on the 303...
> 
> I dont want to put you off in your 303 studies and i'll try not to 
> write a thesis now but i've come to the conclusion that it's the 
> sequencer part of the 303 that gives it the biggest part of it's 
> characteristic sound that we all love.

It's the combination of the synth and the sequencer. You just won't get the
full range of 303 sounds/patterns from anything else - including another
synth with the 303 sequencer or a 303 synth clone driven via midi.
My own 303 clone is only indistinguishable from the original when it is
driven by the original sequencer.

> IN fact i'd go as far as saying 
> that both 303's sound worse over midi compared to when they are 
> programmed with their own sequencer.

I found this too...

> irrespective of what modular components 
> i use[ie 
> i can change over the filter module to a different sounding LP filter 
> or even change VCO from an ASM-1 vco to a LM13700 vco ]the resulting 
> sound still has that certain,303ness to it.

You really need to add an accent output to the 303 to get the best results.
This can easily be taken from the non-inverted output of the accent
flip-flop.
303ness is still no substitute for 303ity.

> Both my 303's internal sequencers also have this other quirk which 
> may be to do with the way they follow the incoming Sync 24 
> pulses,i'm not sure but what happens is that no 2 bars are absolutely 
> the same timing wise.I proved this when trying to sample a 303 
> pattern and i could visually see the difference in the waveform on my 
> akai..The timing is only a tiny bit out[real small amounts]but i fell 
> that this certainly adds to the all important groove of the pattern 
> where as over midi you get a more static pattern.

I noticed this when I tried to exactly recreate a 303 pattern using Seq303
via midi - it just didn't move the same way.

The 303 cpu has the din sync inputs connected to its IO lines. The IRQ line
of the cpu is connected to an oscillator generates a pulse every 2ms.
I believe that the IRQ handler performs the scanning of the keyboard and
switch matrix, and the led driving.
The handling of sync inputs is not in the interrupt handler, but is carried
out in the normal program loop.
This will lead to timing variations depending on where a clock pulse arrives
relative to the interrupt generating oscillator - if the interrupt routine
has just started when a clock pulse arrives, it will continue to completion
before the pulse is processed.
I haven't put a frequency counter on the cpu clock osc, but I suspect its
clock speed is in the order of 500kHz.
You could test this theory by stopping the IRQ oscillator during playback,
and observing whether the leds go out and the timing is improved.

Colin f