[sdiy] Polyphonic keyboard scanner

Veronica Merryfield veronica.merryfield at shaw.ca
Fri Jul 31 23:29:44 CEST 2009


The DPX is a TOG and divider based synth. Every key will produce a  
note. Some nice notes in the construction articles on how this is done  
to make reasonable sound. Of note, for it's day, the chorus was really  
lush without being wobbly. Had one once.

The Transcendent polysynth inspired my to do a polysynth in days gone  
by. Like that one, mine was done in logic. The basic idea is to use a  
ram as a down counter with addresses corresponding to keys, so as the  
keys are scanned as an address, so is the RAM. The RAM starts off with  
all ones once a key down start is detected and then each time round  
the loop the RAM count is decremented until the key is down, giving  
the time of flight or velocity. The address gives the pitch, the RAM  
content the velocity and the keydown event the gate. Each key moves a  
contact between two rails such that down, up and in flight give  
different voltages. A window comparator gives the logic signals. At  
one point I was thinking about making the ADSR release proportional to  
keyup flight but never did it. The complexity comes in deciding which  
voice for which note when all the voices are used. I can't remember  
what the Transcendent used but I used a wrapping counter in the note  
assigner but like the transcendent, I had a switch for either stealing  
the oldest note or no stealing. The voices in the Transcendent were  
CEM based. I used CA3080s and transistors for mine.

My logic board was a very messy affair, but once I got it working, it  
was reliable. I wouldn't do it that way again although I learnt a lot  
doing it. I would either use logic in an FPGA or do it in software.

The originals where black and white to. Very rarely did ETI have  
colour inside as I remember.

Claim to fame; I was offered a job about 15 years ago now at Akai in  
Oxford by Tim Orr who designed the Powertran/ETI Transcendent stuff.

They ETI articles are at CAG.

Veronica

On 31-Jul-09, at 1:24 PM, Mike Gorman wrote:

> David,
>
> There are at least 3 designs from ETI that should fit the bill here.
>
> The Polyphonic keyboard from July 1979,
> The Transcendent DPX from August to November 1979 (I think this was  
> the one
> that Tony mentioned)
> The ETI Polysynth from December 1980 to March 1981
>
> The last one might be the best bet, as I know this could be expanded  
> to 8
> voices (Mostly CEM based voice modules IIRC).
>
> I think the ETI articles are all available on CAG, though the scans  
> may only
> be monochrome, which can be awkward at times.
>
> Regards
>
> Mike Gorman
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David G.  
> Dixon
> Sent: 31 July 2009 17:53
> To: 'synth diy'
> Subject: [sdiy] Polyphonic keyboard scanner
>
> I know this will earn the scorn and derision of the programmers on  
> the list,
> but I'm wondering:  Does anyone here know of a polyphonic keyboard  
> scanner
> circuit based on logic IC's (non-programmable)?  Is this even feasible
> without a massive pile of chips?  If so, I'd be very interested to  
> learn
> what the algorithm is.  The approaches I've been thinking about are  
> all
> fairly complicated (one scanner (counter + 2 multiplexers), but 8  
> separate
> latches and ladders with data comparators to compare the count to the
> latched data for preventing double latching and controlling note-off  
> -- I
> haven't worked out all the gory details yet).  I'd like to design  
> something
> that will send out 8 CV's, 8 triggers, and one gate.
>
>
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