Quantiser

[List, Christopher]

I had never seen this schem before. Interesting. The one on my site works
the same way but using more "modern" IC (probably more expensive, but lower
parts count and smaller board). 
"My" circuit wasn't mine (I should've made that clearer on the web page), it
was what I came up with after Juergen made a post (a couple of years ago)
describing the quantizer that you are talking about - very similar to your
post below (but not as much detail)!
I just used a monolithic flash DAC IC, and IC S&H chips to get the same
thing done. Because I only use the top 7 bits of a 12 bit dac, the accuracy
of the steps does not have to be tuned for each bit. Note that a stepped
ramp osc. can be made just using S&H chips and op-amps (no DAC), but scaling
all the steps is not as easy.

Once again...
http://www.mindspring.com/~clist/resource/circuits/cv_quant.html

- CList

 

-----Original Message-----
From: Caloroso, Michael E [mailto:CalorosoME at corning.com]
Sent: Wednesday, October 27, 1999 4:59 PM
To: '[*Synth-DIY]'
Subject: RE: Quantiser

> >The Oberheim Minisequencer is the correct schematic to download....
> 
> >Oberheim produced this as a standalone product (very rare) and also put
it into the 2-voice....
> 
> Can somebody explain this circuit? I can't read the component values and
names, due to low scan resolution. Is there a better scan somewhere?
> It seems to contain a R-2R DAC and a two channel multiplexer, but what
signals are input to the DAC? Where does the clock come from?
> Why the two channel multiplexer?
> 
Ahhhh... neat circuit, just figured it out.  This thing has gotten my
curiosity too.

I'm referring to the lower left hand corner of the schematic at
http://www.synthfool.com/miniseq.jpg

The 32 bit counter's lowest six bits feed the DAC, while the seventh bit
forms A7 & A7* to toggle between the two sequencer CV signals in the
multiplexer.  This way only one DAC is needed for each sequencer CV, that's
why there's a two channel multiplexer.  The output of the DAC is a cyclic
ramp waveform with discrete steps, which can be reset by the sequencer
clock.  The counter's clock at pin 1 comes from the two NOR gates and I
assume it's a much higher frequency than the sequencer clock.

Now look at the multiplexer.  A7 & A7* select which sequencer CV goes to the
opamp input.  The opamp compares the selected sequencer CV to the DAC
signal.  When the ramping DAC signal is greater than the CV, a positive
spike is generated (via AC coupling on the opamp output) which enables the
NOR gates controlling the CMOS switches.  The DAC signal is also routed to
the CMOS switches, thus the *instant* the DAC signal is greater than the CV,
the switch is turned on for a very short transition and charges the cap on
the switch's output.  While the switch is off, the output is held constant
by the charging cap that is part of the sample-and-hold circuit.  Therefore
the output of the sample-and-hold is the quantized signal.

This cycle repeats for each CV at a much higher frequency than the sequencer
clock.

When the DAC ramp cycles back to zero, the diode on the output of the opamp
shunts negative transitions, thus the CMOS switches are only turned on with
positive spikes.  

That's how the quantizer works.  The DAC's ramp waveform is the heart of the
system.  If you wanted to change the scaling of the quantizing you could
just range and scale with the trimpots on the 1458s on the output of the
R-2R network.

Very clever circuit.  Flexible too, lots of I/O points, even a CV for
controlling the sequencer clock.  You can even transpose the sequence CV
with the keyboard CV.  And not a single obsolete IC anywhere.  A very good
candidate for a DIY project.

MC
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