[sdiy] how you got started with your current µC?

[Dave Manley]

It was a long time ago...

In high school I scrounged jobs to get enough money to buy a PAiA 2700 
modular kit sometime around 1975.  The one with the 'button' keyboard 
that used an angle-cut piece of resistive plastic and per-note "feelers" 
that tuned each key.

http://3.bp.blogspot.com/-K0UMSCfRx_c/T8AoH1m1bEI/AAAAAAAEg24/RVvHwpjOpqo/s1600/IMG_3001_002.JPG

To be honest it sucked.  At a local junk dealer I found for ~$20 an 
ivory two-manual organ keyboard that had been pulled out of who knows 
what (not Hammond).  I cut that down to make a single manual 4-octave 
keyboard.  Around then John Simonton released a simple key scanner PCB 
(EK-3, $14.95) which along with a custom resistor network (not R-2R) 
allowed you to digitally control Hz/V VCOs.

http://web.archive.org/web/20051225084727/http://www.my-scifi-stuff.com/paia/images/PAIA%20jpgs/PAIA%20page%2023.jpg

I used these in a single voice synth.

Later for a UW-Madison class I needed a project.  The lab had a 
PDP-11/20, an ASR-33 teletype, a DEC writer, and a Hazeltine ADM3 
terminal.

http://s7.computerhistory.org/is/image/CHM/x1507.98p-03-01 (octal based 
address input!!!)
http://www.columbia.edu/cu/computinghistory/la36.jpg
http://www.pdp8.net/asr33/asr33.shtml (110 baud!!!)
http://www.retrotechnology.com/herbs_stuff/adm3a.jpg

I dug up the old 2720 VCO schematics and cut down the design to make a 
single analog board with 4 saw-only VCOs plus 4 minimal VCAs.  No VCF.  
A second board with 4051's and caps after the 'PAiA DAC' created a 
multi-channel CV source.  It was a combination of wire-wrap for the 
digital and point-to-point perf board wiring for the analog.

The 11/20 had memory mapped IO, so this was used to scan the keyboard 
(which had a typical diode matrix) and also drive the DAC and refresh 
the multi-channel S/H CV source.

PDP assembly was written to implement a voice assignment algorithm and 
SW-based ADSR.

It was a ridiculous amount of work for a 3 or 4 credit, one-semester 
course.

Yesterday, I did a little searching online and found the schematics for 
the PDP-11/70.  The CompSci department had one of these and it was 
biggest/baddest computer on campus.  Looking at it now in retrospect it 
was ridiculously simple compared to what can be done in a small FPGA 
today. It was page after page of hand drawn schematics of 74xx logic, 
but then my first job wasn't too different - page after page of hand 
drawn schematics on D-sized sheets of paper.  Maybe if I'd known the 
Prophet-5 used the same basic architecture, or after the bit-slice class 
I took, that Eventide Harmonizers used bit-slice I would have tried to 
interview with either of them, but it was probably too late at that time 
- the DX7 was on the horizon and analog was dead.

-Dave

On 2015-09-25 16:37, rsdio at audiobanshee.com wrote:
> Sounds interesting, Dave. Can you tell us more about the polyphonic
> synthesizer? How did it interface to the PDP-11/20?
> 
> 
> On Sep 24, 2015, at 3:43 PM, Dave Manley <dlmanley at sonic.net> wrote:
>> I missed out on the PDP-8 but spent many late night lab hours in 1982 
>> coding a 4-voice polyphonic controller on a PDP-11/20. The addressing 
>> modes and instruction set made for easy assembly coding.
>> 
>> -Dave
>> 
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