Kevin Dowd's Synth
Speaking of synthesizers and Color Computers...
How about a....
Quad Channel Home Built Synth
Hardware & Software By
Kevin Dowd
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Here's a four-channel synthesizer that Kevin Dowd designed and built in the 1980s. Three channels are based on General Instruments chips from Radio Shack. The fourth channel is a damped low-frequency sine wave that Kevin used for a bass drum. The board was programmed through an editor Kevin wrote in Basic on the color computer.
It was driven by a real-time executive that he also loaded on the color computer and ran in the background. It would grab a non-maskable interrupt on a timed basis. All details aside, here's a clip made by this board. Funky!
(below from an email I recieved from Kevin)
Here are the basics: the interface to the CoCo is through a general-purpose I/O card. There are four sound sources. Three are General Instruments digital sound generators, programmable via digital I/O. They each share data lines from the Color Computer, with two of the digital output lines actings as a chip selects. The fourth sound source is a damped, low frequency sine wave that acts a bass drum. The sine wave is produced continuously. The envelope of the bass drum comes from damping the wave over a handful of cycles. Looking at the board, I'm recalling that I used the TL082 op amp as a variable gain amplifier, and that the gain was inversely controlled by a charging capacitor. It appears that I was doing some voltage regulation in the lower middle part of the board. I'm pretty sure that I had a -15V source that I needed to pare down to -5V. Power came in on the screw terminals. The lower right part of the board is a clock circuit for the General Instrument chips.
The whole thing ran from a screen based editor on the color computer, written in basic. I have included three pages of the editor printout. It starts with declarations of sounds, declarations of measures, and the definition of a sequence of measures. On the second page, the measures are fleshed out with the sounds declared above-- into 32 slots per measure. The third page shows how sounds were programmed; basically, the registers of the General Instruments chips are exposed. There were three sound generators per chip, plus noise, digitally mixed. I also wrote a background executive running on the 6809 non-maskable interrupt on a timed basis to dish out commands to the synthesizer board. As I recall, the software was rock solid. I used the board primarily for percussion in a number of recordings in the mid 1980s.
Now that the nostalgia is out of the way, I'm pretty sure I would start somewhere else if I were building a new sound card for the Color Computer. Certainly, my software was not like Lyra or UltiMusE3. And the hardware was limited to what the General Instruments chips could do, which was very 8-bit, lo-fi Atari kinds of sounds. Endearing, perhaps. But... thinking about building a hardware synth from scratch in 2012. What fun!
Kevin Dowd
Coco Synth Board
