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Back to ProjectsAn Idea Whose Time Has Passed?
When I first saw the Oberheim digital 2-voice keyboard schematic, my reaction was, 'This looks do-able' -- because I immediately saw that the number of ICs could be reduced and the wiring simplified. (Yeah, I know...a PIC or other microcontroller could do it even simpler, but I know me: I'd get so involved with the journey I'd lose sight of the destination -- like I did 2 decades ago. Note the end of my ID.)
Below, for comparison, you see the relevant portion of the original circuit and my revision. (Reduced in size and not meant to be actually readable.) In the drawings which follow, I have for the most part omitted IC pin numbers, leaving that for when this gets built -- if ever. Also, most resistor and cap values are 'to be determined.' This is by no means a finished (re)design.
Click to view full-size Left Channel Logic
(The original schema uses '*' to designate NOT -- inversion or active low. I've used the overbar. I've also renamed CLOCK* to /STROBE because that's what it does.)The purpose of that cluster of 6 XORs (1 ½ ICs), 4-input NOR (½ IC), 2-input NOR (¼ IC), and 2-input NAND (¼ IC), in the original is to compare the value in the 74C174 latch with the keyboard scanning count and provide an active low at the NAND output when the two values are equal. This can be done by a single chip, a 74HC688, called an 8-bit equality detector. This reduces the number of chips and also eliminates the hassle of interconnecting all those XORs, NORs and NAND -- twice! The 74HC688 can be used in other designs that compare groups of bits. Since it compares two 8-bit values, here two P inputs and two Q inputs are held at 0, Gnd, as is the /Enable. (Curious that inputs be designated 'Q' -- that's usually reserved for flip-flops, latch and counter outputs.)
There's one problem with this parts reduction, though. The 74HC family runs on 5 volts, so I can't use the 9 volts the original design calls for. This is fine with me, as I don't understand the use of all those non-symmetric supply voltages in the original, anyway. (The original Digital to Analog Converter looks a bit screwy, too. I've made changes to that, as well.) I've choosen a 74HC4316 for the Sample and Hold switch. This is like a CD4016 analog switch, but includes level translation, so a positive-only logic level can switch on and off a +/- signal. Each switch also has lower 'On' resistance than the 4016. The voltage from the DAC, with Octave Up/Dn and a Tune or Pitch Bend would have to be offset to keep it within the range permitted by the 'HC4316 +/-5V supply.
The RIGHT CHANNEL logic is the same as the LEFT, with its corresponding 'R' signals substituted for 'L.' Note that either circuit alone can function as a mono keyboard if the xTAKEN input to the 4-input NOR at the bottom left is held low. Note also that 'Left' and 'Right' do not refer to pressed key locations on the keyboard, but to which CV and Gate channel has 'First' priority as set by the MODE switch. The two channels could just as legitimately be called A&B or 1&2.
If my understanding of the logic is correct, the pulse that clocks the 74HC174 happens only when a key is first pressed, to store the scan count for comparison on suceeding scans. This can possibly be used to derive a Trigger, but the pulse would have to be stretched. There will be leftover gates and inverters with which to do this. (Or not.)
Full Left-Right Schematic
(Hmmm...it might be worthwhile to eliminate the 4316 and the opamp that follows it and build a separate DAC for each channel and hang them off the 'HC174s outputs. I've never entirely trusted S/Hs anyway. I gotta think about this some more. It might also be worthwhile to just forget it, save up my Quatloos and buy a PAIA MIDI2CV8, and use my Korg Poly-800 to control my modular stuff, which right now is controller-less.)
Revisions Esthetic changes to all schematics. Separate Analog and Digital +5V and ground. Scan counter changed from CD4024 to CD4520.
