knudsen@ihnss.UUCP (02/10/84)
It's easy to upgrade the 6-bit D/A (digital-to-analog) converter circuit in the Coco to 7 bits, less easy to 8 bits. The increased resolution gives better music and sound, AND can increase joystick resolution to 128 points (double what you get now). One of the PIA chips feeds lower byte bits 7 thru 2 to the D-A resistor network. Bit 1 is the RS232 data output, but can be easily added to the sound as well. This bit is on pin 2 of the 8-pin op-amp (a 741 or equivalent) near the RS232 socket. Simply connect the right value of resistor from that point to Test Point 7, which is the D/A summing junction. (Alternately to the TV soun multiplexer input, or to the resistor nest itself; but on an E board, the first- mentioned points are only a couple inches apart and out in the open). The right value of resitance is about 640 K. Best bet is to use a small 1 Megohm trimpot and tune it so that a D/A number of 2 is halfway between 0 and 4. A 'scope and a tight BASIC loop are handy for this. Music and game sounds will be improved by 6 dB in signal/noise ratio. You must write your own successive-approximation routine to get the double Joystick precision -- a good routine was published in Byte c. 1981, very fast too, used for speech analysis input! Greedy hardcores may go for the 8th bit by inserting aresistor between PIA bit 0 and the cassette input comparator (about 3K?) and running the bit to the summing junction thru a 1.3 Meg resistor. You'll have to reprogram this PIA bit for output, etc. I haven't tried this; it involves some cutting & desoldering. The 7 bit shceme is just adding two wires. I tried it and it's good. Music freaks will probably add external "real" 8-bit D/As anyway (Radio Shack, $2.50), but these won't help your joysticks, unless you plug them into your outboard addition also. Mike K. PS: You can ripoff a full byte under the keyboard connector from the other PIA, which scans the key columns. Pretty noisy while the keys are being scanned, tho...