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Path: utzoo!watmath!clyde!floyd!harpo!ihnp4!ihnss!knudsen
From: knudsen@ihnss.UUCP
Newsgroups: net.micro.6809
Subject: Improve Coco Sound and Sticks
Message-ID: <1921@ihnss.UUCP>
Date: Thu, 9-Feb-84 19:05:49 EST
Article-I.D.: ihnss.1921
Posted: Thu Feb  9 19:05:49 1984
Date-Received: Fri, 10-Feb-84 07:12:14 EST
Organization: AT&T Bell Labs, Naperville, IL
Lines: 33

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...