Philharmonic 12 - A 12 Voice Hardware Music Synthesizer for the TANDY Color Computer and Dragon
This cartridge reproduces the functionality of the Speech Systems "Symphony 12" cartridge.
NOTE: As of 2018-12-24, this design has not been built and tested. Caveat Emptor!
Reproduction Changes
Although the schematic on page 27 of the Owner's Manual was followed to implement this reproduction, the schematic was incomplete, and so a few changes were made to minimize parts count and fill in the gaps.
The decoding of the MC6821 /CS2 line was defined as "pin 15 74ls138 $ff60", which implied that $ff60 would represent a low level on pin 15 of the IC (a 3-8 decoder), which is value "0". That meant input lines A,B, and C must be low levels. Since address lines 0 and 1 select the specific MC6821 register, it is assumed address lines 2,3, and 4 are sent to inputs A, B, and C. Furthermore, since the unit decodes to $ff60, which is above the /SCS space, a full address decode must be done. A 74LS30 8 input NAND gate was used to decode an $ffxx access, and that output was used to gate the '138. Address lines 7, 6, and 5 were still not decoded, but must be set to 011. Thus, A7 was sent to a negative select line on the '138, while A5 and A6 were sent to active high select lines on the MC6821 (which are tied to +5V on the original schematic). THis change should not affect functionality.
Another difference lies in the clock signal for the sound ICs. The original schematic ties the sound clock to 6809 E clock, which provides a stable .895MHz signal on all computers except for the Color Computer 3. On the CoCo 3, the E clock can be set to .895MHz or 1.79MHz. This will affect the pitch of the audio considerably. Thus, a stable .895MHz oscillator was included on the PCB design, though a jumper cna still be used to select the E clock if desired.
Finally, the AY-3-8912 ICs used in the original design are harder to find than the larger cousin, the AY-3-8910. Thus, this design used the larger and cheaper ICs.
License
Copyright (C) 2018 RETRO Innovations
These files are free designs; you can redistribute them and/or modify them under the terms of the Creative Commons Attribution-ShareAlike 4.0 International License.
You should have received a copy of the license along with this work. If not, see http://creativecommons.org/licenses/by-sa/4.0/.
These files are distributed in the hope that they will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the license for more details.