Not too long ago, it came to my attention that Pere Serrat had modified the ROM for the Orchestra 90cc to make it work with Dragon computers. This inspired me to design and build a compatible board that would run the original software, and be useable in both Dragons and Color Computers.
The first thing I had to do was figure out how the original worked. I had an original cart in my possession, but didn’t want to ruin the label by cracking it open. Luckily, the manual for the original is out there in .pdf form, and includes a schematic. Here it is…
As you can see from the schematic, there isn’t a lot of circuitry there. Some TTL logic for address decoding, a couple of data latches, a couple of resistor DACs, a ROM chip, and some audio output circuitry.
The system uses two 8-bit DACs driven by the latches (using data bus signals) to create the sound waves. It’s stereo, one DAC per channel. The decoding logic decodes for two addresses used to write the latches. The ROM contains the software, and is tied to the Color Computer bus in the usual fashion, using !CTS as the output enable line, with Q and !CART tied together to autostart the program.
The output circuitry (after the DACs) consists of a handful of capacitors, and an op-amp circuit that feeds both channels mixed together back through the SND line so that the sound can be included with the computer’s built in sound or RF output.
I decided to use modern surface mount components (mostly) for size considerations, and because I just like working with them more than through hole now.
I figured a 5v tolerant CPLD could be used to replace all the logic on the board, and selected the Xilinx XC9572XL in the VQFP-64 package. Bourns SMT resistor packages would do nicely for the DACs. And the original op-amp is still manufactured and available in a nice little SMT package, so why not just stick with it.
After a first prototype, which worked fine, I made a few minor changes to the capacitors I was using. The result was the REV 02 board, which is the final version.
I used a 32K ROM, to hold software for both Color Computers and Dragons. The platform can be selected using the slide switch beside the stereo jack. Below are pictures of the assembled board, and pictures of it in a case.
In a Tano Dragon
And here are some links to line-in recordings of a couple of Orchestra 90 tunes using the REV02 boards. Headphones or good speakers recommended to hear everything.