D. Kupke, 8 Lomond Drive, Toronto, Ontario, M8X 2W3
Superboard II
OSI Superboard manuals hint at a lot of hidden features but fail to reveal how to implement them. Through research and discussions with other Superboard owners, I uncovered one of these features: sound.
All you need are resistors, diodes, one capacitor and a tape recorder. If you own a tape recorder, the rest of the hardware amounts to less than a dollar, and the software is exercise for your fingers.
Hardware
Sheet 13 of OSI's circuit diagrams refers to sound and joysticks. The drawings are correct, but omit the values for resistors R67 to R71. The little diode-resistor network is a simple four-bit (R2 to R5) D/A converter routed to pins 11 and 12 of J4; the connector is on the left-hand side of the board. R67 is just a pull-up resistor. R68 and R71 form the ladder; each resistor is twice the value of the preceding one (see Fig. 1).
Good soldering practice should produce clean joints without runover onto adjacent foil patterns (see Photo 1).
You are now ready to attach the speaker to pins 11 and 12 of J4, the connector on the left side of the board, and here are a few suggestions.
The output from this little D/A converter is low, and a two-inch speaker (8 ohms) produces a feeble sound. You can build a small amplifier using an LM380N as suggested by Rod Hallen in "Audio for Your Microcomputer" (Microcomputing, February 1980, p. 32). (I used my tape recorder as an amplifier.)
Connect an audio cable to pins 11 and 12 with a phone plug on the other end and plug it into the auxiliary input of your tape recorder. Now you can plug a large extension
speaker into the tape recorder and get volume to spare. The trick is to put the recorder in the record mode. You don't even need to put a cassette into your tape recorder. Just push the little tap, which is normally moved by the cassette, with your finger to engage the record button.
I routed all connections from J4 to the back of my home-built case for the Superboard. Pins 11 and 12 terminate in a phone jack; the remaining pins, 1-10, terminate in an RS-232 connector (see Photo 2).
When you turn on the system and the tape recorder, you will hear a high-pitched sound interrupted by a low-pitched sound every time you touch the keyboard. This in- dicates that everything is working as intended. What remains is to develop some software to produce the desired sounds. You can't turn the sound off before you run programs, so plug the speaker in just before entering RUN.
Software
Even though OSl's Microsoft BASIC is fast, it limits the sound spectrum.
From the memory map in the user manual you get the location of the keyboard port (DF00,57088), so you can use Listing 1a to activate the speaker. This produces a simple, low-frequency square wave. Modifying the program (Listing 1b) only increases the frequency by a small amount, and the sound approximates that of a small airplane.
This small change in BASIC shows audibly that the frequency is related to the efficiency of your programming. Although you can experiment with this BASIC pro- . gram and use it for sound effects, machine language is the solution.
If you have the extended monitor, so much the better, but the standard monitor by OSI is adequate. Listing 2 shows a short machine-code program that follows the lines of Listing 1a. The value for the delay loop is stored at location 0028H. A good value to start with is 80H. Load this register with hex values 80 to C4, and you'll get all the tones of one octave. Listing 3 contains a machine-code program that simulates the sound of a siren. Again, the value of the maximum frequency must be loaded some- where on page zero.
Conclusion
In my application for a simple way to get sound from your Superboard, you use only the middle four bits, because the board is prepared for this arrangement. It's easy to convert the whole eight-bit word. The incentive for this improvement is that different four-bit words produce different sounds, but also different levels of volume. A little program I wrote to produce a whole octave in response to keyboard entries 0-7 works fine but suffers from the above-mentioned volume problem. Rodnay Zaks' 6502 applications book shows a similar program, but it needs some modifications to suit the OSI Superboard.
