How to Make Music with the TS 2068, Part 1

Digital computers like the TS 2068 deal with two voltages. These are 0v and +5v, or rather something is true (+5) or something is false (0). We could also call this switched on or off. A popular process today, is to control external equipment with analogous properties and multiple control voltages, with digital computers.

In music, controlling electronic instruments with computers is rapidly growing into a bold new frontier Instruments like music synthesizers, some home organs, and the newer compact Casio-type potable keyboards, can be manipulated by digital computers to some degree. They can send information along data and address busses to define tone pitches, tonal colors, and envelope shaping. The possibilities are endless. Entire songs or note sequences can be stored in memory, and recalled instantly.

As we mentioned before, computer operate in only two voltages. We pose a problem here, because most modern electronic keyboard instruments like synthesizers are voltage controlled.

This means that say a one-volt control signal is sent to the synthesizer’s tone oscillator. This would register a low C note. As the voltage is increased, the steps and half-pitch steps rise, while a control voltage of two volts would produce a C one octave above and so on. Other portions of electronic instruments are also voltage controlled like the tone shaping-filter’s cut off frequency, the voltage controlled amplifier, and the envelope generators.

A solution to interfacing the digital computer to an analog music synthesizer, is a DAC (digital-to-analog converter). Most DAC’s have two sides. On one side you feed in the binary numbers, and from the other side you can extract an analog voltage, which corresponds in some manner to the binary number. There is one DAC that you could build from a kit, and it won’t deplete your cash flow either. For $24.95 (plus $2.00 for shipping) you could put together the 8785 Linear Digital-To-Analog Converter from PAIA Electronics (1020 W. Wilshire Blvd., Oklahoma City, OK, 73116). The DAC kit comes with a PC board, all the necessary electronics, documentation on what to do with it after you build it, and a brief set of instructions. It is not a kit for the first time kit builder. You need to be pretty handy with a soldering iron before you start out. After the DAC has been constructed, you will need to obtain the necessary cables and connectors (not supplied) to interface the DAC to the rear edge of your 2068’s PC board. Gladstone Electronics (90 Furhmann Blvd., Buffalo, N.Y., 14203) carries a complete line of cables and connectors that are compatible.

There are eight possible data lines used in the 8785 to convey control voltage information. This essentially provides only the output for a monophonic synthesizer, or one that produces one tone at a time. To provide adequate voltage outputs for a polyphonic synthesizer, you may need to consider a TTL chip called a “one-of-16-decoder” which allows for connection of up to 16 separate 8785’s. PAIA also markets a QuASH (quad addressable sample-and-hold) kit, which drives four seperate monophonic lines from one DAC.

A complete understanding of the 2068 MPU and memory mapping is essential in developing adequate software needed to drive a computer controlled electronic music system. To send data to the DAC requires storing a byte to that address. More musically satisfying results require more sophisticated programs. You could simply POKE the values and numbers that correspond to voltages in BASIC, but in order to fully utilize the synthesizer’s potential, it calls for machine code routines.

If you would like to further explore this area of interfacing a digital computer to electronic music instruments, there are two books that are quite interesting and informative on the subject.

1. “Controlling Exponential Systems” by John Simonton. Polyphony, Box 20305, Oklahoma City, OK 73156.
2. “Musical Applications of Microprocessors” by Hal Chamberlin. Hayden Book Co., Rochelle Park, New Jersey