Exclusive software to tie a range of micros into SCI's popular monophonic — by Robert Penfold

The SCI Pro-1 monophonic synthesizer has been popular for a number of years, and it has an excellent specification. It includes a built-in sequencer, but this is of limited value as it has a capacity of just 40 notes and all notes have to be of equal duration. However, there is a digital interface which enables the instrument to be controlled by an external microprocessor system, such as a home-computer, but in order to use this interface it is necessary to open up the instrument and make three minor modifications. This just entails removing the microprocessor from its socket, adding a link wire, and fitting a 10k resistor — this is all detailed in the Pro-1 manual, and there is little point in going into this procedure in detail here.

Having done this, a cable must be fitted to the digital interface socket, which is a 16-way DIL socket (header plugs to fit these are readily available). A ribbon cable is fitted to the header plug; only a nine way cable is required. Connections are made to pins one to nine — the Pro-1 must then be reassembled, and as the ribbon cable is very thin it should be possible to take it out between the two sections of the case.

In use, pins one to six are fed with a binary number which selects the desired note. This gives a range of over five octaves — some two octaves or so better than the Pro-1 keyboard. Pin seven is fed with a positive gate pulse, and this is the equivalent of holding down a key. Pin eight is fed with a negative latching pulse, but if the circuit is fed from a latching output port this pin can simply be tied to ground, along with the ground line from pin nine.

Programs

There are several home-computers that can directly drive the Pro-1's digital interface, and the programs provided here are for the VIC-20, Commodore 64, BBC model B, and Atari 400, 600XL, 800, and 800XL. In the case of the first three the interface is driven from the user port, and in the case of Atari the joystick ports are used as output ports (use joystick ports one and two in the case of the 400 and 800 machines). Below are the appropriate methods of connection for each machine.

The facilities offered by the programs vary somewhat, the program for the unexpanded VIC-20 being the least sophisticated. This enables up to one hundred notes to be entered, and provides editing facilities. The other programs enable music to be saved and loaded, and also have a review facility. With the BBC and Commodore 64 programs the maximum number of notes is 500. All three programs permit the gate time and note duration to be programmed (in one hundredths of a second in the case of the BBC, sixtieths for the Commodore 64 and VIC-20, and fiftieths for Atari). Notes are entered as numbers from 0 to 63, with one corresponding to the lowest note (C) on the Pro-1 keyboard. Successive notes rise in semitones. In each case the sequence can be played once or made to loop. All the programs are largely self explanatory in use, and there would be no point in giving detailed operating instructions here.

Of course, apart from use with the Pro-1, the programs would also apply to a synthesizer being controlled via a D/A converter and the CV input, or any other instrument having a suitable digital interface.

[Note: multiple page program listings are not included here (at least for now), due to the extensive cleanup required from OCR.]