The Poor Man’s Disc
Logic Controlled Tape Decks

by David Elliott

Most computer hobbyists work on a shoestring budget, and finding the money to buy expensive disc drives can be difficult. But anyone who has to rely on cassette systems for storing and retrieving programs knows the frustration of having to search through tapes for the required program, and the inherent unreliability of the tape system when using domestic audio cassettes. One reason for this unreliability is the variable quality of the tape in most cassettes, and some so-called ‘digital’ tapes are just as bad, with drop outs caused by pinholes in the magnetic coating and even folds in the tape being common. Everyone has their favourite tape, and I use TDK-C46 tapes, which I have found to be very high quality.

Nevertheless, the average hobbyist is unlikely to be able to discard cassette tape as his storage medium until the cost of disc systems reduces drastically, and so a way has to be found to make the system more flexible and more reliable. The introduction of the cheap logic-controlled cassette deck onto the hi-fi market led us to consider controlling such a deck from the Nascom output port, and writing a cassette operating system to give many of the features of disc drives at a fraction of the cost, whilst retaining the standard Nascom tape format, allowing complete compatability with standard tapes.

This is how E.C.O.S. was born. The Elliott Cassette Operating System is an attempt to enable all the hard work of locating, storing and reliably retrieving programs from tape to be carried out by the computer. The hardware couldn’t be simpler, as most logic-control led decks have a convenient remote control socket enabling easy interfacing with the computer. The deck selected was a Scott 665DM which costs £75, but many similar decks are on the market at around £75 – £80. Audio quality is not paramount, and can even be said to be a disadvantage in some respect , as the program information is encoded as a series of audio tones of 1200 hz and 2400 hz, and it would be better to suppress frequencies much outside this range. Another problem encountered was that of level matching. The standard cassette interface on the Nascom expects to receive a high level signal (1 – 2 volts peak to peak), but most hi-fi decks are intended to feed a high quality amplifier and are therefore designed to give a relatively low level output, typically 50 – 100 mv into 600 – 50K ohms. This was the case with the Scott deck, and a simple two stage transistor amplifier had to be interposed between the output and the Nascom cassette input in order to produce the required level. Another alternative would have been to use the headphone output, but this was not as reliable.

Control of the cassette deck was via the remote socket, and it was found that all the front panel buttons controlling the tape transport were brought out to an