More micros and their music capabilites — Gary Evans looks at the Dragon, Laser 200 and Comx 35.

Last month we looked at the sound generation capabilities of a number of the best selling home micro-computers. Since then, however, two new computers have arrived on the scene — one offering colour computing at £70! Before looking at the sound facilities offered by these newcomers, though, we'll examine the sound function of a more established and very popular machine — the Dragon 32.

Dragon's Roar

Last month we explained that most machines offering all but the most basic of sound facilites make use of a special sound generating IC, such as General Instruments AY-3-8910. Other designs, most notably the Spectrum, merely switch one of their outputs on and off at varying rates to produce a monotone at a fixed amplitude. There is another approach, however, and that is to use a Digital-to-Analogue converter together with some specialised software to produce the tones. It is this approach that has been adopted by the Dragon's designers.

The use of a D-to-A means that the Dragon is restricted to a single sound channel, but that the amplitude of the noise produced by the computer can be varied over a range of values. This means that the Dragon is quite adept at producing simple tunes but that chord generation and complex sound effects are more of a problem to produce.

In common with a number of other machines, the sound output of the Dragon is fed to the TV's speaker and the machine does not therefore suffer from any limitations imposed by the small loudspeakers found on so many small computers — the quality of the sound will be limited only by the audio section of your TV.

At Your Demand

The basic Dragon sound command is

SOUND p, d

where p is a number from 1 to 255 that specifies the pitch of the sound (89 corresponds to middle C) and d specifies the duration of the note in units of 1/50th of a second.

The second command associated with sound production on the Dragon is the versatile PLAY command, which is followed by a string that selects the Tempo, Volume and Octave at which a sequence of notes will be played, then followed by the notes themselves. Thus

PLAY "T100V20defg"

would play the sequence defg in the lowest octave available on the Dragon. The Tempo parameter can vary from 1 to 155 although the higher values would only be used for special effects. The level of the sound output is controlled by the number following the V and can vary from 0 to 31.

The PLAY facility of the Dragon offers a very convenient way of transcribing music to play on the machine, but if attempting any original composition it can take quite a bit of trial and error with the PLAY string to get the exact sound required.

A major disadvantage with the sound technique used on the Dragon is that while the computer is producing sounds it cannot do anything else. This is not a disadvantage if the machine is only used to play tunes, but if, for example, it's required to add a tune or sound effects to an animated game, this can be a problem. There is no way around this difficulty apart from the use of a specialised sound generating IC that can be programmed to produce a noise and then continue to do so while the computer gets on with other things.

The Dragon, then, adopts a different approach to the production of sounds and offers two versatile commands for the generation of simple tunes. Chords and sound effects are more of a problem and while generating any sound the machine cannot get on with any other tasks.

Laser 200

The Laser is a new colour computer that retails for only £70 and offers a fairly good sound facility for the price. The basic form of the computer's sound command is

SOUND p, d

where p is the pitch (frequency) of the note to be produced and d the duration. Tables 1 and 2 show the values of p and d that produce a wide range of tones and note durations.

The sound command is used with the Laser's READ and DATA commands to build up the sequence of notes to be sounded. Thus the following program could be used to sound a set of notes.

10 FOR 1 = 1 TO 7
20 READ X
30 SOUND X, 7
40 NEXT
50 DATA 16, 18, 20, 21, 23, 25, 27, 28
RUN

The laser does not offer the facility to vary the amplitude of the note produced, instead these are sounded at an acceptable level on the machine's internal speaker, sensibly mounted on the top surface of the computer so that the sound is not muffled by the surface on which the Laser is being used.

Once again the Laser offers a single sound channel — chord generation not possible — and no sound effects facility. At the price of £70 it does offer a worthwhile performance.

Comx 35

The Comx 35 is a £120 colour computer with 35K of memory. It is built around the fairly unknown RCA 1802 MPU. The machine offers a range of sound commands, the basic form being

MUSIC (p, o, a)

where p is the pitch of the note to be sounded (varying from 1 to 7), o is the octave in which the note is to be sounded (from 1 to 8) and a is the amplitude of the note.

Once such a command has been issued the note will continue to sound until it is either modified or turned off with

MUSIC (0, 0, 0)

This is much the same as the system used on the Oric computer (see p. 54). For a finer control of frequency the Comx offers the tone command that takes the form TONE (p, o, a) where o and a are as before while p now determines the pitch of the note produced on a scale from 1 to 128.

In addition, noise can be produced with a command that takes the form

NOISE (f, a)

where f determines the frequency band in which the noise is generated while a gain controls its amplitude. As with MUSIC and TONE this command must be switched off with

NOISE (0, 0)

Finally, a global volume command can be used to vary the sound's output, overriding any of the amplitude statements embedded in the commands described so far. This takes the form

VOLUME (x)

where x can take one of four values with one producing the quietest sound and four the loudest.

The Comx thus has a fairly comprehensive sound facility and allows crude sound effects to be produced with its noise command. It is still limited to a single tone channel, though, and while capable of producing simple tunes may place a number of constraints serious music composition.