Another micro comes under the detailed scrutiny of Gary Herman — ES&CM's computer expert

Before there were BBC micros and Spectrums, there were Ataris. The Atari 400 and 800 were sturdily built micros whose excellence was only undermined by their dogged refusal to fall in price. That doggedness, and the persistent belief on Atari's behalf that true riches were to be found in an elusive market for expensive ROM cartridge games, nearly brought about the company's downfall. However, Atari and its mighty parent, Warner Communications, have — they hope — come bouncing back with a new range of machines. Regrettably, the Atari 600 XL (excel — get it?) is the only one of the range so far available in Britain and any assessment of the company's strategy and likely future must be arguable, based as it is on this lowest-price machine.

One thing is clear. Atari have decided that the wealth of existing software is too great a resource to render obsolete and have, in consequence, built their new machines to be as compatible as possible with the old ones. Looks apart and with the exception of a number of extra graphics screens (and the addition of a built-in software facility to test the machine on power-up), the 600 is practically identical to its forebears.

Click Innovator

The 600's major innovation is the inclusion of on-board BASIC. The earlier Atari's BASIC had to be bought in cartridge form — which was inconvenient, unnecessary and expensive. The 600's BASIC is barely distinguishable from its predecessors, but it is there when you turn the machine on.

So what does the 600 offer the computer musician? Like other Atari machines, the 600 uses the so-called 'pokey' sound chip controlled from BASIC by the single command 'SOUND'. The virtue of this arrangement is its simplicity but — as with other aspects of Atari machines — one does get the feeling that sacrifices have been made for the sake of ease of use. The chip has four channels (or 'voices' as Atari calls them) numbered 0 to 3 in BASIC. Although independently programmable, using two or more channels does noticeably slow up the music synthesizing process. Likewise, multi-channel sound generation seems to introduce some unwelcome clicks into the noises and tones emanating from your TV loudspeaker. And on one last sour note, despite a nominal spread of five or six octaves, only two or three octaves around middle-C are really usable for musical purposes.

A typical SOUND command might be:

10 SOUND 0, 60, 14, 15

which would produce C-above-middle-C at maximum volume on channel 1 (numbered 0). In general, SOUND is followed by a number between 0 and 3 representing channel, a number from 0 (highest) to 255 (lowest) representing frequency, an even number from 0 to 14 representing distortion and a number from 0 (off) to 15 (highest) representing volume.

Pure Distortion

Distortion is the most interesting feature of Atari sound. Values 10 and 14 give what the manual calls a 'pure' tone. This is, in fact, a pulse waveform of regular frequency. Other distortion values give similar tones with what an oscilloscope shows to be periodically variable pulse widths. The result — at low frequencies — are a variety of buzzing tones. At high frequencies, these begin to resemble sounds characterised by 'spiky' pulses: white-noise where there is a high degree of variability in pulse width and plucked or struck string instruments (eg, clavier or piano), where the pulse widths do not vary greatly. The following program demonstrates this:

10 FOR DIS = 0 TO 14 STEP2
20 FOR V= 150 TO 0 STEP -15
30 SOUND 0, 0, X, V/10
40 NEXT V
50 FOR PAUSE = 1 TO 1000: NEXT PAUSE
60 NEXTV

The program also reveals that at very high frequencies (the second '0' in line 30) the distortion values producing 'pure' tones (10 and 14) make barely any sound at all. The other values produce piano-like plinks or snare-drum-like beats. Increasing the frequency value slightly will produce more useful sounds.

In fact, the Atari sound facility has some unique and interesting features. Distortion is perhaps the key word, for it can be introduced not just by setting a direct value but by allowing the volume setting to go above its ostensible limit of 15. This alone produces some curious (if so far unanalysed) rhythmic effects, but it can also be combined with settings which allow frequency and distortion levels to sweep repeatedly through their ranges. In the simplest form, a programme like:

10 FOR X = 0 TO 65355:
SOUND 0, X, X, X: NEXT X

produces a complex sound repeating over and over for some minutes which, if not exactly music, is much more than just random noise. Small changes to the above one-liner (in the FOR... NEXT loop or in the SOUND statement itself) have fascinating results. Adding other sound statements and regular pauses can create rhythmic effects that might once have taken a composer (like Stockhausen) days.

Last Words

At its current price (around £150) the Atari is an attractive machine for general use. Musically, it's limited when set against the Commodore 64 or the BBC, but within its limitations there is still fertile territory especially for those interested in rhythmic effects.