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Article from Electronic Soundmaker & Computer Music, January 1984 |
Generating envelopes
In response to popular demand — some micro-music software. Gary Herman describes an envelope generator
With Texas Instruments now virtually giving away their TI-99/4A computer, it's almost worth buying one just to see what it will do. In fact, the TI-99/4A is a reasonable machine — the BASIC is quirky (good on editing, OK for graphics and sound, not too good on anything else) and the machine is surprisingly slow (especially since it uses a 16-bit processor), but it's reliable and often pleasant to use. There is a dearth of software and a corresponding lack of interest in the machine's sound facilities which — considering the £99 price tag — are quite impressive. The programme presented here goes some way to remedying that situation. It is easy to rewrite for other machines offering a sound command which handles frequency and volume, definable functions and numerical arrays.
The programme uses 10 functions (defined after some trial and error) or a series of input statements to load an array (VOL(X)) with values which determine the envelope of a note. There are also some elementary graphics to give a rough picture of the eventual envelope. For simplicity, the graphics statements (lines 30-70, lines 520-610 and lines 1470-1490) can be omitted (they will, in any case, need to be changed if the programme is to run on other machines). The Texas uses a SOUND command which specifies a duration variable, a frequency and a volume. Channels (there are three) are identified by position in the statement. Obviously, the programme will be of most value to a machine with no facility for controlling envelopes. The major obstacle to portability is the fact that on the Texas the volume parameters range from 30 (quietest) to 0 (loudest). Most other machines — more logically — allows volume to range from 0 (quietest) to 15 (loudest). Line 690 will need to be adjusted to cope with different sound commands and the functions defined in lines 910, 960, 1010, 1060, 1110, 1160, 1210, 1260, 1310 and 1360 will need to be adapted to take account of the volume control features of other computers (as will the manual setting section of the programme in lines 1430-1530). The principles of the programme's operation are, however, good for most BASICS.
As it stands, the programme is of most interest as an analytic or educational tool. It allows you to control the parameters of an envelope and see and hear what the effect is. The routines can, of course, be included in a more elaborate compositional, sequencing or real-time playing programme. In such contexts, the VOL array will probably need to be two dimensional — VOL(4,7) will give the 7th value for the envelope of the 4th note, for instance — but the method of setting the envelope for each note will be the same. The essential parts of the programme could be incorporated in a subroutine. The programme includes a repeat feature (lines 480-510 and lines 670 and 710) which repeats a note 10 times. This can be dropped or enhanced (for example, the number of repetitions could be varied by inputting a value for D in line 510). I find it useful to hear a note repeated. Incidentally, the best effects using this programme are obtained by enveloping noise which, on the Texas, is achieved by entering -1, -2, -3, -5, -6 or -7 in answer to the 'note frequency' request in line 80.
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