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Alpha-Syntauri

A computer-based digital music synthesiser

Article from Electronics & Music Maker, December 1981


The Alpha Syntauri system.


The Alpha Syntauri is described as 'a keyboard-based digital playing and recording system with a microcomputer interface'. The first appearance of the system was at a Computer Music Festival held in Philadelphia in August last year, where it attracted considerable attention by virtue of the fact that its target market was the average musician rather than the tax-juggling member of a 'super group'.

The components needed for the primary system are the 61-note Alpha Syntauri keyboard with relevant software, a 48K Apple II Plus, a disc drive with 16-sector controller, a colour or monochrome monitor, and hardware oscillators. A multiway lead connects the music keyboard to a scanner interface card plugged into one of the expansion connectors on the Apple motherboard. The primary system is illustrated in Figure 1. The hardware oscillators are, in fact, old friends of ours, as they are derived from the two Mountain Hardware digital oscillator boards used in the Apple Music System reviewed in the May issue of E&MM. The 16 oscillators available from this system are individually programmed from a 256 byte waveform table, which assigns pitch and amplitude information over one period of the waveform to be produced by each oscillator. The oscillators receive a waveform update every 32 microseconds, which means that direct memory access to the waveform table has to occur every 2 microseconds.

Figure 1. The primary Alpha Syntauri system.


This comparatively high sampling rate of 32kHz (20kHz is a norm for the majority of other systems) gives the system an excellent theoretical frequency response (up to 15.6kHz). The main Alpha Syntauri performance program, Alpha III, translates traditional analogue synthesiser control patterns into real-time control of the Mountain Hardware oscillators by using processing loops. The programs embedded in the process loop are written in 6502 assembly language to provide the necessary speed for the system. Two other languages, Applesoft BASIC and integer BASIC allow user-selection of parameters to control the processing cycles and setting-up of the initial process loops.

Programming



The set-up program is basically to configure where the interface cards are in the Apple, and, once that's done, a master file can be loaded into RAM to prescribe instrument definitions according to a waveform table and set of envelope parameters. Using Alpha Syntauri's preset master file, Alpha, six banks of ten instruments are available on file, with one bank being loaded into RAM on power-up. Each of the ten instruments per bank can be immediately called-up by entering the number 1 to 10 on the Apple keyboard.

Envelope parameters preset in Alpha can also be reassigned to other instruments in the same way, which means that from the beginning each bank can provide a hundred variations using their simple command language. The decay and release times of the software ADSRs are varied when the sustain pedal is depressed and the degree of variation is selected by an appropriate key entry. Of the six banks in Alpha, four actually turned out to consist of the same presets, which suggests that Syntauri still have some way to go in constructing a decent library of presets. However, the ease of calling-up presets, whether they are Syntauri's or your own stored on disc, means that real-time playing is a cinch on stage. Further user-control is derived from the velocity-sensitive Alpha Syntauri keyboard. Two sets of key contacts are used in the keyboard, and, typically, the attack rate and volume are inversely proportional to the time between contact closures. The actual changes to attack rate and volume are handled by a look-up table which is loaded automatically by the Alpha III software into a specific memory location. Flexibility in the velocity-sensing results can be further achieved through reworking the look-up table from which the velocity-sensing results are ultimately derived. In addition, being a general design, the parameters of the envelope which are affected by the velocity-sensing process may be altered from the attack rate/target volume to, for instance, the attack target volume/decay rate.

The main process loop contains what is called an unconditional JSR (jump to a user-written subroutine). This allows any user familiar with 6502 assembly language to devise his/her own special effects and controls which are then assessed during the process cycle. An example of a JSR might be to process the note information through a table for frequency modulation of the sound. The current Alpha III program utilises this modification, and provides frequency modulation by any of 253 possible knob-selectable complex waveforms which result from sequences of logical ANDs between the knob-defined 'FM mask' (a value entered from the Apple game paddles which defines the modulation signal) and the changing envelope as updated by the envelope routine for the instrument currently selected. As a result, this gives different timbres and effects for different envelope presets too, and adds further variations to user-selected presets from the master file. The impression gained so far from the system is that Syntauri have elected to make the interaction with the user as straightforward as possible, and a near approximation to the knob-selected options offered by analogue systems.

This somewhat unorthodox approach to digital synthesis is probably due to the role played by the composer and synthesist Laurie Spiegel in the development of the Alpha Syntauri system. She feels that the feedback loop present in analogue synthesisers (move this knob to get a different sound, i.e., instant response) is a necessity not really present in digital synthesis. The Alpha Syntauri goes a long way towards providing this feedback by means of a bar graph representation of the frequency content of whatever is being entered on the Alpha Syntauri keyboard. The five octave range of the keyboard is displayed along the x-axis of the VDU, and each division of the octave is allocated a different colour. The amplitude of each note is displayed on the y-axis and indicates envelope shapes as well as reiterated cycles and looped ADSRs. The potential in this as a teaching aid is terrific, but its use as a serious means of feedback is only really possible once you've convinced yourself that you're not looking at Liberace's 'dancing fountains'!

Configuration



The Alpha Syntauri is actually configured as an octophonic system, with two Mountain Hardware oscillators for each note. Each of these is programmed by a waveform table, but the waveforms are separated by 0.5Hz to get a chorus effect. In software it's also possible to program an alternative Leslie-type effect. One oscillator is programmed with a standard ADSR envelope, whilst the other is used as a percussive waveform generator and produces what Syntauri call a 'PFSF' envelope. The percussive waveform is constructed to give the hammer action to a keyboard or the blowing sound to a flute.

This second waveform gives some splendid sounds on presets like RMI electric piano, pipe organ or vibes (you can hear examples on demo tape 4), but applying the same PFSF envelope to instruments that don't characteristically produce a percussive envelope (such as their preset 'strings') doesn't seem a particularly clever way of selling your product. Considering how good strings and brass sounds are from poly-synths, ranging from the Poly-Moog to the Prophet 5, more effort should have been put into creating realistic presets — that's if Syntauri really feel that it is important to go along with the convention of trying to imitate conventional instruments.

These criticisms are no great thing, though, for the Alpha III program also enables the user to synthesise waveforms. These can be constructed from sinewaves, as in the case of the Apple Music System, but also with sawtooth, square and triangle waveforms, all of which can be applied to the construction of any number of harmonics as well as the fundamental pitch. It has to be said at the outset that the Alpha III waveform program has the same limitation as that in the Music Player program of the Apple Music System, and that's that all harmonics have the same amplitude envelope. Looking at Figure 2, it's quickly apparent that any attempt to synthesise a realistic violin sound from the first five harmonics is only possible if some variation can be made between the envelopes of the upper harmonics and those of the fundamental and octave.

Figure 2. Harmonic envelopes of a violin note A440.


Apart from programming waveform tables via additive synthesis, it's also possible to go half-way through additive synthesis and then start subtractive synthesis by specifying a negative number to lower the weighting given to a particular harmonic. Again, this subtractive element of the Alpha Syntauri reflects Laurie Spiegel's belief in subtractive synthesis as being more natural to musicians than additive synthesis.

VDU Synthesis



Waveform synthesis is also assisted by an analysis program built into Alpha III. This displays the effect of an added harmonic on the overall waveform by updating the display of the waveform in a slow scan across the VDU screen. Though the Apple doesn't have the computing power of the Fairlight CMI, paradoxically the slow sweep resulting from this limitation permits the process of additive synthesis to be shown much more clearly than the over-complex if, undeniably, pretty 3-D waveform plot displayed by the Fairlight.

With the present software version of Alpha III, the analysis program also allows any waveform stored on disc to be separated into its harmonic components, a digital filter algorithm being executed for each of the first twenty harmonics, and their relative amplitudes displayed on screen. It doesn't take much imagination to see that this program could be adapted to analysis and reconstruction of real sounds input via an A/D converter, though to make any real sense of this Syntauri would need to add a routine for prescribing individual harmonic envelopes.

Conclusions



As you'll have realised now, Syntauri have paid a great deal of attention towards developing a system which is really dynamic, and this also includes a portamento pedal that glides between notes according to values entered in software. Since Alpha Syntauri is at an early stage of its development, and bearing in mind the very reasonable price of the software, it's hardly surprising that there are some areas of the system needing a lot more work. This includes the current facilities for 'recording' music on disc. At the moment, you're limited to putting all your eggs in one basket, so to speak, as all eight possible parts have to be written on to disc at the same time, i.e., it's akin to recording any polyphonic keyboard on 2-track tape.

Mind you, whilst playing back the notes file it's possible to transpose the keyboard over six or more octaves, change the tuning from well-tempered to International or Just, play along with the pre-recorded piece, vary playback speed between half and double the original, and select a continuous repeat (echo) of any section of it. All this is pretty impressive, but it would be rather cool to add what the Apple Music System provides, and that's to enter one part at a time, but in real time from the Alpha Syntauri keyboard, i.e., using the system as a 8-track sequencer. Also, bearing in mind that the Apple Music System has something approaching a music composition language entered from the Apple keyboard, it is perhaps surprising that the Alpha Syntauri doesn't have the facility to use the music files created by the Music Editor in the former system as the source of notes for the Alpha III program.

The approximate UK price of the Alpha Syntauri keyboard and software is £750, and distribution is being handled by Personal Computers Ltd, (Contact Details). Anyone interested in knowing more about this excellent system should contact John Hopprich at PCL, on (Contact Details).

If what I've been looking at represents the primary Alpha Syntauri system, then the quaternary system should really be something!


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Publisher: Electronics & Music Maker - Music Maker Publications (UK), Future Publishing.

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Electronics & Music Maker - Dec 1981

Review by David Ellis

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> Working with Video


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