by Sergio Franco
(excerpts)
1. Importance of Real-Time in Electronic Music
With the advent of electronic music, the roles of the composer and the performer have been placed in a new perspective. Traditionally, musicians have tended to specialize in either one role or the other, or have tended to play only one of the roles at a time. This has happened mainly because of the considerable difficulty encountered when composing and performing are carried on simultaneously.
When a musician improvises at an instrument, he is essentially composing music in real-time. This means he is thinking and evaluating very quickly a series of possibilities out of which he makes appropriate choices, which in turn lead to the execution of the various mechanical motions necessary to produce the music he wants.
Because of the difficulties encountered in doing all these things at once, musicians have tended to specialize either in the inception/evaluation process (composition) or in the execution process (performance), and have developed the notation of musical scores to communicate among themselves. Thus, the composer need not be principally concerned with the quick thinking and decision making of the improvisational process, but can pursue his inspection of musical possibilities on his own time scale.
What is even more important, to appreciate the effects of his choices and changes, he does not need to try them out on actual instruments because he can rely on his aural imagery, a faculty he has developed through experience either by playing instruments or by listening to others playing them, or both.
In developing his aural imagery, the musician is certainly helped a lot by the fact that he deals with a limited set of instruments whose characteristics extend over known and predictable ranges.
In electronic music the situation is quite different. Because of the new kinds of sounds that electronic instruments are capable of synthesizing, it is very difficult for the musician to develop the kind of musical imagery that may suffice to assist him in composing on his own time scale and away from his instruments. If the composer is to come up with something musically meaningful at all, it is absolutely essential that he actually hear the effects of his choices, his trials, and his alterations as he makes them, so that he can directly evaluate them in the context of the whole composition. In other words, the concept of direct feedback has come to play a dominant role in the electronic music composing process.
Another important change brought about by electronic music involves the relationship between performer and instrument, and is due to the much greater detail in which the performer is required to control the various musical parameters. While in conventional instruments such parameters as timbres, attacks, decays, etc. are, to a large extent, fixed, built-in features, in electronic music they are left to the discretion of the performer, who must therefore specify and control them directly. If this feature allows on the one hand much more freedom of choice and experimentation, on the other it imposes a more demanding control burden upon the performer, and it certainly renders the communication among musicians more complex than with conventional instruments, where the notation of the musical score is usually adequate.
As a result of the above discussion, it should be clear that in electronic music the distinction between composer and performer ceases to exist, partly because the composer needs to try out his musical ideas on the instrument by himself, and partly because of difficulties of communication among different people.
Furthermore, having accepted the notion of direct feedback as an indispensable ingredient of the improvisational process, it is of paramount importance that the composer/performer be allowed to interact with his instrument on the time scale of the music he is improvising, that is, in real-time.
As shown in the block diagram of Figure 28, the realization of the sound distribution system requires a total of 96 audio gates. With a number of this magnitude, the cost per gate plays an important role in the choice of the circuit realization to be adopted.
An audio gate, besides satisfying such obvious requirements as low distortion, high on-to-off transfer ratio, and low control signal feedthrough, must also exhibit specific transient characteristics in order to ensure proper audio switching.
The last requirement is motivated by the fact that when a sound is switched on or off, spurious partials are generated which may alter the tonal character of the sound considerably, as has been discussed in connection with sound enveloping.
As Fourier analysis reveals, the amount of spurious partials accompanying sound switching usually increases with the rate at which sound builds up or decays. Thus, unwanted switching effects can be easily reduced by employing gates with low switching speeds.
From the viewpoint of sound movement control, however, it is desirable to have fast gates so that sounds can be switched around the performance space at arbitrary rates. A compromise between the two conflicting requirements can be determined experimentally.
Analog gates based on solid-state, electro-optical devices like Raysistors or Vactrols usually exhibit predetermined and highly asymmetrical switching characteristics which render these components unsuited to the present application. The cost of these devices at the time of design was also a key factor that contributed to their rejection.
The smoother for the harnonic tone generator has been real-ized with two CA3080 OTArs in the manner i]lustrated in Figure 1p.This circuit realizes a tvo-pole,maximally-f1at, low-pass prograrnrnable fil-ter. Due to the absenceof the voltage-to-current converter and diode compressor, signal conditioning at the OTArs inputs i-s achieved by means of resistor attenuators, as shown. Tire resj-stcr values have been chosen so that the bal-anced outputs from the progranraable mixer can be tied to
the inputs of the first 0TA directly, without the need for additional ancilfary circuitry. AIso, because of the finite imped.ance presented. by the attenuators, the output from each OTA naust be buffered. by a high input-imped.ance decoupler. This task is performed. by the pair of FET-transistor
source fol-lowers. The voltage-offsets associated vi-th the fol-lowers $o not affect the dc level of the filter because each follover is placed insid.e the feed.back loop of the correspond.ing OTA.
Digital Waveshape Generation
Because of the discretization introduced by the sampling technique, the control of tonal qualities can be exercised only over a limited bandwidth. According to the well-known sampling theorem, the order of the highest harmonic that can still be represented with the sampling process equals half the number of samples contained within one waveshape period.
Thus, if the harmonic generator is to span a wide tonal scope, the number of sampling intervals should be as large as possible. A high sampling density, however, requires a commensurably high frequency for the sequential scan of memory as well as a large memory size to accommodate the samples.
As it has already been pointed out in connection with the relaxation oscillator, the accurate generation of an exponential frequency over a range of three decades becomes more and more difficult as the range is shifted in the direction of higher frequencies.
A compromise has been achieved with the choice of a time resolution of 32 samples per period and an amplitude resolution of 15 levels per sample. Since the tonal bandwidth is determined solely by the number of samples, the amplitude resolution need not be as high as the time resolution.
It should be noted, however, that the presence of the mixer results in an effective increase of the amplitude resolution because it allows for the programming of arbitrary ratios between corresponding samples of the incoming waveshapes.
(Sergio Franco was one of the people that designed the Sal-Mar Construction)
This excerpt is shared for educational and research purposes only, to facilitate discussion and understanding of electronic music history. If you are the copyright holder and object to its publication here, please contact me directly, and I will remove it immediately.
No comments:
Post a Comment