| Popis: |
One of the main interests of the Structured Sound Synthesis Project (SSSP) is to develop a highly interactive environment to serve as an aid in the composition of music. We would like to do this within the context of a small, inexpensive, accessible system. In order to resolve the conflict between the high computational demands of sound synthesis and these design objectives, we have developed a specialpurpose digital sound synthesizer. Work on this project began in January 1977, and the device was functioning in a limited state by December of that year. The design owes much to the Dartmouth synthesizer (Alonso et al., 1976) as well as the VOSIM oscillator (Kaegi and Tempelaars, 1978). Generally described, the synthesizer consists of one "real" oscillator which is time-divisionmultiplexed so as to function as sixteen oscillators. There are two aspects of the device which we see as particularly significant. First it was designed so as to incorporate in hardware five important techniques of sound synthesis: fixed waveform, frequency modulation, additive synthesis, waveshaping, and VOSIM. Secondly, the device can be easily interfaced to most digital computers. The generators are essentially fixed sampling rate, accumulator-type digital oscillators. The sampling rate of each oscillator is 50 kHz, with a system bandwidth of 20 kHzi. Dynamic range is well over 60 dB (and should improve with further adjustment) while frequency resolution is approximately .7 Hz (linear scale) over the entire bandwidth. This will shortly be improved to enable resolution of less than one "cent" at even extremely low frequencies'. The output signal of each oscillator can be fed to one of four analogue output busses which may then either be fed directly to an amplifier, or to a channel distributor (Fedorkow, 1978; Fedorkow, Buxton and Smith, 1978). The waveform output by each generator may not only be user defined-up to eight waveforms available at one time -but one may switch waveforms in mid-cycle. This is possible since the sixteen oscillators share a 16k buffer of 12-bit words to store waveforms. This 16k of RAM is partitioned into eight 2k blocks, one for each of the eight possible waveforms defined by the user or system. Apart from this memory configuration, this synthesizer is particularly interesting because the oscillators may be used to generate sounds according to five different synthesis modes: fixed waveform, frequency modulation, VOSIM, additive synthesis, and waveshaping2. This goes a long way towards a "universal module"-that is, all modules of a uniform type (with the resulting ease of conceptualization and communication). While this is in direct contrast with analogue synthesizers, a very wide repertoire of sounds is possible (including all phonemes in Indo-European languages, for example). We shall now present in greater detail the actual design of this device. |
