This research group has presented a series of papers concerning multi-transputer
audio processors. At the 1990 ICMC, a prototype module, consisting of 16
x T800, was demonstrated [Bailey et al., 1990]: A T800 transputer has four
communication links that permit to form a ternary tree. Four transputers
form a single element and four of these are situated
on a single 3U printed circuit board, 16 transputers
in total. The original rationale behind this design was that a real-time
system should not require a large amount of memory for storage, therefore,
there is no memory other than 4k-byte integrated to each transputer, 64k
bytes per board in total. The prototype was subsequently developed to an
audio processor using 10 of these cards; 160 transputers.
An application of real-time additive synthesis, using recursive sine oscillators,
for the network was reported and demonstrated at the 1994 ICMC [Itagaki et al., 1994].
1.2 Granular Synthesis
Granular synthesis was originally proposed as a representation of "acoustical
quanta" [Gabor]. More recently applications in digital form and granulation
of sampled sound are proving of increasing interest to composers. These
have led to at least one commercial company manufacturing a suitable custom-designed
audio platform for its wider exploitation [Bartoo et al., 1994]. The signal
processing requirements of this application set tough demands for both hardware
and software engineers. In particular mapping suitable algorithms onto a
variety of parallel architectures provides an interesting challenge.
2. IMPLEMENTATION
2.1 Preliminary Experience
Our research group has been studying granular synthesis as a part of
on-going investigations into real-time audio synthesis using a multi-transputer
based audio processor. This has been inspired by the multi-processor application
of granular synthesis [Bartoo et al.]. In the initial phase of this project,
the granulation of a short fixed length sound sample has been implemented
on a segment of our tree-structured network. Parameters that describe a
grain; its amplitude, frequency, frequency range, ramp (amplitude envelope),
duration and duration range are controlled from a PC keyboard. From this
experiment we have ascertained that it is possible to implement nine voices
of real-time granular synthesis, using a set of short fixed sound samples,
on a sixteen-transputer network.
As a first step towards improving control flexibility, two coefficients;
amplitude and frequency, can be communicated to the network through a MIDI-to-Transputer
interface. Possibilities exist for controlling other key parameters through
MIDI using programme change or systems exclusive messages. It is anticipated
the final configuration of this system will allow the real-time performance
of at least 13 voices of granular synthesis on the 160 transputer network,
controlled either via MIDI or the host PC, with the option to process source
data dynamically as a stream of digitised sound information rather than as
a fixed length sample, as is the case at present.
2.2 Revised Implementation
In a prototype configuration, some of the transputers are assigned for
sound sample storage. This has not proved a particularly efficient way of
usage, since only 800 samples in 32-bit floating point format can be accommodated
to a transputer. This is also against the original rationale: a large
memory storage should not be required for real-time systems, however it has
become clear that the minimum memory requirements for granular synthesis
are greater than that provided internally via the configuration of 16-transputer
cards. For the revised configuration, a transputer with 128k- or 256k-byte
of external memory is connected to three transputers at the tree-bottom level
that share this extra memory. At a moment, 9 of these transputers with
external memory are available that cover a third of the network and provide
at least 27 voices. The sound samples stored in the memory are refreshed
through the other end of the network as shown below.
figure 1: Diagram of Granular Synthesis System
3. SUMMARY
A real-time granulation of sound is implemented onto a part of the 160
transputer network. A preliminary configuration, some transputers assigned
for memory storage, works with fixed short sound samples. The revised configuration,
using transputers with external memory, would allow more flexible applications,
such as time-shifting and time-stretching of long sampled sound.
4. ACKNOWLEDGEMENTS
The authors acknowledge the generous donation of processors from INMOS
Ltd., UK and financial support from the University of Durham.
REFERENCES
[Bartoo et al.] Bartoo, T., Murphy, D., Ovans, R. and Truax, B. 1994. "Granulation
and Time-shifting of Sampled Sound in Real-time with a Quad DSP Audio Computer
System." Proceedings of the 1994 International Computer Music Conference,
pp. 335-337
[Bailey et al.] Bailey, N.J., Bowler, I., Purvis, A. and Manning, P.D.
1990. "An Highly Parallel Architecture for Real-time Music Synthesis and
Digital Signal Processing", Proceedings of the 1990 International Computer
Music Conference, pp. 169-171
[Gabor] Gabor, D. 1947. "Acoustical Quanta and the Theory of Hearing."
Nature 159 (4004) : 591-594
[Itagaki et al., 1994] Itagaki, T., Purvis,
A. and Manning, P.D. 1994. "Real-time Synthesis on a Multi-processor Network."
Proceedings of the 1994 International Computer Music Conference, pp.
382-385