Page  354 ï~~Electroacoustic Scoring with Phase-vocoding Instruments using the E-Scape composition system T M Anderson & PR Kirk, Music Technology Group, University of York, UK EMail: tmal, Abstract: A composer can use the E-Scape graphical electroacoustic scoring system [1] to construct instruments which incorporate phase vocoding analysis and resynthesis of sound, and complex manipulations of the analysis files. These instruments are presented to the composer in the same way as instruments incorporating synthesis or other sound manipulation processes. An instrument is then implemented as a network of primitive processing units on the University of York MIDAS system [4], at present running on a network of UNIX workstations. Graphical displays of the resultant sound spectrum at various stages within an instrument can be provided by;linking in frequency spectra displayprocesses. An example instrument is presented, along with a graphical score which uses it. The composer is thus provided with a direct visual mapping from the score to the analysis-file processing parameters used in each event. Spectral manipulation techniques involving the processing of phase vocoder analysis files have been successfully employed in electroacoustic composition [5]. These files consist of amplitude and frequency values for a partial (spectral component) in each of a number of frequency 'channels'. These values are presented in a series of overlapping time windows ('frames') taken from the sampled input sound. To manipulate such files, composers have hitherto had to specify command line parameters for custom designed processing programs. The latest version of CSound now provides spectral data type unit-generators which enable a composer to:- impose a (static) frequency spectrum envelope, proportionally mix two (static) spectra, differentiate successive frames, display analysis data, and filter each channel. These additions to CSound are welcome but are relatively high-level operations. Thus their flexibility is somewhat limited, as is the facility for continuous variation of processes since all control parameters are i-rate. lnu.rm- 'bend 3 band 2 'band I 'band 1 cnplaici.ani 'enetyi e stretch Stretch i.brtch tlt I S fle name' I factor' I Wctor' | factor' I frequency Instrument-telate named: 'Frequency Stretches Fig. I 354

Page  355 ï~~In the E-Scape composition system, [1] a composer can design instruments graphically from modules, which when unravelled, specify a network of primitive processes on a variety of synthesis systems. One such is the MIDAS system under development at the University of York [2, 4]. This enables a network of primitive processes (UGPs) to run on an arbitrary number of processor nodes, which communicate data via a fast LAN-like network. Several UGPs are under development [3] to enable time-windowing, FFT analysis and resynthesis processes to be constructed by a high-level user. UGPs to perform data processing operations on analysis files are also planned, which will be capable of accepting continuously variable control data. Frequency domain displays of analysis data could in principle be provided by display-UGPs running on a MIDAS graphics node. Thus a composer can design an E-Scape instrument which processes analysis files in arbitrarily complex ways (and converts them back to sound). An E-Scape score can then allow time-varying control of any instrument input parameter. Each instrument input receives data from a score pattern, which also provides a visual indication of the data variation. Fig 1. shows an example instrument which stretches the frequency spectrum of a (previously analysed) soundfile in various ways. The signal path runs from top left to bottom right. The analysis-file name may be changed (via the score) during an event, providing a crude capability to migrate from one sound to another (the smoothness depending on the window overlap). Each window of data is then split into three frequency bands, which are then independently processed. Each band is 'stretched' by multiplying the frequency value of each partial by a variable factor controlled from the score. For each band a frequency is also specified, below which no stretching will be performed. This is also controlled from the score for band 1. The top right module performs an average (weighted according to amplitude) of the frequencies of any partials which may now exist in the same channel. This is necessary as the frequency of a partial in band I may have been multiplied so it now occupies one of band 2's channels, in which there still may exist a partial. Finally inverse-FFT and window-merging UGPs produce sound output, and analysis data from the start and end of the processing chain is sent to two 3D frequency domain display-UGPs. The modules shown may be UGPs, or may be constructed out of simpler UGPs, but in either case the composer may connect and use them as modules without needing to know their inner construction. 2. 1.8 1.6 1.5 1.2 ] Q 1, IV, -,........lgo 1 1 1 1 1go 1 et 0...,.. 2.4 band 2 stretch o 1.8 1.6 1.4 1.2 1. 200 400 600 800 1000 1200 1400 1600 Fig. 2 The simple E-Scape score example in fig. 2 changes the stretch factor in each band during the course of two score events (the second starting at 900ms). The composer can see any time domain changes directly in the score, while a separate screen would show the two frequency domain displays. The 'band I start frequency' and 'analysis file name' parameters are not scored in this example, and therefore assume their default values as specified in the E-Scape instrument. Thus, the score visually presents the changing analysis-file processing parameters used in each event, and enables a composer to easily alter them. [ 1] Anderson, T.M. "E-Scape: An Extendable Sonic Composition and Performance Environment" Proc. ICMC, Glasgow, 1990. [2] Anderson, T.M., Hunt, A.D., Kirk, P.R., McGilly, P., Orton, R. & Watkinson, S.A. "From Score to Unit Generator: A Hierarchical view of MIDAS". Proc. ICMC, San Jose, 1992. [3] Katrami, A.I., Kirk, P.R. & Orton R, "Deconstructing the Phase Vocoder". Proc. ICMC, San Jose, 1992. [4] Kirk, P.R. & Orton, R. "MIDAS: A Musical Instrument Digital Array Signal Processor". Proc. ICMC, Glasgow, 1990. [5] Wishart, T. "The Composition of Vox-5". Computer Music Journal, Vol 12, No 4. 1988. 355