Proc. of the 16 th Int. Conference on Digital Audio Effects (DAFx-13), Maynooth, Ireland, September 2-4, 2013 DAFX-1 CHROMAX, THE OTHER SIDE OF THE SPECTRAL DELAY BETWEEN SIGNAL PROCESSING AND COMPOSITION Arshia Cont, Carlo Laurenzi Marco Stroppa Ircam 1 place Stravinsky 75004 Paris University of Music and Performing Arts Urbanstr. 25 70182 Stuttgart, Germany Arshia.Cont@ircam.fr Carlo.Laurenzi@ircam.fr stroppa@mh-stuttgart.de ABSTRACT Spectral delays have been used for a long time as a way to colour and shape spectral characteristics of sound. Most of available software is controlled by drawing an envelope on a window that represents spectral bins, and by setting a max- imum delay time. Despite its comfort, such a simplistic approach does not imply any methods for allowing symbolic manipulations on spectral data that are often required by composers and sound designers. Chromax proposes an alternative dynamic parameterization of spectral delays, allowing fine and complex compositional manip- ulations. It implements a bin-synchronous spectral processing using the new Gen~ technology available in Max6 [1], and pro- vides algorithms to dynamically specify a filter, a delay and a feedback level for each bin of a processed sound. 1. INTRODUCTION Spectral delays may be considered as an extension of traditional delays. An incoming sound is subdivided into frequency bands and each band can be assigned a different delay and played back. In addition a feedback can be added. Even if alternative implementations have been proposed [2], the frequency subdivision is usually performed via a Fast Fourier Transform (FFT). Spectral delays may become computationally demanding if the number of bands is very high, raising efficiency issues and accordingly reducing control over parameters. Moreo- ver, since the user must set delay times and feedback values for each frequency band, a large amount of data needs to be speci- fied; this raises issues of interface design. The currently available spectral delays are either standalone ap- plications (such as iZotope Spectron [3], or the now discontinued NI Spektral Delay, [4]), or special objects for Max or Pure Data. The interface is usually conceived as a time vs. frequency win- dow (time is the Y-axis). The user draws an envelope over fre- quencies and sets a value for the maximum delay. A second win- dow may be used to control feedback levels. 1.1. Control strategies The need for better control strategies than those available on commercial software was already mentioned in a paper by Kim- Boyle [5]. He implements a spectral delay as a Max object using pfft~, and provides some macros to manipulate the frequency window, such as increment or decrement by a small amount the entire contents of the buffer or write a value to a specific set of bins [5, p 43]. He also mentions the automatic computation of delay values by performing an FFT analysis of a control signal, which may not be the same signal as that processed [5, p. 43]. From a purely compositional standpoint, however, these controls remain quite basic. For instance, it would be hard with them to compose a precise rhythm. Gibson [6], drawing examples from his own works, points out that a spectral delay can be also used as a compositional tool and that delay times can be set at once for many bands in the form of a Max message. His paper, however, does not delve into the de- tails of how high-level compositional data are mapped onto low- level spectral parameters and the main control still seems to be window-oriented. 1.2. Time issues A further complexity stems from the need of accurate timing and constraints imposed by Short-Time Fourier Transforms (STFT), where the smallest time unit is the hop-size, that is typically ! of the analysis window if a Hanning window is used, which de- pends on the sampling rate (SR) and the window size (WS). For instance, for a SR of 96KHz and a WS of 8192 points, the time resolution is 21.3 msec. Although this value seems to be very small from the point of view of a composition (it corre- sponds to a little more than a hemidemisemiquaver of a sextuplet with a MM of 120!), when planning rhythmical patterns with a large amount of feedback, round-off errors may rapidly lead to unsynchronized sloppy rhythms that composers would reject. Designing musically satisfying, albeit computationally reasona- ble time scaling algorithms, while giving the maximum freedom to the composer, is a challenge and implies a trade-off between scientific precision and aesthetical needs. Kim-Boyle [5] also mentions another problem of temporal limi- tations, when spectral delays are used with stereo files for spatial purposes at the resolution of inter-aural time differences (few msec). Finally, when dynamic delays are allowed, the process becomes further complicated, since such delays must correspond to the timing constraints of the STFT so that feedback values remain rhythmically coherent. A common side-effect when missing this consideration is timing offsets after each feedback cycle. 2. SPECTRAL GENERATOR Chromax is a spectral generator that provides separate templates for the delay and the feedback windows and a large set of param- eters. Chromax achieves high-level generation of low-level spec-