The Stereo Time Delay in Tandem Configuration Frans Absil The Tempo-Synchronised Stereo Time Delay Effect in Tandem Configuration Frans Absil∗ June 2013 Abstract This document will demonstrate the creative use of two or more stereo time delay units in a tandem (series) configuration. In case of tempo-synchronised stereo delay plugins with cross-channel feedback, rhythmically panned echo patterns will be achieved, when exciting the effect set-up with percussive (pulse-like) sounds. Diagrams and score examples will illustrate the basic mono and stereo time delay effect, the difference between in-channel and cross-channel feedback, and the more complicated double delay effect chain with tempo- synchronised delay times. Keywords: audio signal processing, music production, sound effects, mixing engineer- ing, tempo-synchronised time delay effect, cross-channel feedback, syncopated rhythms. 1 Introduction Audio mixing engineers use a variety of sound effect processors, either as hardware rack com- ponents or as software plugins on a Digital Audio Workstation (DAW computer) [1, 2, 3, 4]. Time delay effects are widely used processors in such a suite. They generate a series of time- delayed copies of the input signal, that are mixed with the original input signal. In case of a stereo time delay the effect operates on two separate audio channels in parallel (the right and left channel). Time delay settings may be absolute (in milliseconds) or tempo-synchronised (i.e., a fraction of the musical tempo unit, such as the duration of a quarter note or an 8th triplet note). The feedback level determines the number of echoes (from a single delayed version to an infinite echo series); the delayed versions will be of diminishing amplitude (audio signal strength). The stereo time delay has two in-channel feedback circuits, enabling different feedback levels for right and left channel. In case of a stereo time delay there may also be cross-channel feedback, where the left channel effect output is fed back into the right delay channel and vice versa for the right channel. A combination of both in-channel and cross-channel feedback may be achieved by balancing the four feedback level settings; in case of a tempo-synchronised delay this set-up yields right-left panned rhythmical echoes. Mixing engineers use multiple time delays in parallel. Time delay settings may be set for specific audio effects; either in the Haas region (typically 5-20 milliseconds), the slapback region (typically 30-50 milliseconds) or audible echoes (typically more than 100-200 ms). Audio signals are then routed into one or more of these time delay effects. ∗ All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, with- out prior permission in writing from the Author. (See last page for additional information.) c 2013 F.G.J. Absil, www.fransabsil.nl 1/13 The Stereo Time Delay in Tandem Configuration Frans Absil e ? Stereo Delay 1 ? Stereo Delay 2 ? Compressor e? Figure 1: The series of two stereo time delays in tandem configuration and compressor may be used as either an insert on an audio or MIDI instrument channel or placed on an Aux bus. Tandem configurations of audio effects are also used frequently; examples are the series con- figuration of an equalizer with a compressor, a time delay unit in series with a reverb unit, or two different reverb processors.1 However, the tandem configuration of two (or more) time delay units is rare. Undoubtedly this effect has been used in music production, but its potential for creating complex percussive rhythms is not described in the literature. The alternative for achieving this result is using ded- icated multi-tap time delay effects (such as the Delay Designer plugin in Logic Pro [5]). However, in such a plugin many individual delay taps will have to be programmed and their parame- ter settings finetuned (level, panning, filtering) to achieve a syncopated panning rhythm. The same effect (and many more) can be easily achieved with setting only a very limited number of plugin parameters in the stereo time delay tandem configuration. This document will illustrate the principle of applying multiple stereo time delay effects with cross-channel feedback in tandem configurations. Complex percussive patterns can be exciting this set-up with a pulse in one (or both) of the audio channels.2 The final, complete set-upthat we will discuss here, is shown in diagram in Fig. 1: it consists of two stereo delays in series with a compressor at the end of the chain. This set could be used as in insert on an audio or MIDI instrument channel; alternatively, it is the set-up of an auxiliary bus, that receives sends from other audio signal channels. This document is structured as follows: first, the basic single stereo delay unit is described. Diagrams and score examples will illustrate the working of the cross-channel feedback. Then the tandem configuration will be demonstrated. Finally, suggestions for further experimenta- tion will be given. Have fun reading this document and then, please start experimenting with your audio equipment! 1A number of these tandem effect processor configurations have been demonstrated by mixing engineer Dave Pensado in his weekly episodes of ‘Into the Lair’ on the online video podcast channel http://www.pensadosplace.tv. This is what triggered my own thinking and experimentation with two stereo time delays in series. 2In case of limited parameter sets for the time delay units (e.g., no stereo version available, no cross-channel feedback), the set-up herein can also be created with multiple mono time delay units on separate Aux channels with specific level and pan settings. However, that makes the design of panned rhythmic patterns far more complicated. c 2013 F.G.J. Absil, www.fransabsil.nl 2/13 The Stereo Time Delay in Tandem Configuration Frans Absil ? e - e- Delay ∆t - BPF - Mix - e 6 FB Figure 2: The single (mono) time delay effect. The delay time ∆t may be synchronised to the tempo. The bandpass filter (BPF) has input parameters for low and high frequency cut-off [fco,L,fco,U ], the feedback (FB) level (in %) determines the number of signal repeats (echoes). The mix (Mix) setting determines the wet/dry output balance. 2 The basic time delay configuration Let’s start by explaining the fundamentals and working of the time delay effect. This will familiarize you with the terminology and the diagrams that will illustrate the effect on the audio signal. Some of you may want to skip this part and move on to the tandem configuration section. 2.1 The mono time delay effect The basic single time delay effect is shown in diagram in Fig. 2. The mono audio signal enters the processor on the left. A copy of this signal is output after a time delay ∆t and mixed with the direct signal (the Mix block will determine the wet/dry balance). A bandpass filter (BPF) will have both a highpass (low frequency cut-off) and a lowpass (high frequency cut-off) filter in order to colour and equalize the delayed signal. The feedback (FB) returns a fraction of the delay output back into the unit. The time response of this time delay effect to a single pulse, entering the effect at time t = 0 s, is shown in Fig. 3. Without feedback (FB = 0) there is a single delayed pulse echo at time t = ∆t. As the feedback value increases (going from FB1 to FB4 in the diagram) the number of echoes increases, while their strength is continuously decreasing (echo attenuation). The example shows a tempo-synchronised time delay with delay setting ∆t = 3 time units (e.g., 3 beats in 4/4 meter). The tickmarks along the timeline represent time units (beat markers); for a 1 4/4 meter 18 beats are shown (4 2 measures). 2.2 The stereo time delay effect The stereo time delay with in-channel feedback is shown in Fig. 4. This may be considered as two mono time delay effect units in parallel, each of them processing an audio signal channel (left and right). The delay time ∆t, feedback level (BF) and wet/dry mix level (Mix) may be different for the right and left channel. The bandpass filter (BPF) settings are equal for both channels (two blocks are shown in the diagram but they have identical parameter settings). The response of the stereo time delay effect with in-channel feedback to a pulse signal on both channels at time t = 0 s (1st beat) is shown in Fig. 5. The echo signal strength is no longer shown in the figure (there are no arrows; the closed circles represent the time instances of the echo signal). The right audio channel has delay time ∆tR = 2 time units (or beats), the left channel has delay time ∆tL = 3 time units. For the given feedback levels this leads to a finite c 2013 F.G.J. Absil, www.fransabsil.nl 3/13 The Stereo Time Delay in Tandem Configuration Frans Absil Input pulse 6 Single echo 6 FB1 = 0 e u ∆t 6 6 Multiple echoes 6 FB2 >FB1 e u u ∆t 2∆t 6 6 6 6 FB3 >FB2 e u u u ∆t 2∆t 3∆t 6 6 6 6 6 FB4 >FB3 e u u u u u6 ∆t 2∆t 3∆t 4∆t 5∆t Figure 3: The pulse response of the mono time delay effect with in-channel feedback. The audio signal pulse enters the effect unit at time t = 0 s (1st beat in 4/4 meter, open circle).