DAFX: Digital Audio Effects, Second Edition

6.2 Variable Speed Replay

Introduction

Analog audio tape recorders allow replay with a wide range of tape speeds. Particularly in fast forward/backward transfer mode, a monitoring of the audio signal is possible which can be used to locate a sound. During faster playback the pitch of the sound is raised and during slower playback the pitch is lowered. With this technique the duration of the sound is lengthened if the tape is slowed down, and shortened if the tape speed is increased. Figure 6.1 illustrates a sound segment which is lengthened and shortened, and the corresponding spectra.

Figure 6.1 Pitch shifting: Variable speed replay leads to time compression/expansion and compression and expansion of the spectral envelope.

Signal Processing

The phrase “variable speed replay” denotes that what has happened initially during the time period nTs, in is now happening during

6.1

at the relative speed v, where Ts, in and Ts, replay are the initial and replay sampling periods. Time expansion corresponds to v < 1. A straightforward method of implementing the variable speed replay is hence to modify the sampling frequency while playing back the sound according to

6.2

where fs, in and f