DAFX: Digital Audio Effects, Second Edition

6.1 Introduction

In this chapter we discuss several time-domain algorithms which are a combination of smaller processing blocks like amplitude/phase modulators, filters and delay lines. These effects mainly influence the pitch and the time duration of the audio signal. We will first introduce some basic effects like variable speed replay and pitch-controlled resampling. They are all based on delay-line modulation and amplitude modulation. Then we will discuss two approaches for time stretching (time scaling) of audio signals. They are based on an analysis stage, where the input signal is divided into segments (blocks) of fixed or variable length, and a synthesis stage where the blocks of the analysis stage are recombined by an overlap and add procedure. These time-stretching techniques perform time scaling without modifying the pitch of the signal. The fourth section focuses on pitch shifting, and introduces three techniques: block processing based on time stretching and resampling, delay line modulation and pitch-synchronous block processing. Block processing based on delay line modulation performs pitch shifting by scaling the spectral envelope of each block. Pitch-synchronous block processing performs pitch shifting by resampling the spectral envelope of each block and thus preserving the spectral envelope. The last section on time shuffling and granulation presents a more creative use of time-segment processing. Short segments of the input signal are freely assembled and placed ...