9.1 Schroeder reverberator  (Page 9/2)

Introduction

Reverberation is a property of concert halls that greatly adds to the enjoyment of a musical performance. The on-stage performer generates sound waves that propagate directly to the listener's ear. However, sound wavesalso bounce off the floor, walls, ceiling, and back wall of the stage, creating myriad copies of the direct sound that are time-delayed and reduced in intensity.

In the prerequisite module Reverberation , you learned how the comb filter structure can efficiently create replicas of a direct-path signal that are time delayed and reduced in intensity. However,the comb filter produces replicas that are time delayed by exactly the same amount, leading to the sensation of a pitched tone superimposed on the signal. Refer back to Reverberation to hear an audio demonstration of this effect. Put another way, the impulse response of the comb filter contains impulses withidentical spacing in time, which is not realistic.

The Schroeder reverberator (see "References" section) uses a combination of comb filters and all-pass filters to produce an impulse response that more nearly resembles the random nature of a physical reverberant environment.

This module introduces you to the Schroeder reverberator and guides you through the implementation process in LabVIEW. As a preview of what can be achieved, watch the screencast video to see and hear a short demonstration of a LabVIEW VI that implements the Schroeder reverberator.The speech clip used in the video is available here: speech.wav (audio courtesy of the Open Speech Repository, www.voiptroubleshooter.com/open_speech ; the sentences are two of the many phonetically balanced Harvard Sentences , an important standard for the speech processing community).

Structure of the schroeder reverberator

The screencast video presents the structure of the Schroeder reverberator and describes the rationale for its design.

All-pass filter

The Schroeder reverberator uses all-pass filters to increase the pulse density produced by the parallel comb filters. You perhaps are familiar with the frequency response of an all-pass filter: its magnituderesponse is unity (flat) for all frequencies, and its phase response varies with frequency. For example, the all-pass filter is used to create a variable fractional delay as described in Karplus-Strong Plucked String Algorithm with Improved Pitch Accuracy .