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Pitch Shifter with Single-Sideband AM

Module by: Ed Doering. E-mail the author

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Summary: Pitch shifting makes an interesting special effect, especially when applied to a speech signal. Single-sideband amplitude modulation (SSB-AM) is presented as a method to shift the spectrum of a source signal in the same way as basic AM, but with cancellation of one sideband to eliminate the “dual voice” sound of conventional AM. Pre-filtering of the source signal to avoid aliasing is also discussed.

Table 1
LabVIEWq.png This module refers to LabVIEW, a software development environment that features a graphical programming language. Please see the LabVIEW QuickStart Guide module for tutorials and documentation that will help you:
• Apply LabVIEW to Audio Signal Processing
• Get started with LabVIEW
• Obtain a fully-functional evaluation edition of LabVIEW

Overview

Amplitude modulation (AM) of a source signal divides the signal's spectrum into two copies, with one copy shifted towards higher frequency and the other copy shifted towards lower frequency; refer to AM Mathematics for a complete treatment of basic AM. The shifted and dual spectrum makes an interesting special effect when applied to a musical instrument or the human voice, creating the sensation of two different people speaking the identical phrase, for example.

If one of these spectral images could somehow be cancelled out, AM seems to be a feasible way to implement a pitch shifter, a device or algorithm that shifts the source spectrum higher or lower in frequency. When this special effect is applied in real time, you can speak into a microphone and sound just like one of "Alvin and the Chipmunks."

As an example of what you will be able accomplish by applying the techniques presented in this module, listen to this original speech clip speech.wav and its pitch-shifted version speech_shifted.wav (speech clip 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).

Single-Sideband AM (SSB-AM)

The screencast video of Figure 1 develops the basic theory of single-sideband (SSB) modulation, a technique borrowed from communications systems that provides a way to apply amplitude modulation with spectral image cancellation.

Figure 1: [video] Single-sideband modulation for pitch shifting
Figure 1 (mod_am-shifter-theory.html)

As an exercise to ensure that you followed each step, draw a block diagram or flow diagram to show how the original signal is modified to produce the final shifted signal. Your diagram should include directed lines (arrows) to show signal flow, and should use symbols (blocks) for the multipliers, cosine and sine oscillators, Hilbert transformer, sign changer, and adder.

Pre-Filtering to Avoid Aliasing

Pre-filtering the source signal ensures the shifted spectrum does not alias, since the source signal typically fills the available bandwidth. The screencast video of Figure 2 discusses the aliasing problem as well as the techniques you can use to design a suitable pre-filter.

Figure 2: [video] Pre-filtering to avoid aliasing
Figure 2 (mod_am-shifter-prefilter.html)

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