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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.
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.
"This online course covers signal processing concepts using music and audio to keep the subject relevant and interesting. Written by Prof. Ed Doering from the Rose-Hulman Institute of Technology, […]"