Study and Implementation of Two Voice Warping Algorithms: Pitch Shifting

1
Study and Implementation of Two Voice Warping
AlgorithmsPitch Shifting Time Stretching

• EEL 6586 Project Presentation
• Deng, Chengyu
• Wang, Dexiang

2
Outline

• Pitch analysis
• Voice warping applications
• Pitch shifting
• Time stretching
• Pitch shifting algorithm
• How to change pitch
• General approaches phase vocoder VS PSOLA
• Improve frequency resolution
• Formants consideration
• Time stretching implementation
• Software exhibition (real work we have done)

3
Take a Look at Pitch

• The perceived fundamental frequency of a sound
  (definition)

Pitch period

• Due to glottis excitation
• An important identification of male or female,
  adults or children
• Accompany with lots of harmonics

4
Voice Warping Applications

• Pitch shifting maintain time duration but
  upscale or downscale pitch
• Change mens voice to womens OR vice versa
• Create chipmunk or Mickey mouse like sounds
• Lots of applications in movie industry
• Time stretching keep the pitch unchanged but
  shorten or stretch time duration
• Help with word identification
• Create some extremely short or long period of
  voice which can hardly be spoken by normal people

5
How to Change Pitch?

• Naïve idea
• Down-sample or up-sample the speech signal
• Problems
• Time duration also gets changed
• Formants get moved as well
• We should generate the same number of samples but
  only scale the pitch

6
Two General Approaches

• Phase Vocoder
• Manipulate the signal in frequency domain
• Phase is an important feature to determine the
  pitch and its harmonic position
• More accurate, higher fidelity, but longer
  computation
• Time domain scaling ((P)SOLA, etc)
• Manipulate the signal in time domain
• Precise pitch detection is a critical
  prerequisite
• Shorter computation, but lower quality
• (P)SOLA (Pitch) Synchronous OverLap/Add

7
Basic Algorithms We Used for Pitch Shifting

• Frequency domain process (more accurate)
• Use short time frequency transform
• And overlapped windows
• Scale the frequency axis to change the pitch and
  harmonics positions
• Upscale discard high frequency components to
  avoid aliasing (human cannot feel difference)
• Downscale put zeros as high frequency components

8
Improve Frequency Resolution

• Due to the accuracy limitation of discrete
  fourier transform
• Cannot precisely represent peak components

• Example
• A frequency point exactly on 50th sample
• B frequency point in between 50th and 51st
  samples

• Solution
• Utilize phase difference between two successive
  windows to compute exact frequency bins (final
  report will have more details)

9
Formants Consideration

• Deal with formant movement issues
• Lose vocal tract information
• Upshifting pitch -gt smaller vocal tract (shape)
  effect
• Downshifting pitch -gt bigger vocal tract (shape)
  effect
• Solution
• Calculate formants envelop (LPC)
• Normalize magnitudes before frequency scaling
• Scale frequency axis
• Recover formants envelop

10
Time Stretching Implementation

• Still take advantage of frequency domain
  manipulation
• Stretch time duration
• Interpolate additional samples between original
  frequency bins (upsampling in frequency domain)
• Linear interpolation instead of SINC function
  interpolation (for convenience of computation)
• Shrink time duration
• Compression of original frequency bin samples
  (downsampling in frequency domain)

11
Put All Together (Building Our Software
Implementation)

• Windows platform / Visual C
• Self-developed framework algorithms
• Formant position maintenance (LPC formant envelop
  calculation)
• Time stretching
• Borrowed idea and some source codes from DSP
  website
• http//www.dspdimension.com/ for elementary
  frequency shifting algorithm
• http//www.koders.com/ for Levinson LPC algorithm

12
Introduction to Our GUI Functions

• Set target parameters needed by pitch shifting
  and time stretching process
• Click Process Voice File to assign the
  original voice file and altered voice file
• Waiting for process completion
• Click Play Voice File button to hear the
  effect of altered voice

13
Introduction to Our GUI Functions

• Advanced setup
• Change the parameters used in our algorithms
• LPC order
• Window size
• Overlapped percentage of the windows

14
Demo Show (Question session follows)