Speech/Audio Signal Processing

1. 1999 MATLAB Conference, Singapore Speech/Audio Signal Processing • J.-S. Roger Jang (張智星) • CS Dept, Tsing-Hua Univ, Taiwan • (清華大學 資訊系) • http://www.cs.nthu.edu.tw/~jang • jang@cs.nthu.edu.tw

2. Outline • Wave file manipulation Reading, writing, recording ... • Time-domain processing Delay, filtering, sptools … • Frequency-domain processing Spectrogram • Pitch determination Auto-correlation, SIFT, AMDF, HPS ... • Others Formant estimation, speech coding

3. Toolbox/Blockset Used • MATLAB • Simulink • Signal Processing Toolbox • DSP Blockset

4. To Read a Wave File • To read a MS wave file (PCM format only): wavread y = wavread(file) [y, fs, nbits] = wavread(file) […] = wavread(file, n) […] = wavread(file, [n1, n2]) [y, fs, nbits, opts] = wavread(file) • If it is stereo, y will be a two-column matrix.

5. To Read a Wave File • Example： [y, fs] = wavread(‘singapore.wav’); subplot(2,1,1), plot((1:length(y))/fs, y); xlabel(‘Time in seconds’); ylabel(‘Amplitude’); • Exercise： Plot the waveforms of the two channels in “flanger.wav”.

6. Solution to the Previous Exercise [y, fs] = wavread(‘flanger.wav’); subplot(2,1,1), plot((1:length(y))/fs, y(:,1)); subplot(2,1,2), plot((1:length(y))/fs, y(:,2));

7. To Play a Sound • To play sound using Windows audio output device: wavplay, sound, soundsc wavplay(y, fs) wavplay(y, fs, ‘async’): non-blocking call wavplay(y, fs, ‘sync’): blocking call sound(y, fs) soundsc(…): autoscale the sound • Example： [y, fs] = wavread(‘singapore.wav’); sound(y, fs); • Exercise： Follow the example to play “flanger.wav”.

8. To Read/Play Using DSP Blocks • To read/play sound using DSP Blockset: DSP Blockset/DSP Sources/From Wave File DSP Blockset/DSP Sinks/To Wave Device • Example: • Exercise: Create a model as shown above. Frame-based operation!

9. To Write a Wave File • To write MS wave files: wavwrite wavwrite(y, fs, nbits, wavefile) “nbits” must be 8 or 16. “y” must have two columns for stereo data. Amplitude values outside [-1,1] are clipped. • Example： [y, fs] = wavread(‘singapore.wav’); wavwrite(y, fs*1.2, 8, ‘testout.wav’); !start testout.wav • Exercise： Try out the above example.

10. To Record Speech/Audio • To record wave files: 1. Use the recording utility under Win95/98/NT. 2. Use “wavrecord” under MATLAB. 3. Use “From Wave Device” under Simulink; it is good for real-time signal processing (dspstfft_nt.mdl) • Example： 1. Go ahead and try Win95/98 recording utility! 2. Try “wavRecord01.m” 3. Try “slWavRecord01.mdl” • Exercise: Try out the above examples.

11. Time-Domain Speech Signals • A typical time-domain plot of speech signals: Amplitude: volume or intensity Frequency: pitch

12. Time-Domain Signal Processing • To control the play of a sound: • Normal: sound(y, fs) • High volume: sound(2*y, fs) • Low volume: sound(0.5*y, fs) • High pitch (and faster): sound(y, 1.2*fs) • Low pitch (and slower): sound(y, 0.8*fs) • Exercise: • Try “playwave.m” and change some of its parameters.

13. Time-Domain Signal Processing • Take-home exrecise: How to get a high pitch with the same time span?

14. Synthetic Sounds • Use a sine wave generator to produce sounds Single frequency: Multiple frequencies: Amplitude modulation: • Exercise: Create the above models.

15. Delay in Speech/Audio • What is a delay in a signal? y(n) --> y(n-k) • What effects can delay generate? Echo Reverberation Chorus Flanging

16. -k z Single Delay in Audio Signal • Block diagram: Input a Output u(n) y(n) = u(n) + a*u(n-k) • Simulink model: • Exercise: Create the above model.

17. -k z Multiple Delay in Audio Signal • How to create “karaoke” effects: a Input Output y(n) u(n) 2 3 y(n) = u(n) + a u(n-k) + a u(n-2k) + a u(n-3k) ... • Simulink model:

18. Multiple Delay in Audio Signal • Parameter values: • Feedback gain a < 1 • Actual delay time = k/fs • Exercise: • Create the above model and change some parameters to see their effects. • Modify the model to take microphone input (so you can start singing karaoke now!) • Use a “configurable subsystem” to include all possible input files and the microphone. (See next page.)

19. Multiple Delay in Audio Signal • How to use “configurable subsystem” block? 1. Create a library (say, wavinput.mdl) 2. Get a block of “configurable subsystem” 3. Fill the dialog box with the library name

20. Audio Flanging • Flanging sound: • A sound similar to the sound of a jet plane flying overhead, or a "whooshing" sound • “Pitch modulation” due to a variable delay • Simulink demo: • dspafxf.mdl (all platforms) • dspafxf_nt.mdl (for 95/98/NT)

21. Audio Flanging • Simulink model: Original spectrogram: Modified spectrogram:

22. Signal Processing Using sptool • To invoke sptool, type “sptool”.

23. Speech Production • How is speech produced? Speech is produced when air is forced from the lungs through the vocal cords (glottis) and along the vocal tract. • Analogy to System Theory: Input: air forced into the vocal cords Output: media vibration System (or filter): vocal tract Pitch frequency: frequency of the input Formant frequency: resonant frequency

24. Source Filter Model of Speech • The source-filter model of speech production: Speech is split into a rapidly varying excitation signal and a slowly varying filter. The envelope of the power spectra contains the vocal tract information. Two important characteristics of the model are fundamental (pitch) frequency (f0) and formants (F1, F2, F3, …)

25. Frame Analysis of Speech Signal Speech wave form : Zoom in Overlap Frame

26. Spectrogram • Spectrogram (specgram.m) displays short-time frequency contents: Wave form : Spectrogram :

27. Real-time Spectrogram • Try “dspstfft_nt”: Spectrum: Spectrogram:

28. Pitch and Formants • Pitch and formants can be defined visually: Pitch period = 1/f0 First formant F1 Second formant F2

29. Spectrogram Reading • Spectrogram Reading • http://cslu.cse.ogi.edu/tutordemos/SpectrogramReading/spectrogram_reading.html Waveform: Spectrogram: “compute”

30. Pitch Determination Algorithms • Time-domain: • Auto-correlation • AMDF (Average Magnitude Difference Function) • Gold-Rabiner algorithm (1969) • Frequency-domain: • Cepstrum (Noll 1964) • Harmonic product spectrum (Schroeder 1968) • Others: • SIFT (Simple inverse filter tracking) • Maximum likelihood • Neural network approach

31. Autocorrelation of Each Frame • Let s(k) be a frame of size 128. 1 128 s(k): s(k-h): h=30 x(30) = dot prod. of overlapped = sum(s(31:128).*s(1:99) Autocorrelation x(h): Pitch period 30

32. Autocorrelation via DSP Blockset • Real-time autocorrelation demo: • Exercise: Construct the above model and try it.

33. Pitch Tracking via Autocorrelation • Real-time pitch tracking via autocorrelation: pitch2.mdl

34. Formant Analysis • Characteristics of formants: • Formants are perceptually defined. • The corresponding physical property is the frequencies of resonances of the vocal tract. • Formant analysis is useful as the position of the first two formants pretty much identifies a vowel. • Computation methods: • Peak picking on the smoothed spectrum • Peak picking on the LP spectrum • Factoring for the LP roots • Fitting of mixture of Gaussians

35. Formant Analysis • Track Draw: • A package for formant synthesis with options to sketch formant tracks on a spectrogram. • http://www.utdallas.edu/~assmann/TRACKDRAW/trackdraw.html • Formant Location Algorithm • MATLAB code by Michelle Jamrozik • http://ece.clemson.edu/speech/files.htm

36. Speech Waveform Coding • Time domain coding • PCM: Pulse Code Modulation • DPCM: Differential PCM • ADPCM: Adaptive Differential PCM (dspadpcm.mdl) • Frequency domain coding • Sub-band coding • Transform coding • Speech Coding in MATLAB http://www.eas.asu.edu/~speech/education/educ1.html

37. Conclusions • Ideal tools for speech/audio signal processing: • MATLAB • Simulink • Signal Processing Toolbox • DSP Blockset • Advantages: • Reliable functions: well-established and tested • Visible graphical algorithm design tools • High-level programming language yet C-compatible • Powerful visualization capabilities • Easy debugging • Integrated environment

38. References [1] “Discrete-Time Processing of Speech Signals”, by Deller, Proakis and Hansen, Prentice Hall, 1993 [2] “Fundamentals of Speech Recognition”, by Rabiner and Juang, Prentice Hall, 1993 [3] “Effects Explained”, http://www.harmony-central.com/Effects/effects-explained.html [4] “TrackDraw”, http://www.utdallas.edu/~assmann/TRACKDRAW/trackdraw.html [5] “Speech Coding in MATLAB”, http://www.eas.asu.edu/~speech/education/educ1.html