2. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 1 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing EE2F1 Speech & Audio Technology Lecture 2 Martin Russell Electronic, Electrical & Computer Engineering School of Engineering The University of Birmingham

3. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 2 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Pulse Code Modulation (PCM)  How many quantization points?  How many samples per second (sample rate)? Quantization error Sample point Quantization point

4. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 3 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Quantisation  Each sample is stored as a computer “word” with a given number of bits  More bits give more levels: Number of bits Number of levels Quality 664Poor – just intelligible 8256Tolerable speech 138192FM Radio 1665536Good - CD

5. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 4 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sample Rate  All sound so far sampled at 44.1kHz (why?)  Hence Nyquist frequency = 22.05kHz  44.1kHz and 16 bit quantisation used on audio CDs (we’ll talk about CDs later)

6. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 5 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Effect of sample rates Samples per secondNyquist frequency 44,10022,050Hz 22,05011,025Hz 16,0008,000Hz 8,0004,000Hz 40002,000Hz

7. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 6 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Speech sounds

8. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 7 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Speech sample rates

9. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 8 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Audio compression  CD quality audio sampled at 44.1kHz, 16bits  Hence 176 kbytes per second  3 minute song requires 30 megabytes  So, an ISDN line at 128 kbits per second is ten times too slow for CD quality audio  Hence need for compression –Lossless (e.g. for computer data files) –Lossy (e.g. for images and audio)

10. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 9 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing  -Law Compression  Use non-linear quantisation –Constant quantisation noise w.r.t signal amplitude  Most common scheme called  -Law compression  8 non-linear bit  -law achieves same performance as 12 bit linear  8 bit  -law at 8 kHz sample rate used in US digital phone lines

11. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 10 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Non-linear quantisation  Quantization points arranged non-linearly Sample point Quantization point

12. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 11 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Adaptive Delta PCM (ADPCM)  Delta PCM –Code changes in signal, not absolute values  Adaptive Delta PCM (ADPCM) –Adapt quantisation step size –Compute change in signal value –Compare with previous step size –Increase or decrease step size accordingly  ADPCM applications –Speech coders –Quality too poor for music or other quality-critical applications

13. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 12 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing MPEG  Exploits knowledge of human audio perception….

14. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 13 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Frequency analysis  1kHz square wave

15. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 14 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Spectrum cross section  1 kHz square wave Fundamental at 1 kHz 3 rd harmonic at 3 kHz 5 th harmonic at 5 kHz 7 th harmonic at 7 kHz

16. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 15 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sums of sine waves Sine wave Original, 3 rd harmonic Original + 3 rd harmonic

17. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 16 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sums of sine waves Original, 3 rd harmonic, 5 th harmonic Original + 3 rd harmonic + 5 th harmonic

18. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 17 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sums of sine waves (contd.) Original, 3 rd harmonic, 5 th harmonic, 7 th harmonic Original + 3 rd harmonic + 5 th harmonic + 7 th harmonic

19. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 18 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing More formally… fHz square wave fHz sine wave 3fHz sine wave ‘Weight’

20. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 19 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Fourier analysis  Adding a set of weighted sine waves results in a complex periodic waveform  In fact any periodic waveform can be expressed as a sum of weighted sine waves  The mathematical technique which calculates which sine waves are needed, and what each should be multiplied by is called Fourier Analysis

21. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 20 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Spectrum of a square wave f 3f 5f 7f 9f … a a/3 a/5

22. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 21 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Recorder

23. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 22 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Phase  Additional information needed to encode phase differences  In fact, the values in a spectrum are complex numbers – the imaginary part encodes phase  We shall ignore phase in most of what follows Phase difference

24. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 23 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Aperiodic signals  For signals which are not periodic, calculate spectrum over a short time ‘window’  ‘Slide’ analysis window forward in time  Spectrogram display

25. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 24 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Wide-band and narrow-band  Long analysis window: –Good frequency resolution –Poor temporal resolution –‘Narrow-band’ spectrogram  Short analysis window: –Poor frequency resolution –Good temporal resolution –‘Wide-band’ spectrogram

26. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 25 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Wide-band & narrow-band spectrograms wide-band narrow-band

27. EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 26 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Next week  Filters  Human hearing