COM325 Computer Speech and Hearing

Transcription

1 COM325 Computer Speech and Hearing Part III : Theories and Models of Pitch Perception Dr. Guy Brown Room 145 Regent Court Department of Computer Science University of Sheffield g.brown@dcs.shef.ac.uk SLIDE 1

2 1. Introduction COM325 COMPUTER SPEECH AND HEARING Definition: Pitch is that attribute of auditory sensation in terms of which sounds may be ordered on a musical scale (American Standards Association). Pitch is related to the repetition rate of a signal. The repetition rate of a sinusoidal tone is its frequency, and the repetition rate of a complex tone is its fundamental frequency (F0). In this part of the course, we will: Present some theories of pitch perception, which fall into two main classes - timing and pattern recognition theories; Discuss experimentally observed pitch phenomena which support or contradict these theories; Outline a computational model of pitch perception. SLIDE 2

3 2. Why is pitch important? Intone languages, pitch variation signifies phonetic or syllabic distinctions. For example, in Shona (spoken in Zimbabwe), kutshera is to draw water, whereas kutshera is to dig (underscore and overscore indicate low and high pitch). All languages use pitch variation to convey meaning. Try saying this lecture is really interesting to express: (i) sarcasm (ii) incredulity (iii) agreement Pitch is a cue for determining the number of acoustic sources present in a mixture, and for grouping sound components which originate from a single source. Pitch is an important compositional element in music. Pitch range is a good cue to speaker gender and, to a lesser extent, age. Pitch is a characteristic of non-speech sources that resonate when struck. SLIDE 3

4 3. The pitch of a sinusoid The pitch of a pure tone is related to its frequency, although other factors such as duration and (to a lesser extent) intensity can influence perceived pitch [1]. The smallest change in frequency that can be detected is called the frequency difference limen (DLF). This is measured by presenting listeners with two tones with slightly different frequencies in sequence, and asking which has the higher pitch. The DLF is remarkably small; at 1 khz the DLF is 2 Hz (i.e., 0.2%). Audio demo: dependence of pitch on duration You will hear tones of 300, 1000 and 3000 Hz in bursts of 1, 2, 4, 8, 16, 32, 64 and 128 periods. The percept changes from a click to a tone. Note whether or not you hear a pitch for each condition. Q. How many periods were necessary to establish a sense of pitch? SLIDE 4

5 3.1. Relationship between frequency and pitch Units of pitch are mels (from melody ). The pitch of a 1000 Hz tone is arbitrarily set at 1000 mels. Relationship between pitch (in mels) and frequency is derived by asking listeners to adjust the frequency of a tone so that it has half the pitch of a reference tone of equal loudness. The growth of perceived pitch is less rapid than the change in frequency (the same was true of the relationship between loudness and intensity). SLIDE 5

6 3.2. Pattern recognition and timing theories The two main pitch theories - pattern recognition and timing theories - are inspired by place and timing mechanisms of frequency coding in the auditory nerve. Theories of pitch require more than an explanation of how frequency is coded - they must also describe how a pitch percept is computed from neural signals. Hence, the pattern recognition and timing theories propose differing accounts of processing beyond the auditory nerve. Channel Number Time [ms] Timing: intervals between phase-locked spikes in the auditory nerve Channel Number x 10 4 Place: position of maximum displacement on basilar membrane SLIDE 6

7 3.3. Frequency coding by place Pitch may be coded by the position of the peak in the auditory excitation pattern. However, excitation patterns may hardly differ at the peaks; figure shows auditory response for tones of frequency 1000 Hz and 1005 Hz (a frequency difference greater than the DLF). Place coding predicts that DLF should vary in the same way as critical bandwidth; discrimination should be good at low frequencies where bandwidth is narrow, and poor at high frequencies where bandwidth is wide. Firing rate (spikes/sec) Channel Centre Frequency [Hz] Not a perfect fit to the data - some other mechanism is involved. SLIDE 7

8 3.4. Frequency coding by timing Timing is preserved by phase-locking in the auditory nerve. A range of fibres may be phase-locked, since center frequencies continuously overlap. However, auditory nerve fibres cannot fire more than a few hundred times per second. How are time intervals above this rate coded? A fibre need not fire on every cycle. If fibres fire every n cycles, intervals accumulate at multiples of the tone period. Firing every 5 cycles Firing every cycle Firing every 3 cycles Time (ms) See diagram - for a 1 khz tone, we get intervals at 1 ms, 2 ms, 3 ms and so on. A process which looked for the greatest common divisor of these intervals would correctly identify the frequency as 1 khz. SLIDE 8

9 3.5. Place coding vs. timing coding The timing theory can account for small DLFs if we assume that variability in the timing of spikes is reduced by averaging over many fibres. Phase locking is only maintained in the auditory nerve up to 4 khz or so. Above this frequency the DLF increases considerably. Timing mechanisms also necessary to explain pitch perception for very short tones, which would generate a blurred place representation. So, likely that auditory system uses timing and place coding. Plausible that timing mechanisms dominate up to 4 khz, and place mechanisms dominate thereafter. SLIDE 9

10 4. The pitch of complex sounds Place theories fall down badly for complex sounds. The classic demonstration of this is the missing fundamental. Consider a harmonic series with F0 f Hz; that is, the stimulus consists of a series of pure tones with frequencies nf Hz, where n is 1, 2, 3, 4 and so on. This sound has a pitch corresponding to the fundamental frequency (F0). Now, suppose the component at F0 is removed. Listeners still hear a pitch at f Hz. Amplitude Pitch heard at f Hz f 2f 3f 4f 5f 6f Frequency Amplitude Pitch still heard at f Hz f 2f 3f 4f 5f 6f Frequency Since there is no energy at the F0, this phenomenon is known as virtual pitch. Q. Why does this experiment present problems for theories of pitch based on place coding? SLIDE 10

11 Audio demo: the missing fundamental You ll hear a complex tone with a fundamental frequency of 200 Hz, consisting of 10 harmonics. First, the complex is presented complete, then without the fundamental, then without the lowest two harmonics, and so on. Q. Did the pitch of the complex change? Q. The bandwidth of telephone speech is approximately 300 Hz to 3 khz. Comments? SLIDE 11

12 4.1. Pattern recognition theories of pitch perception How can virtual pitch arise? Perhaps the auditory system uses the whole excitation pattern to compute the pitch; it might hypothesize a range of pitches, and find the one with the best fit to the harmonics in the excitation pattern. Such pattern recognition models of pitch are not dependent on place theories of coding (e.g., the pattern that is presented to the pitch mechanism may have been derived from spike intervals). What distinguishes pattern recognition models from other models is the assumption that the pattern contains resolved harmonics. SLIDE 12

13 4.2. Resolved and unresolved harmonics A resolved harmonic is represented as a separate peak of activity at its frequency. If two harmonics lie within the same critical bandwidth, they are not separately resolved in the output of the auditory filter array. Since critical bandwidths are narrow at low frequencies and wide at high frequencies, we see resolved harmonics at low frequencies and unresolved harmonics at high frequencies. The precise point at which harmonics become unresolved depends on the fundamental frequency of the stimulus. For example, if the fundamental is 200 Hz, harmonics will become unresolved at the frequency at which the critical bandwidth exceeds 200 Hz (at around 1500 Hz). Amplitude Amplitude Low frequency: narrow critical bands, resolved harmonics Frequency High frequency: wider critical bands, unresolved harmonics Frequency SLIDE 13

14 4.3. Timing theories of pitch perception Pure timing theories propose that pitch results from unresolved harmonics. The response in mid- and high-frequency regions of the auditory filter array is amplitude modulated. CF = 100 Hz (resolved) CF = 2 khz (unresolved) 250 Amplitude Amplitude Time [ms] Time [ms] The figure shows the response of two auditory filters to a harmonic complex with F0 of 100 Hz. The output of the filter with CF = 100 Hz is a single resolved harmonic, but when CF = 2 khz several harmonics interact in the same filter. The time between pulses in the 2 khz channel is 10 ms, which corresponds to the F0; so amplitude modulation could provide a cue to pitch. SLIDE 14

15 4.4. Beating COM325 COMPUTER SPEECH AND HEARING Amplitude modulation occurs because of beating between adjacent harmonics. Adding two tones that are close in frequency produces a waveform which has an AM rate equal to the difference in frequency between the tones Hz 20 Hz Audio demo: beats You will hear two pure tones with frequencies of 1000 Hz and 1004 Hz, first presented separately and then presented together. The sequence is presented twice. Q. What is the frequency of the beat in this example? SLIDE 15

16 4.5. Pattern recognition theories vs. timing theories Pattern recognition theories Signal Auditory periphery Timing theories Frequency analysis Resolved harmonics Timing analysis Unresolved harmonics Frequency Calculation of best fitting fundamental Time Calculation of most frequent interval Pitch estimate SLIDE 16

17 4.6. Challenges for theories of pitch perception Pitch of resolved harmonics only. It is possible to perceive a pitch based only on resolved harmonics, i.e. when there is no possibility of interaction between components. Pitch of unresolved harmonics only. It is possible to perceive a pitch based only on unresolved harmonics. Dominance. The 3rd, 4th and 5th harmonics tend to dominate the pitch percept. Mistuned harmonics. If a single component of a harmonic complex is mistuned so that its frequency is not an exact multiple of the F0, it can be heard as a separate tone. Q. Do the above findings support the pattern recognition theory or timing theory? SLIDE 17

18 5. A computational model of pitch perception Many models of pitch perception has been proposed, but we ll concentrate on one; the correlogram [2]. See [3] and [4] for other models. This model performs an autocorrelation on the output of each channel of an auditory model, defined as: N acg( τ) = xt ()xt ( τ) t = 1 where x(t) is the signal and N is the window length over which the autocorrelation is computed. The parameter τ is the autocorrelation delay (lag). You should recognise this as the convolution of x(t) with itself. The autocorrelation has a maximum at zero lag, and for a signal with period p it attains its next maximum at a lag of p. It also has periods at 2p, 3p, 4p and so on. Summing the autocorrelation functions of each channel gives rise to a pooled autocorrelation; the biggest peak in the pooled function occurs at the pitch period. SLIDE 18

19 5.1. Computing a correlogram Auditory Nerve Correlogram Channel Number Channel Number Time [ms] Autocorrelation Delay [ms] Harmonic complex with F0 = 100 Hz. Summary correlogram shows peak at 10 ms lag, indicating that this is the pitch period. Note the duplicate peaks at 20 ms and 30 ms. 3.5 x Summary Correlogram Autocorrelation Delay [ms] SLIDE 19

20 5.2. Why does the correlogram work? Channels that are responding to a particular frequency component show a peak in the autocorrelation function at the period of that frequency, and also at multiples. For example, consider the first four harmonics of a 200 Hz fundamental: Harmonic Frequency [Hz] Time lags at which an autocorrelation peak occurs [ms] , 10.0, 15.0, 20.0, , 5.0, 7.5, 10.0, , 3.33, 5.0, 6.66, , 2.5, 3.75, 5.0, 6.25 Each channel has a peak at 5 ms (period of the 200 Hz fundamental). Higher channels also have a peak at this period because they beat at a frequency corresponding to the difference between adjacent harmonics (also 200 Hz). Q. Is the correlogram related to the timing or pattern recognition theory of pitch perception (or both)? SLIDE 20

21 5.3. Explaining pitch phenomena The figures show that the correlogram can account for the missing fundamental (B,D), pitch of resolved harmonics only (A,C), pitch of unresolved harmonics only (B,D) and dominance (C,D). Both signals have F0 = 100 Hz. A Hz Hz Hz 2100 Hz Hz Hz B Channel Number Channel Number C x Autocorrelation Delay [ms] D x Autocorrelation Delay [ms] Autocorrelation Delay [ms] Autocorrelation Delay [ms] SLIDE 21

22 5.4. The pooled autocorrelation function and pitch strength The height of the peak in the pooled autocorrelation function can be interpreted as a measure of pitch strength. 10 x IRN 1 iterations Iterated ripple noise (IRN) is created by adding a time delayed random noise signal to itself [5]. A weak pitch is apparent for one iteration, becoming more salient as the number of iterations is increased. Noise in IRN out Autocorrelation Delay [ms] 8 x IRN 10 iterations z -n The correlogram shows the right pattern. Examples are for IRN with delay of 10 ms Autocorrelation Delay [ms] SLIDE 22

23 5.5. What is a good model of pitch perception? Good models of pitch perception should not only perform as well as humans, but they should make the same mistakes too. Audio demo: circularity in pitch judgment This demonstration uses a cycle of complex tones, each composed of 10 partials separated by octave intervals. The tones are windowed with a raised cosine: Moving the frequencies of the partials upwards in steps results in an ever ascending scale, which is an acoustic analogue of Escher s staircase visual illusion. Decibels Log frequency SLIDE 23

24 6. Summary COM325 COMPUTER SPEECH AND HEARING Pitch is largely determined by the repetition rate of a signal (frequency for tones, fundamental frequency for complex sounds). Theories of pitch are influenced by two possible mechanisms of frequency coding in the cochlea; timing and place. Simple place-based theories of pitch cannot apply to complex sounds (e.g., the missing fundamental). Timing theories rely on beating in frequency regions where individual harmonics are not resolved. Pattern recognition theories rely on resolved harmonics. Neither theory fits all of the data - it is likely that the auditory system uses both mechanisms. A computational model of pitch perception which combines periodicities in resolved and unresolved harmonic regions can account for the majority of psychophysical pitch phenomena. SLIDE 24

25 7. References COM325 COMPUTER SPEECH AND HEARING [1] B.C.J. Moore (1989) An introduction to the psychology of hearing, Academic Press. [2] M. Slaney & R. Lyon (1993) On the importance of time - a temporal representation of sound. In Visual Representations of Speech Signals, Ed. Cooke, Beet and Crawford, Wiley. [3] D. Hermes (1993) Pitch analysis. In Visual Representations of Speech Signals, Ed. Cooke, Beet and Crawford, Wiley. [4] W. Hess (1983) Pitch determination of speech signals, Springer. [5] W. A. Yost, R. A. Patterson & S. Sheft (1996) A time domain description for the pitch strength of iterated rippled noise. Journal of the Acoustical Society of America, 99, pp SLIDE 25

26 Tutorial questions COM325 COMPUTER SPEECH AND HEARING 1. Run the MAD demonstration called auto, which illustrates fundamental frequency analysis by applying autocorrelation directly to the signal waveform (i.e., no auditory filters are involved). Answer the tutorial questions associated with this demo. 2. Run the MAD detuning demonstration, which demonstrates the effect of mistuning a harmonic on the pitch of a complex tone (see slide 17). 3. Play with the MAD demonstration called vowelexplorer. This allows you to generate a mixture of two vowel sounds, and to see the corresponding basilar membrane response and correlogram. When you can clearly hear the pitch of each vowel, do you see two clear peaks in the pooled correlogram function? 4. Use the MATLAB function irn to generate iterated ripple noise (IRN). Write a program that uses a loop to generate IRN with the same delay but with the number of iterations varying between 0 and 20. Play each signal using soundsc - does the pitch become more salient as the number of iterations is increased? 5. Write a MATLAB function that measures the frequency difference limen (DLF). Your function should present the listener with a reference tone of fixed frequency, followed by another tone whose frequency is slightly above or below that of the reference. Your function should ask the listener to indicate whether the second tone was lower or higher than the first, and record the results for several trials. Use the tone function. SLIDE 26