Duda Vowels

Description

The following correlograms are shown here:

1. /a/ vowel (140 Hz fundamental, 5% 6 Hz vibrato)
2. /i/ vowel (132 Hz fundamental, 5% 6 Hz vibrato)
3. /u/ vowel (125 Hz fundamental, 5% 6 Hz vibrato)
4. All three vowels with no variation (world's most boring cocktail party)
5. 5% FM-Three vowels with sequential 5% pitch vibrato
6. 100% AM-Three vowels with sequential 100% amplitude modulation

Vowel sounds can be thought of as periodic signals with very special spectral patterns. Their spectral envelopes are dominated by the formant resonances, which are basically independent of the fundamental frequency. Through learning spoken language, people become particularly sensitive to the spectral envelopes for the vowels in that language.

In his doctoral thesis, Stephen McAdams showed that when different vowels having the same fundamental frequency were mixed, the resulting mixture did not have a vowel-like quality; however, when the glottal pulse train for any one vowel was frequency modulated, the sound of that vowel would "emerge" as a separate, recognizable sound stream [McAdams84]. The effect was very strong, again suggesting that the auditory system makes central use of the comodulation of harmonic components to separate sound sources.

Several experiments were performed to see how both amplitude and frequency modulation of the glottal pulse train affected the auditory perception and the correlograms for the vowel mixtures. All experiments used the same three synthetic vowels: /a/ (as in "hot"), /i/ (as in "beet") and /u/ (as in "boot"). These vowels were synthesized using a cascade model due to [Rabiner68]. Specifically, a pulse train with fundamental frequency f0 was passed through a cascade of two single-pole glottal-pulse filters, three two-pole formant filters, and a first-difference stage to simulate the effects of radiation. The glottal-pulse filter has poles at 250 Hz, and the formant resonances had a 50-Hz bandwidth. The following table lists the fundamental and formant frequencies:

	F0	F1	F2	F3
/a/	140	730	1090	2440
/i/	132	270	2290	3010
u/	125	300	870	2240

The correlograms of these individual vowel sounds and their mixture are shown on the video tape. One notes at once the distinctly different visual patterns of these three vowels. The horizontal organization (rows of energy blobs) reveal the formants, the high frequency formants for the /i/ being particularly distinctive. The vertical organization (columns of energy blobs) reveal the distinctly different pitch periods, which are about a semi-tone apart. Note, however, that there is no clear pitch period in the mixture, whose low-frequency organization is murky. The mixture sounds comparably murky; like a dissonant blend of unidentifiable tones.

A variety of different synthesized vowel mixtures were produced by modulating the glottal pulse trains in different ways. The standard method was to create 4-second pulse trains for the three vowels as follows:

Vowel	0.0	0.5	1.0	1.5	2.0	2.5	3.0 3.5	4.0 (sec)
/a/			MMM
/b/				MMM
/c/					MMM

For example, the pulse train for /a/ was held steady for 1 second, modulated (M) for the next 0.5 second, and then held steady again for the remaining 2.5 seconds. Signals with the following kinds of modulation were generated in this fashion (not all are shown on the tape):

1. Sinusoidal 6-Hz frequency modulation: 0.2%, 0.5%, 1%, 2% and 5% percentage modulation.
2. Sinusoidal 6-Hz amplitude modulation: 5%, 25%, and 100%.
3. Step amplitude change: -6dB, -3dB, +3dB, +6dB. In these experiments, the amplitude was changed for 0.5 seconds and then restored to its initial value.
4. Step frequency shift: .1%, .2%, .5%, 2%, 5%. In these experiments, once the frequency was shifted, it was held steady, rather than returning to its original value.

The perceptual character of these signals can be summarized as follows:

1. The steady vowel mixture sounds like an uninteresting, dissonant, buzzy chord with no vowel qualities. With 5% frequency modulation (vibrato), the vowels clearly and effortlessly emerge from this mixture. At 1% modulation the effect is still clear, but 0.5% is marginal. When one knows what to listen for, changes can be heard (or at least imagined) with .2% and even .1% modulation.
2. Frequency shift is even more effective than sinusoidal modulation. The vowels "come in" much like the harmonics do when a periodic wave is built sequentially. Furthermore, the ear tends to hold on to the higher-pitched sounds, so that the /i/, with its prominent high-frequency formants, clearly persists as a separate sound stream even after all signals are again steady.
3. When the frequency shift is 5% or 1%, one has a clear sense of both the direction and amount of pitch change. With the .5%, .2% and .1% shifts, one is aware that something has changed, but the pitch seems the same.
4. Sinusoidal amplitude modulation (tremolo) is not particularly effective in separating sounds. Although 5% and 25% modulation patterns were certainly noticeable for the vowels in isolation, they produced inaudible to marginal changes in the mixture. At 100% modulation the vowels "emerge," but not as well as they do with 5% frequency modulation.
5. A 6-dB step change of amplitude (100% up, 50% down) is clearly audible, including the offset when the last vowel returns to its original level. A 6-dB increase causes the vowel to stand out. However, a 6-dB decrease creates an unsettling awareness of change, but with no well-defined vowel sound. In fact, one frequently hears the vowel only when its amplitude is restored to its original value. The effect is still obtained with a 3-dB change (40% up, 30% down), but it is beginning to be marginal.

All of the changes that could be easily heard could also be easily seen in videotapes of the correlograms. Even the small 1% frequency shifts produced clearly visible motions. While the formants can also be seen in the correlograms, the untrained eye is not naturally sensitive to these spectral patterns. That is, recognizing the vowel from viewing the changes is not obvious, and would require an ability to read correlograms similar to the ability of trained spectrogram readers to identify vowels in spectrograms [Zue85]. However, it seems likely that a vowel-recognition procedure that worked on the correlograms of isolated vowels would also work on the fragments of correlograms separated by comodulation.

However, these experiments also revealed a phenomenon that might limit a recognition strategy based on unsophisticated motion detection procedures. Low-frequency beats between the three fundamental frequencies produced moving patterns in the correlogram even in the absence of any modulation of the pulse train. The difference between motion due to beats and motion due to comodulation or other temporal changes in the acoustic input needs to be better understood before grouping models based on comodulation can be developed.

Transcript

1. /a/ vowel (140 Hz fundamental, 5% 6 Hz vibrato)
2. /i/ vowel (132 Hz fundamental, 5% 6 Hz vibrato)
3. /u/ vowel (125 Hz fundamental, 5% 6 Hz vibrato)
4. All three vowels with no variation (world's most boring cocktail party)

Transcript

1. 5% FM-Three vowels with sequential 5% pitch vibrato

Transcript

1. 100% AM-Three vowels with sequential 100% amplitude modulation