Listener Ratings Research Articles

Operant Preference of Retarded and Normal Males for Rate of Narration

This study investigated the listening rate preferences of retarded and normal boys. A story was recorded at the 180 wpm rate and modified by a speech compressor-expander mechanism to obtain additional rates of 90, 120, 240, and 360 wpm. These rates were paired throughout several sessions and simultaneously offered to each S. By programing these stimuli by conjugate reinforcement, the investigator was able to obtain continuous data relevant to each S’s preference for a particular speed of speech. The results disclosed that of the normal boys, 5 preferred the 180 (normal) wpm rate, 3 the 240 wpm rate, and 1 the 120 wpm rate. Of the retarded boys, 1 chose the 360 wpm rate, 3 the 240 wpm rate, 2 the 120 wpm rate, and 3 the 90 wpm rate, all rejecting the normal speech rate. Data from one retarded and one normal boy were inconclusive in so far as establishing a specific preference.

Preferences of Phonetic Stimuli

Eight types of recorded phonetic stimuli were presented to Finnish-speaking and Swedish-speaking students of Finnish universities. Four of these stimulus types consisted of wholly artificial, partly controlled and partly randomized sequences of Finland-Swedish speech sounds; two were more similar to Finnish and two to Swedish, having been obtained by scrambling either the vowels or the consonants of samples of Finnish and Swedish texts, respectively. It was found that (1) the ratings of Finnish and Finland-Swedish listeners were relatively consistent when the stimuli differed radically from Finnish and Finland-Swedish, but tended to vary more when stimuli were more similar to these two natural languages; and that (2) the consonant/vowel ratio and syllabic structure had a greater influence on aesthetic preference than the front vowel/back vowel ratio.

Voice-excited vocoders for practical speech bandwidth reduction

In spite of their great potential for bandwidth saving in long distance telephony, vocoders have not found wide-spread acceptance. Two major problems have retarded their application. First is their strong electrical accent. Second is the so-called "pitch problem;" namely, deducing the nature of the talker's vocal excitation from his speech waveform. The reliability of this deduction and measurement depends critically upon high input speech-to-noise ratio, particularly between 50 and 200 cps. In many communication situations, this requirement precludes satisfactory operation. This limitation can be removed by a new method known as "voice excitations' which eliminates the necessity for a decision-making pitch detector. The principal advantage of voice excitation is its insensitivity to input signal-to-noise ratio and equalization. A voice-excited vocoder (VEV) with a 720 cps (250-970 eps) baseband and 17 spectrum channels low-passed to 25 cps each, covering the band 970-3700 cps, has been built and evaluated. The test shows an average PB-word intelligibility of 86%, compared to 92% for input speech of the same bandwidth, both with an 18 db signal-to-noise ratio. Quality tests indicate that listeners rate VEV speech "as good as" the input in about 90% of the test utterances. 0nly 19% of conventional vocoder utterances were so considered. The vocoder performed about equally well for each of the 12 speakers in the quality test. Voice-discrimination tests indicate that voice identity is well preserved. Crucial factors influencing the remade speech quality are the accuracy of spectral flattening and the impulse response of the analyzer low-pass filters. These results indicate that the principle of voice excitation provides the key to practical speech bandwidth reduction.

New Applications of Voice-Excitation to Vocoders

The principle of voice-excitation for a vocoder synthesizer involves generation of a flat spectrum of pitch harmonics or noise from an uncoded subband (“baseband”) of the original speech [M. R. Schroeder and E. E. David, Jr., “A vocoder for transmitting 10 kc/sec speech over a 3.5 kc/sec channel,” Acustica 10, No. 1, 35–43 (1960)]. The necessity for a decision-making pitch detector is thereby removed. The principal advantage of the voice-excited vocoder is its insensitivity to input signal-to-noise ratio and equalization. A voice-excited vocoder with a 720-cps (250–970 cps) baseband and 17 spectrum channels low-passed to 25 cps each, covering the band 970–3700 cps, has been built and evaluated. The test shows an average PB-word intelligibility of 86%, compared to 92% for input speech of the same bandwidth, both with and 18-db signal-to-noise ratio. Quality tests indicate that about 85% of all listeners rate the speech “as good as” the input while only 20% so consider a vocoder using conventional pitch control. The vocoder performed about equally well for each of the 12 speakers in the quality test. Voice-discrimination tests indicate that voice identity is well preserved. Crucial factors influencing the remade speech quality are the accuracy of spectral flattening and the impulse response of the analyzer low-pass filters. Studies of voice-excitation indicate that the baseband, if used for excitation only, can be coded into about 2000 binary pulses per second. Accordingly, the vocoder was modified with the addition of four spectrum channels, and operated with a digitally coded baseband. In total, it is estimated that about 4000 binary pulses per second are required for all-digital transmission. A tape demonstrating these results will be played.

Articulation test measures and listener ratings of articulation defectiveness.

Articulation test measures and listener ratings of articulation defectiveness.

Listener ratings of stereophonic systems

Subjective listening tests have been completed which indicate that stereo in any of the forms compared is preferred over monaural, but normal stereo from two full range speakers well physically separated is, in all probability, the most preferred.