Perceptual Speech Quality Research Articles

Interactions between speech coders and disordered speech

We examined the impact of standard speech coders currently used in modern communication systems, on the quality of speech from persons with common speech and voice disorders. Four standardized coders, viz. G. 728 LD-CELP, GSM 6.10 RPE-LTP, FS1016 CELP, FS1015 LPC and the recently proposed US Federal Standard 2400 bps MELP were evaluated with speech samples collected from 30 disordered talkers. Objective speech quality measures, including the auditory distance parameter based on the measuring normalizing blocks technique, and the perceptual speech quality measure, and subjective impressions of speech coder performance were used to assess the interaction between speech coder and speech disorder. Objective speech quality measures revealed that the performance of the LD-CELP and GSM RPE-LTP coders was not measurably influenced by the type of input speech, and that MELP, FS1015 LPC and to a certain extent FS1016 CELP exhibited degraded performance with speech samples from disordered talkers. Results from perceptual experiments were in contrast with the objective measures of speech quality; ratings of speech coder performance indicated that the listeners are less sensitive to coder-induced distortions with abnormal speech samples.

Interactions between speech coders and disordered speech

We examined the impact of standard speech coders currently used in modern communication systems, on the quality of speech from persons with common speech and voice disorders. Four standardized coders, viz. G. 728 LD-CELP, GSM 6.10 RPE-LTP, FS1016 CELP, FS1015 LPC and the recently proposed US Federal Standard 2400 bps MELP were evaluated with speech samples collected from 30 disordered talkers. Objective speech quality measures, including the auditory distance parameter based on the measuring normalizing blocks technique, and the perceptual speech quality measure, and subjective impressions of speech coder performance were used to assess the interaction between speech coder and speech disorder. Objective speech quality measures revealed that the performance of the LD-CELP and GSM RPE-LTP coders was not measurably influenced by the type of input speech, and that MELP, FS1015 LPC and to a certain extent FS1016 CELP exhibited degraded performance with speech samples from disordered talkers. Results from perceptual experiments were in contrast with the objective measures of speech quality; ratings of speech coder performance indicated that the listeners are less sensitive to coder-induced distortions with abnormal speech samples.

Comb filters for binaural dichotic presentation to improve speech perception by persons with bilateral sensorineural hearing impairment

Intelligibility of speech for persons with sensorineural hearing impairment gets reduced due to increased spectral masking. For persons with bilateral loss, improved speech perception has been reported with binaural dichotic presentation using comb filters having complementary magnitude responses, based on auditory critical bands. In this scheme, spectral components in adjacent critical bands do not mask each other as they get presented to different ears. The objective here was to design a set of comb filters with transition crossovers adjusted for a low perceived spectral distortion. For this, a slowly swept tone presented through the two comb filters should not have a change in its perceived loudness at crossover frequencies. Experiments showed that the change in loudness was negligible with crossover response lying between −4 dB and −6 dB with respect to passband response. With this constraint for response at the crossover frequencies, comb filters with responses corresponding to 18 critical bands were designed as 256-coefficient linear phase FIR filters, using frequency sampling technique. The filters have pass band ripple of 1 dB and stop band attenuation of 30 dB. These filters have resulted in improvement in speech quality and speech perception.

Low rate WI SEW representation using a REW-implicit pulse model

Reducing the bit rate of waveform interpolation speech coders while maintaining the perceptual quality has been the focus of a great deal of research. This letter proposes a new method of slowly evolving waveform (SEW) quantization specifically targeted at low rate coding. The proposed method uses a pulse model whose parameters are implicitly contained in the quantized rapidly evolving waveform (REW) parameters, thus requiring no bits for transmission. Results indicate no degradation in perceptual speech quality when compared to that of the existing SEW quantization method. This retention of perceptual quality is in spite of a 12% reduction in the overall coder bit rate.

Pre-processing method for pitch smoothing

A new pre-processing method for pitch smoothing is proposed. This methodology modifies the residual speech signal such that the pitch period evolves more smoothly without distorting perceptual speech quality. Thus, pitch can be effectively interpolated and transmitted only once per frame while presenting the long-term correlation of speech signal and maintaining the perceptual speech quality.

Psychoacoustically motivated objective speech quality evaluation procedures, PSQM, and improvements

PSQM (Perceptual Speech Quality Measure), measuring speech quality objectively, has been standardized by ITU-T as recommendation P.861. PSQM characterizes the perception of the (degraded) output speech signal of the system in comparison to the (ideal) input speech. A perceptual model is used that maps input and output signals onto psychophysical representations using psychophysical equivalents of frequency (Bark) and intensity (compressed Sone). The quality of the device under test is determined with a simple cognitive mapping from the differences in the psychophysical representation to the perceived speech quality in terms of Mean Opinion Scores (MOS). Within an ITU benchmark testing a limited set of unknown codec distortions the PSQM showed high correlations (around 0.97) between subjectively perceived and objectively measured speech quality. When applying PSQM to a wide variety of real world distortions two major limitations show up: First, dynamic time warping effects, as they will be found, e.g., in Internet telephony, cause a degradation in correlation. Second, the perceptual model that is used within the PSQM method is too simple to account for a wide variety of distortions. This presentation discusses extensions to the PSQM method that allows application to real world distortions.

Perceptual Rankings of Speech Quality Produced With One-Way Tracheostomy Speaking Valves

Perceptual speech quality rankings, mechanical functioning, and maintenance of respiration as measured by oxygen saturation were determined for four different one-way tracheostomy speaking valves. Results indicated significant differences in speech quality rankings, with the Montgomery and Passy-Muir valves ranked significantly better than the Kistner and Olympic valves, and the Olympic valve ranked significantly better than the Kistner valve. The Passy-Muir valve was identified with the best speech quality most often by both listeners and subjects, and exhibited the fewest clinically relevant mechanical problems. Maintenance of respiration was not affected by use of any of the valves studied.

The Effects of Digital Quantization Error on Speech Intelligibility and Perceived Speech Quality

The effects of digital quantization error upon speech intelligibility and perceived speech quality, for normally hearing subjects, were investigated for digitized speech processed to simulate 6-, 8-, 10-, 12-, 14-, and 16-bit integer conversion and 2-, 3-, 4-, 5-, 6-, and 7-bit floating-point conversion. For the integer data, there were no significant differences in speech intelligibility for 8- to 16-bit conversion. Only 6-bit integer conversion at 55 dB SPL resulted in a significant degradation in speech intelligibility. For the floating-point data, there were no significant differences in speech intelligibility for 2- to 7-bit floating-point conversion. However, results of the perceived quality experiment appeared to be more sensitive to differences among the various conditions. Speech processed using 12-, 14-, and 16-bit integer conversion was judged to be superior to speech processed using the 6-, 8-, and 10-bit integer conditions. Speech processed using 5-, 6-, and 7-bit floating-point conversion was judged to be superior to speech processed using 2-, 3-, and 4-bit floating-point conversion.