Ernst Rothauser

Preference evaluation of a large set of vocoded speech signals

Starting from an IEEE Recommended Practice for Speech Quality Measurements and from previous work of the authors, a large set of vocoded speech signals has been evaluated in terms of preference. The set of speech samples has been taken from the vocoder survey of the 1967 Conference on Speech Communication and Processing, Boston, Mass. The test samples are evaluated by several methods: direct comparisons, the isopreferenee method, the relative preference method, the category judgment method, and the absolute preference judgment method. Due to the size of the test material, not all the test samples could be evaluated by all these methods. The test results are discussed and it is shown that, in certain respects, reliable system evaluations pose formidable problems. An effort to rank order the systems, which are described by small sets of test samples of frequently very different quality, for good reasons shows only limited success. The majority of the systems are of about equal preference with only insignificant differences. There are only a few systems that are outside this group and are either significantly better or worse than the rest.

Loudness and annoyance of impulsive office noise

A study has been made on the evaluation of natural impulsive noises by subjective and objective procedures. One special example was used to get an indication of whether existing objective calculation methods might give sufficient agreement with subjective tests. The office noise sample studied bursts from a single typewriter operated at repetition rates between 1.5 and 15 strokes per second. A forced-pair comparison procedure of the constant stimulus type was used for the subjective evaluation. This choice has been prompted by the good results which we obtained with a corresponding method for speech quality measurements. In the subjective tests reported here, we asked for loudness and for annoyance comparisons. Two types of broad-band noise were utilized as reference signals. In addition to this, narrow-band noise centered at 1000 Hz was used to calibrate the two broad-band noises subjectively, and was also compared directly to the test noise. The subjective measurement results of this study are compared with those obtained by several well-known objective methods for noise rating and loudness evaluations.

New Reference Signal for Speech‐Quality Measurements

In a previous paper in 1963, a method was described for the measurement of speech quality. It was proposed to utilize a reference signal consisting of high‐quality speech degraded by additive white noise. Extensive studies have shown disadvantages of this form of degradation. The reference signal differs considerably in its nature from the output speech signals generated by most speech‐processing devices, and in spite of its apparently simple definition it cannot be reproduced very easily, because of the difficulties in measuring the sound level of speech signals. A new reference signal is proposed. Instead of adding a controlled amount of noise to a high‐quality speech signal, the noise is multiplied into the signal. One way to generate the reference signal of the form s(t)[1+k⋅r(t)] is to modify this form to s(t)+k⋅r(t)⋅s(t) and to generate the product with a Hall multiplier. Test results are given and discussed. [Research reported in this paper sponsored by the U. S. Government.]