Interference Test Evaluations of Telephone Speech, Quality

The interference test for speech quality evaluation was investigated for sensitivity and validity with various types of telephone speech. Three components of the telephone system (microphone, transmission channel, and earphone), each with two alternatives (high fidelity or telephone), were used in all possible combinations to obtain eight types of telephone speech. The interference test was sensitive enough to measure the amount by which each telephone component degraded speech, even though all the speech types were highly intelligible according to articulation tests. Consistent and systematic relationships were observed between the interference test results, articulation test scores, and subjective evaluations of the same speech types, indicating that the interference test is a valid procedure for measuring speech quality. Moreover, by simply adding up the interference test measures associated with the telephone components, the measure for a speech type with any combination of the components could be accurately predicted. The results confirm that the interference test yields sensitive, valid, and additive measures of speech quality.