Effect of Transmission Errors on the Intelligibility of Vocoded Speech

Much time and study is being devoted to the improvement of Vocoder speech quality and intelligibility. One factor which is being neglected, however, is the effect of transmission errors on the intelligibility of Vocoded speech. To evaluate this effect, a full duplex 16-Channel Philco Vocoder is employed. Errors are inserted into the PCM output of the transmitter at various rates and in a random fashion. Intelligibility tests are run to determine the loss of intelligibility with increasing error rates. Curves are plotted showing these effects for a number of speakers. Fixed position errors are also inserted within the pitch portion of the bit frame and similar intelligibility tests are run. Results for the random insertion of errors indicate that an error rate of one to two per cent (1 to 2 bits per 100) is tolerable before any appreciable loss in the zero error PB word intelligibility of 82 per cent is apparent. At a transmission error rate approaching five per cent (5 incorrect bits per 100), the Channel Vocoder is capable of a PB word intelligibility of 70 per cent, corresponding to a sentence intelligibility of 90 per cent. Comments on the loss of speech quality with increasing errors are also given. Error insertion in the pitch portion of the frame indicates a surprising ability of the Channel Vocoder to tolerate error while maintaining intelligibility.