Time-frequency scanning in narrow-band speech transmission

Two basic sampling methods for the transmission of intelligible speech with reduced channel capacity have previously been studied. The channel vocoder developed by Dudley is based on frequency band separation or quantization, and Schisser, and Fairbanks, Everitt, and Jaeger have developed a technique of time sampling. Sampling in both time and frequency offers a third major possibility which should be investigated. For this study a narrow-band speech transmission system was constructed which scans the time-frequency plane. The system scans in either a sinusoidal or in a sawtooth manner over a frequency range of 200 to 7000 cps. In order to achieve the scanning, a variable carrier frequency oscillator, having a frequency shift of ± 30 per cent of the carrier frequency and an amplitude modulation of ± 2 db, has been developed. The process of scanning leaves empty spaces in the time-frequency plane, and a four-channel time-delay system which employs the method of dielectric recording has been used to fill the gaps in the time-frequency plane by repeating the signal samples. When signal reiteration is employed with this system, a score of 75 per cent of monosyllabic phonetically balanced word lists was obtained with a scanning filter of 1000-cps bandwidth and a sinusoidal scanning rate of 30 times per second. This intelligibility is appreciably higher than that which can be achieved with a fixed 1000-cycle filter located in the frequency region of maximum intelligibility.