A novel technique for coping with fading and burst noise on HF channels used for digital voice communication has been developed and evaluated. The technique transmits digital voice only during the high signal-to-noise ratio time intervals, i.e., channel "on" times, and speeds up the speech when necessary in order to avoid delays which would hinder conversation. The technique was evaluated using a spoken version of a reading comprehension test. The test involved 15 spoken, 2-min paragraphs processed by a real-time channel vocoder simulation which had been modified to also simulate the on/ off characteristics of a fading HF channel. Based on informal listening tests, four test conditions were selected in order to characterize those conditions of interrupting the speech that would render it comprehensible or incomprehensible. One of the test conditions, 2 s on \frac{1}{2} s off on the average, produced percentage correct test scores comparable to the reference continuous speech case. Since this condition represents a reasonable match to the temporal variations of a disturbed ionosphere, and also represents a more difficult condition than the slower variations of an undisturbed ionosphere, it is concluded that the sped-up speech technique is appropriate for HF digital voice communication systems.
['John T. Lynch']
Evaluation of the Comprehension of Noncontinous Sped-Up Vocoded Speech: A Strategy for Coping with Fading HF Channels