Vocoded speech through fading channels

HF speech communications is possible beyond the line of sight because signals in the 5-30 MHz range are refracted by the ionosphere. The fading these signals suffer and the channel impulse burst noise can be mitigated by a variety of channel equalizer and error correcting coding schemes. If the fade or burst noise is severe enough or long enough, however, speech communication becomes difficult or impossible. This paper describes how vocoders can be designed to overcome these difficulties in a way that is more or less transparent to the users, i.e., the users can talk through fades and not have to repeat themselves. Two methods are discussed; first, variable rate vocoding techniques and second, "speeded-up" speech. The speeded-up speech works as follows. During a deep fade, the transmitter retains the necessary speech parameter information. When the channel returns, this information is transmitted and transformed into speech at the receiver wherein the speaker is perceived to be speaking more rapidly, although with the correct pitch and spectrum. This allows the listener to "catch-up" with the speaker. A demonstration will be given to show how the vocoder performs under several simulated fading situations.