Optimum linear filter for speech communication

Linear filtering at the transmitter can enhance speech intelligibility over a noisy channel when the transmitter is peak-power limited. Griffiths (1968) derived an optimum filter analytically using the articulation index concept and measured an improvement equivalent to 2.5 to 4 dB increase in transmitter power. Thomas and Ohley (1972) derived a different filter empirically and measured a much larger improvement. A direct comparison of these two filters and no filter was made on a peak-limited link. The Griffiths filter proved superior to the Thomas filter by about 3 or 4 dB under marginal conditions; both filters were appreciably better than no filter.