A new design of a digital conference system
Hans Wilhelm Gierlich · The Journal of the Acoustical Society of America · 1988
Two main drawbacks make telephoning using a conference system troublesome: First, noise and reverberation produced at the site of the speaker are transmitted to the listener. Second, to achieve a sufficient speech volume at the listener'site, the necessary amplification for loudspeaker playback leads to instability (feedback) of the whole system. To suppress reverberation and noise, a speech controlled microphone arrangement is used. The implemented algorithm for speech detection is able to distinguish between noise and speech up to noise levels of 70 dB SPL. The noise signal may either be noiselike, impulselike, or sinusoidal. The detection algorithm needs a maximum time of 32 ms to detect a speech signal. To avoid instability, a combination of a special filter arrangement and a speech controlled amplification of microphone and loudspeaker signal is used. Two filters, having a transfer characteristic similar to a comb filter, are inserted into the transmitting and the receiving path. The transfer functions of the filters are inverse. With regard to the tone color, the filters were optimized, taking into account the special effects of signal processing used by the human ear. Hearing tests showed that speech intelligibility is not influenced by the filter arrangement.