Abstract
An all‐in‐one type hands‐free telephone set was constructed by using a single chip digital signal processor integrated circuit (DSP IC). It includes a compact acoustic echo canceler, a sidetone echo canceler, and voice‐swilched variable attenuators. An echo canceler whose length is only 16 ms (128 taps for an 8‐kHz sampling) can suppress the power of an impulse response of the loudspeaker‐microphone acoustic coupling path by more than 15 dB, because the major part of the path for an all‐in‐one telephone set is due to short direct paths. Most of the sidetone can also be suppressed by more than 20 dB by using an echo canceler of 8‐ms length. A speech circuit including these echo cancelers and conventional voice‐switched variable attenuators was composed of only one medium‐performance DSP IC (75‐ns machine cycle, 2K words of ROM and 512 words of RAM, and an additional 1000 ASIC gates}, CODEC IC's, and a few analog IC's. Complete simultaneous conversation is feasible with this hands‐free telephone circuit because the voice‐switched attenuation could be reduced to 10 dB or less.
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