櫛歯形フィルタを用いた拡声同時通話系のハウリング防止法

抄録

In loudspeaking telephony, one method to prevent howlback is the introduction of voice switching circuits in amplifiers. This method, however, leads to deterioration of simultaneous speech quality. The howlback suppressor presented here, consisting of a pair of complementary comb filters, may be utilized to prevent howlback in loudspeaking telephony without sacrifice of simultaneous speech quality. The comb filter can be derived by digital filter synthesis techniques (Fig. 1). In the experimental system, analogue shift registers, commonly known as the "bucket brigade devices, " are used instead of digital shift registers. The allowable excess loop gain depends on the type of comb response chosen (Table 1). For example, filters with fourth-power cosine and sine amplitude characteristics give 13dB excess loop gain. A modified fourth-power response can give as much as 19dB excess loop gain. This method can also be used for loudspeaking telephony for multi-places. The arrangement of comb filters for three places is shown in Fig. 7. From experiments, it may be said that the speech as heard at the receiving terminal is satisfactory for loudspeaking telephony.