在长距离电话叫声中,回声是一种妨害,它是噪声,在低音量时更令人讨厌。因为回声的主观烦恼随延迟和电平增加而增加,所以无回声预防的附属电路(图1a)将令人不满意。在每个4线通路中的270毫秒延迟表示附属电路的延迟。在这组合中,不是所有的谈话能量将从4线通路引向2线通路。某些能量像被延迟540毫秒的讨厌的回声那样沿其它的4线通路返回到谈话者那里。 关于自身适应的回声消声的工作早在1965年就开始了。认真地为工业提议的消声器早在2年之前就讨论了,但是全部的是打算,或价格太高,以至妨害推广应用。 Echoes are a nuisance in long-distance telephone calls, noise, and more annoying at low bass. Since the subjective annoyance of the echo increases with delay and level increase, ancillary circuits without echo prevention (Figure 1a) will be unsatisfactory. The 270 millisecond delay in each 4-wire lane represents the latency of the accessory circuit. In this combination, not all talk energy will be diverted from the 4-wire path to the 2-wire path. Some of the energy is returned to the talker along the other 4-wire paths as the annoying echo delayed 540 milliseconds. The work on echo adaptation to self-adaptation began as early as 1965. Mufflers, which were earnestly proposed for industry, were discussed as early as two years ago, but all were intended or the price was too high to hinder the promotion and application.