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Echo SuppressorsIt is not practicable to remove all impedance irregularities from telephone circuits, and those remaining may under certain conditions cause wave reflections producing serious "echo" effects.20,21 To be troublesome, the reflected wave must return after a certain perceptible time interval, and hence two factors are involved. Troublesome echoes are most likely to occur in long circuits where the time required to travel is appreciable and in loaded cables where the velocity of propagation is low (page 246). In fact, the low velocity obtained with heavy loading limits its use. An early echo suppressor,1 consisted of two high-impedance vacuum-tube amplifier-detectors bridged across the line. Each amplifier-detector was connected to a relay, and, when an alternating speech voltage of sufficient strength was impressed across the detector, the relay was operated, placing a short circuit on the opposite line. This short circuit prevented reflected echo waves from traveling back on the other side of the four-wire circuit to the speaker.22
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Last Update: 2011-05-30