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1.6 SPECIAL TRANSMISSION EQUIPMENT 29Figure 1.6-1. Long-distance connection.The effect of echoes on a talker varies with the echo delay (the signal propagationtime from talker S 1 to H 2 and back to S 1 ). Echoes delayed by less than 20 ms arebarely noticeable, but echoes with larger delays rapidly become very disturbing toa talker. Echo delays increase with increasing length of the four-wire circuit, andtrunks longer than 2000 miles (transcontinental, transoceanic, and satellite trunks)are equipped with echo control devices.1.6.2 Echo SuppressorsLong four-wire analog trunks are equipped with echo suppressors [1,14].Figure 1.6-2 shows echo suppressor units ES-A and ES-B, located at both ends ofa trunk between exchanges A and B (the other exchanges in the connection arenot shown).Figure 1.6-2. Echo suppressors on a long-distance analog trunk. D, Voiceband signal detector;X, switch; H, hybrid circuit; ES, echo suppressor.