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As mentioned earlier in this document, the hybrid is the primary cause of the returned signals that cause echo. But other factors, such as delay, also contribute to echo. The following subsections discuss the types of echo:
- Talker Echo - Listener Echo (CCITT calls this, "Received End Echo")
Talker echo occurs when a talker's speech energy, transmitted down the primary signal path, is coupled into the receive path from the far end. The talker then hears his/her own voice, delayed by the total echo path delay time. If the 'echoed' signal has sufficient amplitude and delay, the result can be annoying to the customer, and interfere with the normal speech process. Talker echo is a direct result of the 2-wire to 4-wire conversion that takes place through 'hybrid' transformers. Please refer to the following diagram:
Listener echo occurs at the far-end by circulating voice energy. This type of echo is also called 'Singing' or 'Near-Singing'. Singing occurs when gain mismatches allow the energy returned from the listener echo path to be greater than the original energy, and when the waveforms are in-phase. This creates an 'oscillation' effect. Near-singing occurs at a point just prior to singing. Near-singing conditions usually create 'hollow' or 'rain barrel' effects. Again, listener echo is generally caused by the 2W/4W 'hybrid' transformers. Please refer to the following diagram:
