Audio Line Echo on 2-wire and 4-wire circuits

Voice Echo and the AVA Audio/Voice Adapter
Generic Echo, Phone Lines, and 2-wire to 4-wire Conversion

Phone line echo (voice echo) is inherent in some telephone systems. It's a natural byproduct of the technologies used, and may be characterized and minimized with proper attention to the system configuration.

Near-end or Far-end?

Echo may be characterised as near-end echo or far-end echo. If it's producted at the near-end of the circuit, the echo time is almost non-existant, or instant. If it's a product of far-end characteristics, there is a significant delay between the spoken work heard in the side-tone and the echo product.

In both near-end or far-end echo situations, the frequent culprits are either impedance mis-match or line levels set too high. While it's a balancing act, line levels may be reduced to control echo yet still be high enough for good voice fidelity. If the levels are too low, then you can't hear the conversation, if they're too high, too much echo is probably heard.

What are some normal levels?

Level settings are installation dependent. While the AVA factory default gains are all set for +0 dB, you may need to change either the Input gain or Output gain to achieve proper output volume with the attached equipment and networks. We always recommend the default setting as a starting point, and changing it as needed by on-site testing.

How to treat echo using AVA configuration

Voice input to the AVA e is amplified by the sum of the preamp gain and the input gain. In general, the preamp gain serves as a coarse setting, while the input gain value supports finer adjustments.

Voice output from the AVA is amplified by the output gain. The closer to +0dB one can achieve, the better the circuit usually performs. For example, try not to set the input gain to +3dB and the output gain to -3dB; a zero setting at both AVA units is usually better.

What about the jitter buffer?

Some technicians falsely believe that jitter delay setting affects echo. It doesn't. More jitter delay means more buffered voice packet capability and increased end-to-end voice delay. Less jitter delay raises the possibility that the buffer with empty due to network delays Audio glitches and pauses may be heard when the jitter buffer is too small. Jitter buffers allow for the variability in link propagation delay, especially noticed when there is an ethernet segment in the network. The delay imposed by larger jitter buffer values may increast the time delay noticed with far end echo problems.

Summary

Anytime there is a line impedance mismatch, 2-wire to 4-wire conversion, or hybrid in the circuit, there is the posibility of unwanted echo. Proper selection of line levels can minimize the problem.



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