1.5 Administrator's Guide

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  4. Commissioning the unit
  5. Configuring Comma device ports
  6. Echo cancellation

Echo cancellation

When two phones, B and C are in a call, and the person’s speech at phone C is reproduced at phone B, some of the audio exiting B’s earpiece re-enters B’s microphone and is returned as echo. Analog lines exacerbate this problem due to additional reflection of audio in the electronic circuitry. The result of audio feedback at B is that person C hears his/her own voice after a short delay i.e. echo occurs.

The person hearing the echo should look to the remote party for the cause of the echo and not to their own phone and system. If both parties experience echo, both systems should be investigated.

In analog phones, both the 2-wire to 4-wire converter (hybrid) and the handset can introduce echo. The Com.X impedance parameters are aligned with South African telecommunications standards. Non-compliant handsets with a large impedance mismatch are likely to introduce noticeable echo. The delay introduced by VoIP based systems can result in the returned echo becoming noticeable to the person who is speaking.

The degree of echo and the delay between the original audio and the echo is a function of the handset, gain settings, potential wiring problems and the audio transmission path.

The Com.X products incorporate a sophisticated Echo Canceller for each telephony channel (BRI, PRI or analog). The algorithm adapts to the detected audio feedback within a configurable timeframe of 16, 32 or 64 milliseconds, and attempts to create an identical image of the echo, which is then removed from the audio signal prior to sending it on to the other party. Generally, a shorter timeframe is preferable since it reduces the time the algorithm takes to “learn” the feedback environment. Typically, echo is reduced to below detectable levels within 1 – 2 seconds. In extreme cases convergence might take 3 – 12 seconds, during which call quality would increase to an optimum.

After primary echo has been canceled in this way, some residual echo may remain (echo present beyond the time-frame the canceler buffered or “non-linear” reflections). Such echo is then removed by a non-linear echo cancellation algorithm, if enabled. This may result in one of the parties’ audio being clipped when both parties are talking. Some users may find this uncomfortable. If this is the case it might be preferable to disable the NLP.

Although the default (16ms + NLP) is normally a good compromise, the Echo Cancellation settings may need to be adjusted on a per-port basis, as the audio environment might differ from one port to the next (e.g. an office environment for one range of extensions vs a factory floor for another and an open plan arrangement for another range of extensions.)

Wiring problems, such as power cables crossing telephone cables, or insufficient earthing of the Com.X or iTAs may add noise to the audio path that may reduce performance of the algorithm.

In diagnosing echo cancellation problems, establish good earth (~0.5 ohm or less, if resistance approaches 1 ohm earthing might be a concern), and proceed to eliminate each of the potential contributing elements from the equation by testing with different phones, connecting phones directly to the Com.X FXS ports, turning echo cancellation off completely to establish a quality baseline and then trying different combinations of echo cancellation, with and without NLP, on the different extensions.

Often the first measure to attempt, often yielding the greatest improvement is to try adjusting the gain on problematic extensions.

Note: In exceptional circumstances, if the echo cancelers take a long time to converge (e.g. > 10 seconds), the acoustics of the environment might be introducing too much echo, long loops may be present in the telco network and / or the handset might be too sensitive.

Adjusting the gain on the port down might help, or fixing the FIR coefficients in the echo canceler for that port might help. After restart, the first call on ports configured thus would take a long time to converge, and there-after the coefficients are re-used for subsequent calls. Due to the dynamic nature of echo, fixing the coefficients is not recommended for most cases.

To fix the coefficients select the ‘Keep EC Coefficients’ option on the FXO/FXS port.

Following this, restart the iTA for the new settings to take effect with:

comma-ls -r

Please note that echo heard by users of the Com.X (as opposed to external parties) is typically caused by the remote party. The Com.X does attempt to cancel the echo from the remote party on BRI, PRI and FXO connections to the telco, but the effectiveness of this cancellation is highly dependent on external conditions which are impossible to predict or control.

Note also that SIP connections (including local SIP phones) are never echo cancelled (it is not technically feasible) – therefore, this is the responsibility of the SIP phone itself or of the upstream service provider.