Howto:Having ECHO problems

Applies To

This information applies to

• all innovaphone products

any build

More Information

Having echo on the Endpoints

General

The presence of echo occurs whenever the replicated signal delay exceeds 10msec , and becomes apparent to the speaker as reflected voice when the delay exceeds as little as 16 msec.

It is manifested to the far end as an altered replica of the speaker original.

On traditional PSTN networks to end to end delay of a voice call is only a few milliseconds. On IP networks the delay is much higher due to packetization. Even small packets of 20ms increase the delay so that echo becomes apperent and needs to be cancelled. Processing delay VOIP gateway and the delay of the IP network itself increase further the end to end delay.

Causes of echo

Echo may be caused in several places within a network:

• On analog lines the 2 wire to 4 wire conversion is not perfect and generates echo.
• In the handset microphone and receicer have an acoustic connection.
• Strong echo occurs during handsfree operation.

Echo Cancellation

Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the received signal.

Once the echo is recognized, it can be removed by 'subtracting' it from the transmitted or received signal.

This technique is implemented using a digital signal processor (DSP):

By correlation the received voice signal witht the transmitted voice signal, the echo canceller can create a model of the echo path which is in turn used to estimate the echo.

This estimation is then subtracted from the incoming voice signal allowing normal speech to pass through.

The core of the algorithm is an adaptive filter. The length of that filter affects the processing power needed and defines the max echo length that can be cancelled.

Echo cancellation is done at the egde of the IP network, in the VOIP gateways and the IP phones. That ensure that the delay introduced by the IP network is not seen by the echocanceller.

Echo cancellation in innovaphone products is never done over the IP network with its large and variable delay.

Line Echo Cancellation

The echo delay on a telephony line should be below a few milliseconds. However, todays PSTN networks are not alway pure TDM networks, they use internal IP or ATM technology so that higher delays of e.g 30ms can occur. So for VOIP gateways it is important that they can handle echo after a reasonable long delay.

Maximum Echo cancellation - maximum echo length (echo tail)

IP400                     16msec
IP3000                    24msec
IP302,IP305,IP800,IP6000  64msec
IP1200                    32/64msec

For the IP1200, due to added delay when using repeater chains, 32ms have been found to not be sufficiant to cancel echo under any possible cirumstances. So the echo canceler has been changed from 32ms to 64ms from build 07-6060069 upwards. Unfortunately, this reduces the maximum number of concurrent calls to from 12 to 10 for a single base. This depends on an updated DSP code, so it is not a configuration option.

Acoustic Echo Cancellation

On IP phones the echo path is very limited (form the phone to the handset ), so the maximum echo length is not critical. That changes in handsfree mode, then the echo length depends on the size of the room. Since that phone and people in the room may move around, the echo changes during a call.

This type of echo can be reduced with a simple echo reducer. A echo reducer implements half duplex comminications ( mute the phones micro if the speaker is active ). This is done on the (old) IP200.

New phones ( IP200A,IP110,IP230 etc ) use an acoustic echocanceller that adapts continously during a call to room echo of up to 96ms.

Troubleshooting Echo Cancellation

It is no solution to make volume adjustments on the Endpoints for solving the Echo problem

If you encounter an echo problem – pleas check this :

Do you meet the Echo problem always ?

• between IP phones ? Simple echo tests using IP phones on one floor or even in one room may lead to the conclusion that there is echo. In this case there a 2 voice pathes, one direct acoustic path without delay and one over the IP network with delay. Echo tests between IP phones need to be done between IP phones without a direct acoustic path.

• between ISDN and IP phones ? Echo perceived on the IP phones results from the echo canceller on the ISDN gateway, echo perceived on the ISDN phone results from the echocanceller of the IP phone.

• between analog phones and/or Dect phones to IP phones or ISDN (connected an analog adapter IP21,IP22,IP24)?

• Echo on a gateway from one interface to the other ( e.g. PRI1 to PRI2 ) can be avoided if the PCM switch is used.

If this problem is with analog or Dect Phone - occurs with very old Dect phones in very few cases, - the best solution here is - eliminate this phone. ( IP600 with G.711 DSP doesnt provide echo cancellation and relies on a echo-free DECT phone )

If you have a general voice qualitity problem you may have a problem in your IP network, but is another story.