View Full Version : Echo debugging

10-30-09, 09:08 AM
Echo Debugging

Echo can be re-created easily using one trunk interface and two POTS interfaces. The line interface must be connected to a Telco line and the POTS interface must be connected to two analog phones (assuming a Daytona card is used). If two POTS are not available (in the case of an InLine), two other trunk interfaces or a play port and a record port can be used instead.

The connections to create the echo are from phone 1 to trunk in half duplex and from trunk to phone 2 in half duplex. With this type of connection, the audio going out from phone 1 will go into the trunk (to the outside world) and the audio from the trunk (from the outside world) will go to phone 2. But since this scenario creates echo, the audio from phone 1 will also go to phone 2. To avoid this, echo cancellation is needed.

Echo cancellation requires an echo cancellation DSP port and a reference port, which is a DSP port with any DSP application enabled on it.

The connections to cancel the echo are from phone 1 to reference port and also from phone 1 to
trunk 1. Then, a connection from the reference port to the echo cancellation port as well as a connection from trunk to the echo cancellation port. The last connection is from the echo cancellation port to phone 2. There are 5 connections in all and have to be established in half-duplex.

Once the resources are connected, the echo cancellation port has to be setup by calling PK_EC_ Initialize. Once the echo cancellation port has been initialized, a call to PK_EC_Enable and PK_EC_Disable will enable or disable echo cancellation.

-> Echo is caused by your transmition going over a one pair cable, which has both transmit and receive super imposed on the same pair, then being switched to a 2 pair cable which happens inside the telephone for ear and mouth.

-> Echo is on every call but the shorter the distance the less delay and it therefore sounds more like side tone.

-> As the distance becomes greater so does the delay and now it sounds more like an echo than side tone. On a normal telephone connection the echo should be cancelled by the central office.

-> When you use another method other than the teleco lines (IP) you need to deal with the echo. With our cards we use small buffers which are primarily used to keep the delay to a minimum and the echo cancellation feature.

-> Echo Cancellation - it works from the other end of the call, so if I am hearing echo it means that the far end is not canceling out the echo. The echo canceller takes the reference signal (receiving signal) and your transmition signal (with echo) and compares and then omits the echo so the outcome is a clean signal. Remember this is done at the far end to benefit you at the other end.

-> When gain is entered into the equation on a closed loop connection it can cause some instability in the line which will impact the echo cancellation. In pksetup under echo cancellation "Double Talk Threshold" may improve on this.