Voice API Programming Guide — June 2005 103
Recording and Playback
Figure 16. Echo Canceller Operating over a TDM bus
Once the ECR feature is enabled on a board, each voice channel is also permanently assigned two
TDM bus transmit time slots. These time slots are referred to as the voice-transmit time slot and
the echo-cancellation transmit time slot. You can retrieve the time slot numbers for each via the
dx_getxmitslot() and dx_getxmitslotecr( ) functions, respectively. If the ECR feature is not
enabled, the channels are not assigned the echo cancellation TDM bus transmit time slots, and ECR
mode is not possible on any voice channel of that board.
The function dx_listen( ) routes the echo-carrying signal to the voice device. A call to
dx_listenecr() or dx_listenecrex( ) routes the echo reference signal to the voice channel and
simultaneously activates ECR mode. The resulting echo canceller uses the echo reference signal to
estimate the echo component in the echo-carrying signal, and subtracts that estimate from the echo-
carrying signal. This process results in an echo-canceled signal with a greatly reduced echo
component.
For another device to receive the echo-canceled signal output by the echo canceller, it calls
dx_getxmitslotecr() to retrieve the echo canceller’s transmit time-slot number, and calls
xx_listen( ) (where xx_ is ag_, dt_, dx_, or ms_) to connect its receive channel to the echo-
canceled signal.
To return the voice channel to standard voice processing (SVP) mode, the application calls
dx_unlistenecr() on the voice channel to stop the echo canceller, disable ECR mode, and
disconnect the echo canceller’s receive channel.
For technical information on ECR functions, see the Voice API Library Reference.
For examples of echo cancellation configurations, see Section8.12.4, “Echo Cancellation Resource
Application Models”, on page 105.
SCbus
Echo
Voice
Device SCbus
Echo-Generating
Signal
Echo-Carrying
Signal
Echo-Carrying Signal
Echo-Reference Signal
Enabled in ECR Mode
Disabled in ECR Mode
Telephone
Network
Switching Handler
TX
RX
Analog
Device
Signaling
VOX_TX
VOX_RX
ECR_RX
ECR_TX
E
C
R