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Appendix E

Echo Cancellation on QX Boards

Introduction
Hybrid Echo in the Public or Private Network
Echo Canceller in the QX 2000
QX 2000 With Analog Ports
QX 2000 Without Analog Ports

IntroductionTop of Page

This appendix discusses echo cancellation using QX boards. It includes default settings for configurations which use the analog port, as well as for configurations which do not use the port.

Hybrid Echo in the Public or Private NetworkTop of Page

In the Public Switching Telephonic Network (PSTN), the transit network uses four wires. Two wires are used to transmit voice in one direction. The two other wires transmit in the opposite direction. However, the subscriber network uses only two wires to transmit voice in both directions in a full-duplex connection. This scheme is more cost effective.

Because the length of the subscriber's line varies from 0 to 9 km (5.5 miles), the impedance of the line never matches the reference impedance of the line interface circuitry. This non-balanced impedance generates echo on the line interface. Where there are two line interfaces in the connection, two sources of echo may be present:

For example, as shown in Figure 33, if telephone set A is connected to the telephone set B through the telephone network, telephone A receives a near-end echo from line interface A and a far-end echo from line interface B.

If the length of the subscriber A line is up to 9 km (5.5 miles), the delay on the line will be less than 1 ms. However, because the line interface circuitry adds some distortion and multiple reflection, in general the width of the echo is up to 8 ms.


appe0.gif

Figure 33. PSTN Transit Network



The delay of the far-end echo depends on the call type (for example, local call, national call, international call). If the network is a full four-wire network, only the far-end hybrid will generate the echo. The delay of this echo depends on the group delay of all devices used by the network. If the network is using two-wire devices to carry communications, additional echo will be generated.



The echo canceller uses a delay line to store the transmitted signal as a reference. For this reason, the echo cancellation should ideally be implemented near the source of the echo.



Here are some typical echo delay times: 





 Echo Type



 Typical Delay




 Near-end echo



 0 to 8 ms




 Far-end echo



·	 Up to 20 ms for local or regional calls


·	 Up to 40 ms for national calls


·	 Up to 60 ms for international calls


·	 Up to 140 ms for submarine transmissions


·	 Up to 520 ms for one satellite transmissions




















 Echo Canceller in the QX  2000Top of Page





With the release QX 4.0 delivered with Natural Access 2000-1, a new version of echo cancellation is introduced for the QX  2000/100-4L boards. This version provides an enhanced echo canceller with a fast convergence time for applications using host-based speech recognition.



Some applications need to enable speech recognition in full duplex mode immediately after a connection has been established. To do so, the echo canceller has to converge very quickly when the first message is played. The new version is able to converge in less than one second if there is no double-talk signal on the line.






 QX  2000 With Analog PortsTop of Page





With a QX  2000 board (or any analog board), the analog trunk interface of the board adds a local echo, superposed upon the near-end echo (see Figure  34):




appe1.gif


Figure 34.	 QX Board With an Analog Trunk Interface




With this configuration, you must set the pre-delay to 0 ms so that the analysis window can synchronize to the local echo and the near-end echo.



If you are using a PBX for a private call, the far-end echo is probably superposed upon the local echo and the near-end echo, with a delay of less than 20 ms.



If you are connected to the public network, the delay of the far-end echo depends on the equipment used by the operator to make the connection. The connection path may vary with each call. With a 20 ms window, the QX  2000 is able to cancel echo for local and regional calls. 



For national and international calls, a 20 ms window may be sufficient for most calls if the operator network is recent. However, if the network is older, the delay could exceed 20 ms. In this case, the QX  2000 cannot cancel the echo. 



In most cases, we recommend that you use the default echo cancellation mode setting for QX  2000 boards.  The mode is assigned in either of the following parameters, depending upon the call control service you are using: 





 Call Control Service



 Parameter




 NCC



 NCC.X.ADI_START.echocancel.mode=1




 ADI



 ADI.START.echocancel.mode=1
















This setting causes the echo canceller to use internal defaults for filter length and pre-delay.



Refer to Appendix  B for more information about these parameters.






 QX  2000 Without Analog PortsTop of Page





If you are not using the analog port of the QX  2000, but instead are using another interface board through the MVIP or H.100 bus, you may have to include the cross time of the board when tuning the pre-delay of the echo canceller.




appe2.gif


Figure 35.	 QX Board Not Using the Analog Ports




If the interface board is not an analog port (for example, you are interfacing with a T1 or E1 trunk), no local echo or near-end echo will be present. You can tune the pre-delay to integrate the cross time of all numeric equipment so that the boundary analysis window can accurately estimate the echo of the first hybrid on the network.



We recommend that you use the default echo cancellation mode settings for QX  2000 boards.  The mode is assigned in the following parameters, depending upon the call control service you are using: 





 Service



 Parameter




 NCC



 NCC.X.ADI_START.echocancel.mode=2        (Use specified value)
NCC.X.ADI_START.echocancel.predelay=xx        (Output sample delay)
NCC.X.ADI_START.echocancel.filterlength=20        (Lgth of analysis window)




 ADI



 ADI.START.echocancel.mode=2        (Use specified value)
ADI.START.echocancel.predelay=xx        (Output sample delay)
ADI.START.echocancel.filterlength=20        (Lgth of analysis window)
















The value of the pre-delay depends on the crossing time of the port (for example, E1/T1) you are using.



Refer to Appendix  B for more information about these parameters.








 
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