TR-41.3.3/99-05-009
May 13, 1999
Discussion of Echo Control and Measurement of TCLw for PN4352
Author:
Contact:
Michael Knappe, Cisco Systems, Inc.
Michael Knappe
Cisco Systems, Inc.
18581 North Dallas Parkway
Suite 100
Dallas, TX 75024
(972) 813-5206
mknappe@cisco.com
Abstract
The weighted Terminal Coupling Loss, TCLw, is intended to provide a measure of echo
performance under normal conversation. It is possible that echo control devices such as
echo suppressors or echo cancellation with non-linear processing may be used on handset
connections to provide sufficent echo return loss to mitigate increased echo noticeability
associated with longer network delays. This contribution examines methods in which
TCLw might be measured in the presence of such echo control devices on the handset.
Discussion
The primary impact on TCLw on phones with handset echo control devices is the fact
that the measure of echo performance that TCLw is trying to measure would likely be
different under cases of either a single far-end talker or double-talk. The TCLw
measurement is currently set up to represent a single far-end talker. This may provide
idealized and unrealistic performance measurements when non-linear processing on the
transmit side is used as part of the echo control algorithm. It may be more appropriate to
measure TCLw either with non-linear processing disabled or with a near-end signal
present that is a) capable of enabling echo control’s double-talk detector with the
subsequent removal of non-linear processing and b) can be filtered out from the final
return signal so as not to cause undue impact on the accuracy of the TCLw measurement.
The latter may be the only method which could used consistently across products in a
black-box testing setup. A suitable signal may be a pulsed sine wave, but would depend
on temporal characteristics of the double-talk detector.
The ‘proper’ measurement of TCLw then becomes very echo control implementation
specific. This requires further discussion at the May meeting.
Copyright Notice:
The contributor grants a free, irrevocable license to the Telecommunications
Industry Association (TIA) to incorporate text contained in this
contribution and any modifications thereof in the creation of a TIA
standards publication; to copyright in TIA's name any standards publication
even though it may include portions of this contribution; and at TIA's sole
discretion to permit others to reproduce in whole or in part the resulting
TIA standards publication.
1
Cisco Systems, Inc.