ECHO CANCELLATION
6.10 ECHO CANCELLER TESTING
As per G.168 requirements [ITU - T - G.168 (2004) (2004)], the tests recom-
mended are given in Table 6.3 . These tests are identifi ed with a test number. The numbers remained the same across several revisions of echo canceller recommendations, and the results are usually referenced with the test number. In the table test number, the test name and a brief description are given. For more details on these tests, it is suggested to refer to the G.168 recommenda- tion [ITU - T - G.168 (2004) ]. These tests ensure time - dependent echo attenua- tion [ITU - T - P.340 (2000) ] for different power levels, NLP tracking and time responses, double talk conditions, echo attenuation over frequency, and characteristics of fax/modem signals and tones.
Table 6.3. Echo Canceller Tests as per G .168 Recommendation
Test No. Test Name Brief Description Test 1 Steady state residual and
returned echo level test (deleted in revisions)
Test 1 is deleted and incorporated in Test 2
Test 2A Convergence and
reconvergence with NLP enabled
To ensure that the echo canceller converges rapidly
Test 2B Convergence and
reconvergence with NLP disabled
This test case is the same as Test 2A with NLP disabled
Test 2C Convergence test in the presence of background noise
To ensure EC converges in the presence of background noise Test 3A Double talk convergence test
with low canceller - end levels
To ensure that the double talk detection is not sensitive to low - level near - end speech
Test 3B Double talk stability test To ensure that DT detection is sensitive
Test 3C Double talk under simulated conversation
To ensure EC does not produce undesirable artifacts during and after double talk
Test 4 Leak rate test To ensure that the leak time is not too fast, fi lters do not go to zero too rapidly in 2 minutes
Test 5 Infi nite return loss convergence test
To ensure EC has to prevent the unwanted generation of echo Test 6 Nondivergence on narrowband
signals
To verify EC remains converged to narrowband signals after converging on wideband signals
Test 7 Stability test To verify that the echo canceller will remain stable for narrowband signals Test 8 Nonconvergence of echo
cancellers on tones (optional)
To verify EC operation with mono or bi - frequency tones
Test 9 Comfort noise test Comfort noise matches the near - end background
Test 10 Facsimile test To ensure EC converges rapidly on initial fax handshaking sequences and to prevent the unwanted generation of echo by these signals Test
10A
EC operation on calling station side
Adaptation
with CNG, CED, and CSI/DIS Test
10B
EC operation on called station side
Adaptation while TSI, DCS are applied Test
10C
EC operation on calling side during page breaks (further study)
This is for V.29 modem, under study for V.27ter and V.17
Test No. Test Name Brief Description
Test 11 Tandem echo canceller test To ensure EC operating in tandem is not degrading the quality of a call Test 12 Residual acoustic echo test
(further study)
To check the performance in the presence of residual acoustic echo Test 13 Performance with low - bit - rate
coders in echo path (optional)
To ensure on performance with low - bit - rate codec in echo path Test 14 Performance with V.series low -
speed data modems
To ensure no impairments to data modems with EC enabled
Test 15 PCM offset test (optional) To ensure EC works with PCM offset Fax
tone
Modem and fax tone disabler tests
To ensure fax and modem are detected
Table 6.3. Continued
6.10.1 Simulated Tests
Echo canceller test signal generation, testing procedure, and timing sequence are given in G.168. The test signal consists of synthetic speech, pseudo - random noise, and silence zones. Developers/implementers also generate additional test vectors to create various corner cases and actual conversation speech fi les. Such an extended suit is used for internal testing. Most of these tests are useful for actual software development and a simulation environment. These test vectors are used in evaluating the software in Matlab, C, and processor - specifi c assembly. For actual testing in an integrated product, echo canceller test equip- ment is used. Equipment manufacturers use some G.168 test signals and several proprietary signals to help simplify the measurements in a known way.
6.10.2 Instrument - Based Tests
The echo canceller is a four - terminal block with R in , R out , S in , and S out . In the
echo canceller, R in and R out are exactly the same. As shown in the previous
sections, the major part of the electrical echo is created at the two - to - four wire hybrids of SLIC interface and telephones. Most users will be using a two - wire telephone interface and analog telephones. The measurement of echo will have several limitations on the two - wire interface; therefore, four - wire inter- face is more suitable.
Basic Instrument - Based Two - Wire Tests. Some instruments [URL (Sage935) ] perform basic steady state echo cancellation tests on a two - wire
interface. Sage instrument - based basic tests are called echo sounder and
responder tests. The echo sounder generates a signal and measures the echo. The echo responder or echo generator creates the echo. On a two - wire inter- face, it is diffi cult to isolate the leakage. Hence, proprietary test signals are sent
in the beginning to perform end - to - end hybrid calibration before creating the echo test. This is found to be useful to evaluate basic echo cancellation and to measure steady state cancellation. Some more test details are available at [URL (Sage935) ].
Basic Instrument - Based Four - Wire Tests. Four - wire tests are represented in Fig. 6.9 . Four - wire - based tests allow several combinations of echo canceller tests given in Table 6.3 . In Fig. 6.9 (a), two VoIP adapters A and B are used for conducting the tests. In the EC tests of Fig. 6.9 (a), the adapter - A interface works for sending the signal and for measuring the echo. It also works for sending the talker voice and for measuring the talker echo. The other side of adapter - B is the echo creator. In this measurement, EC of adapter - B is under evaluation. For measuring EC performance, it is required to activate all hybrids, phones, and so on. Hence, it is useful to create echo on a two - wire interface of adapter - B. It is also possible to create echo on a four - wire interface as shown with dotted lines on adapter - B of Fig. 6.9 (a). In general, the adapter - B side can be used with two or four wire. Instruments also provide various controls and confi gurations to the echo canceller in maintaining timing relations. An
Figure 6.9. Echo measurements with VoIP systems. (a) Echo measurement on VoIP adapters. (b) Echo measurement through four - wire or Ethernet options.
RS - 232 serial interface is used in most instruments. As shown in Fig. 6.2 , there are three controls to the EC block: H - register, inhibit convergence, and NLP disable. These controls are mapped through the communication interface from the instrument or computer controlling the instrument.
With reference to Fig. 6.9 (b), the measurement on adapter - A should not see a two - to - four - wires hybrid. It is essential to provide the direct interface from the ADC and the DAC. The SLIC hybrid on adapter - A is shown as iso- lated. This type of arrangement is possible in the development phase of the product. In a fi nished product, it is not possible to extract the four - wire inter- face. Alternatively, four - wire interface - capable VoIP systems can be used as shown in Fig. 6.9 (b). In Fig. 6.9 (b), the VoIP gateway with a T1/E1 or ISDN
interface is used. In these four - wire interfaces, the hybrid is not present.
Another simpler option is to use the IP phone. The IP phone does not have any hybrid. The IP phone internal architecture is given in Chapter 2 . The IP phone ADC, DAC path signals are directly interfaced to the headset micro- phone and speaker. This headset interface will go directly to the instrument. In Fig 6.9 , two instruments are shown for distinction of echo generation and measurements. In practice, both sides will be part of the same instrument. Several details on these tests can be found at equipment supplier websites [URL (Sage935) , URL (GL) ].
Echo Measurement through Network Interface. In Fig. 6.9 (b), the Ether- net interface is shown to be going to the instrument. In general, the Ethernet interface is used with several instruments for network measurements. The purpose of the Ethernet interface shown here is for echo measurement. The instrument on the adapter B - side sends the talker signal as RTP packets as if they are delivered from the gateway. Internal representation blocks of this type of measurement are given in [URL (GL)]. The received IP packets (RTP packets) will also be captured directly from the network side of adapter - A. Echo is created on adapter - B side. It is more easy and accurate to measure echo with ideal four - wire interfaces. It is expected that such echo measure- ments will be made on a dedicated local area network. Hence, IP impediment contributors will not be perceived. The system may work like a fi xed round - trip delay system for the purpose of measurement.
6.10.3 Perception - Based Tests
Simulation and instrument - based echo cancellation tests reveal most of the performance issues of the echo cancellers. Certain impediments of NLP, pleas- antness in DT, interactive voice calls with delays, voice calls under background noise, and so on, are evaluated with perception tests. These tests are conducted with audio interfaces. Audio interfaces and listeners are positioned in acoustic silence chambers. A listener observes various conditions of the echo cancella- tion. The perception tests have to be conducted after thorough evaluation and
meeting the echo cancellation specifi cations in simulated and instrument -
based tests. Echo cancellers certifi ed by agencies such as ATT - USA and British Telecom exact - UK validate the carrier - grade credentials of the product. These tests include both measurement and perception - based tests. This type of cer- tifi cation is essential for ensuring better voice quality. The echo cancellers mainly aimed at PSTN – VoIP gateways and long tail - length operations are subjected mandatorily to certifi cation and subjective testing. Certifi ed echo cancellers are widely used and accepted in inter - regional calls with end - to - end delays up to 400 ms.
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VoIP Voice and Fax Signal Processing, by Sivannarayana Nagireddi Copyright © 2008 by John Wiley & Sons, Inc.