6.3 LINE AND ACOUSTIC ECHO CANCELLER

Echo cancellers for voice processing are classifi ed as LECs and AECs based on the source of echo generation. Several techniques and requirements overlap between these two echo cancellers. In general, acoustic echo canceller can be reused as line echo cancellers. Line echo canceller may not cater to all require- ments of the acoustic echo cancellation. The main differences of the line and the acoustic echo canceller are listed in Table. 6.1 .

Table 6.1. AEC and LEC Functional Differences

Parameter Line Echo Canceller Acoustic Echo Canceller Echo source Electrical echo with

wired connections

Acoustic echo with speakerphone mode and mechanical body coupling

Applications Used in VoIP CPEs with FXS, FXO, and VoIP - to - PSTN interfaces

Speakerphone mode of operation for IP phones, WiFi handsets, and soft phones

Echo span Typical 8 – 32 ms, as high as 128 ms

Typical 32 – 128 ms, as high as 256 ms Echo

characteristics

Slowly varying, sparse, typical tail span ≤ 12 ms

Rapidly varying, dispersive tail span, but decays rapidly with echo tail span

Echo return loss ERL > 6 dB Echo can be stronger and varying in characteristics

Processing complexity

Depends on design and tail length

In addition to design and tail length, also depends on sampling rate variations

Voice quality enhancements

Adaptive level control Adaptive level control and background noise reduction

As shown in Fig. 6.3 (a), acoustic echo is the coupling/leakage of voice through acoustic media, usually through air as media in most telephone appli- cations. Acoustic echo is created while using phones in speakerphone mode. Several phones (mobile phones, IP phones, WiFi phones, car phone adapters) are supporting speakerphone mode. With speakerphone, acoustic waves are stronger in strength to reach several feet distance. The strong acoustic waves from the speaker will also be reaching the microphone creating acoustic echoes. The signal level caused by acoustic coupling is much stronger than the usual speech level reaching the microphone. The phones with a speakerphone option come with a built - in acoustic echo canceller. The acoustic echo cancel- ler is mainly governed by P.340 [ITU - T - P.340 (2000) ] recommendations. Many proprietary acoustic echo cancellation techniques [Gay and Benesty (2000) , Gustafsson (2001) , Borys (2001) , Benesty et al. (2001) ] are also used based on the actual application and implementation. In a normal handset - based phone, some amount of acoustic coupling will be happening through air and the mechanical body structure of the handset. This echo coupling is 45 to 50 dB lower and merges with the ideal channel noise. Hence, in the handset mode of operation, the effect of acoustic echo is less.

Line echo cancellers are related to coupling of electrical signals (no acoustic coupling) as shown in Fig. 6.3 (b). Talker echo and listener echo indicated in Fig. 6.1 (a) are related to the line echo canceller. Line echo cancellers are used with foreign exchange subscriber (FXS), and FXO electrical interfaces. In a simple a VoIP adapter with an FXS interface, echo is generated inside the phone and at the SLIC – CODEC telephone hybrid. Line echo cancellation is

Figure 6.3. Echo canceller classifi cation. (a) Acoustic echo creation. (b) Electrical echo cre- ation. © Echo canceller classifi cation and functional blocks.

mainly governed by G.168, G.131, and G.169 [ITU - T - G.168 (2004) , ITU - T - G.131 (2003) , ITU - T - G.169 (1999) ] recommendations. The other names used for the line echo canceller are the electrical echo canceller, network echo canceller, or simply echo canceller referring to the context of the application.

AEC is not always associated with speakerphone mode. Even without a

hands - free mode of operation, acoustic echo can be noticed because of

mechanical coupling though miniature form factor handsets such as mobile and WiFi phones. As shown in Fig. 6.3 (a) and (b), AEC or LEC removes echo

in one direction at the near end (close to echo creation). In the communication between mobile phones and PSTN, the central communication nodes keep both the near - end and the far - end echo canceller. This type of back - to - back echo canceller operation can help remove echoes from the mobile and the PSTN side. The echo canceller on the mobile phone side has to cater to low - level acoustic echo. PSTN side can take care of echo problems, even if mobile phones are not incorporated with an echo canceller and end - to - end delays are more between mobile and PSTN calls.

Figure 6.3 (c) is presented for broad classifi cation of the echo canceller functional blocks and their relation. The functional blocks are common to LEC and AEC. Modem and fax tone detection are not applicable in AEC. Some blocks indicated in the fi gure are explained in next sections of this chapter. The main functional blocks of the echo canceller are as follows:

1. Adaptive fi lter — adapts to the echo path and estimates the closely match-

ing signal to cancel the echo.

2. Double talk detection — indicates the presence of simultaneous conver-

sation from both ends also referred as presence of near end speech and echo.

3. Nonlinear processor (NLP) — removes the nonlinear and residual part of

the echo and optionally inserts matching background as comfort noise.

4. Modem answering and fax tone detection — detect the presence of

modem/fax tones to disable certain functions of the echo canceller. This is presented in Chapter 14 .

5. Monitoring — residual echo estimators, echo residual monitoring, as well

as echo and signal parameters to voice quality monitoring during an active call. Confi guration has to ensure that echo canceller parameters are properly set for various conditions.

In this chapter, echo cancellation operations are given in relation to LEC. The operations and algorithms of LEC are also useful for AEC.