People have been using phones as a means of distant voice communication for more than a century now.Using phones has become sort of a usual thing. We use the phones almost every day and just about everywhere At home, at work,outside,in our cars and so on.
Subscribers use speech quality as the benchmark for assessing the overall quality of a network. Regardless of whether or not this is a subjective judgment, it is the key to maintaining subscriber loyalty. For this reason, the effective removal of hybrid and acoustic echo inherent within the digital cellular infrastructure is the key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing solutions that can reduce background noise and remove hybrid and acoustic echo before any transcoder processing. By employing this technology, the overall efficiency of the coding can be enhanced, significantly improving the quality of speech. This tutorial discusses the nature of echo and how echo cancellation is helpful in making mobile calls meet acceptable quality standards. Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted or received signal. Once the echo is recognized, it can be removed by 'subtracting' it from the transmitted or received signal. This technique is generally implemented using a digital signal processor (DSP), but can also be implemented in software. Echo cancellation is done using either echo suppressors or echo cancellers, or in some cases both Echo cancellation is the process of removing echo from a voice communication in order to improve the voice call quality. Echo cancellation is often needed because speech compression techniques and packet processing delays generate echo. There are 2 types of echo: acoustic echo and hybrid echo.
Echo cancellation not only improves quality but it also reduces bandwidth consumption because of its silence suppression techniqueThe term echo cancellation is used in telephony to describe the process of removing echo from a voice communication in order to improve voice quality on a telephone call. In addition to improving subjective quality, this process increases the capacity achieved through silence suppression by preventing echo from traveling across a network. Two sources of echo have primary relevance in telephony: acoustic echo and hybrid echo. Echoes arise in various situations in the telecommunications network and impair communication quality.
1.1.1 HISTORY OF ECHO CANCELLATION
The late 1950s marked the birth of echo control in the telecommunications industry with the development of the first echo-suppression devices. These systems, first employed to manage echo generated primarily in satellite circuits, were essentially voice-activated switches that transmitted a voice path and then turned off to block any echo signal. Although echo suppressers reduced echo caused by transmission problems in the network, they also resulted in choppy first syllables and artificial volume adjustment. In addition, they eliminated double-talk capabilities, greatly reducing the ability to achieve natural conversations.
Echo-cancellation theory was developed in the early 1960s by AT&T Bell Labs, followed by the introduction of the first echo-cancellation system in the late 1960s by COMSAT TeleSystems (previously a division of COMSAT Laboratories). COMSAT designed the first analog echo canceller systems to demonstrate the feasibility and performance of satellite communications networks. Based on analog processes, these early echo-cancellation systems were implemented across satellite communications networks to demonstrate the network's performance for long-distance, cross-continental telephony. These systems were not commercially viable, however, because of their size and manufacturing costs. In the late 1970s, COMSAT TeleSystems developed and sold the first commercial analog echo cancellers, which were mainly digital devices with an analog interface to the network. The semiconductor revolution of the early 1980s marked the switch from analog to digital telecommunications networks. More sophisticated digital interface, multichannel echo-canceller systems were also developed to address new echo problems associated with long-distance digital telephony systems. Based on application-specific integrated circuit (ASIC) technology, these new echo cancellers utilized high-speed digital signal-processing techniques to model and subtract the echo from the echo return path. The result was a new digital echo-cancellation technique that outperformed existing suppression-based techniques, creating improved network performance. The 1990s have witnessed explosive growth in the wireless telecommunications industry, resulting from deregulation that has brought to market new analog and digital wireless handsets, numerous network carriers, and new digital network infrastructures such as TDMA, CDMA, and GSM. According to the Cellular Telecommunications Industry Association (CTIA), new subscribers are driving the growth of the wireless market at an annual rate of 40 percent. With wireless telephony being widely implemented and competition increasing as new wireless carriers enter the market, superior voice transmission quality and customer service have now become key determining factors for subscribers evaluating a carrier's network. Understanding and overcoming the inherent echo problems associated with digital cellular networks will enable network operators and telcos to offer subscribers the network performance and voice quality they are demanding today
2.4.1 CONTROLLING ACOUSTIC ECHO
In echo cancellation, complex algorithmic procedures are used to compute speech models. This involves generating the sum from reflected echoes of the original speech, then subtracting this from any signal the microphone picks up. The result is the purified speech of the person talking. The format of this echo prediction must be learned by the echo canceller in a process known as adaptation. It might be said that the parameters learned from the adaptation process generate the prediction of the echo signal, which then forms an audio picture of the room in which the microphone is located. Figure 2.8 shows the basic operation of an echo canceller in a conference room type of situation.
During the conversation period, this audio picture constantly alters, and, in turn, the canceller must adapt continually. The time required for the echo canceller to fully learn the acoustic picture of the room is called the convergence time. The best convergence time recorded is 50 ms, and any increase in this number results in syllables of echo being detected. Other important performance criteria involve the acoustic echo canceller's ability to handle acoustic tail circuit delay. This is the time span of the acoustic picture and roughly represents the delay in time for the last significant echo to arrive at the microphone. The optimum requirement for this is currently set at 270 ms—any time below this could result in echoes being received by the microphone outside the ability of the echo canceller to remove them, and hence in participants hearing the echoes. Another important factor is acoustic echo return loss enhancement. This is the amount of attenuation which is applied to the echo signal in the process of echo cancellation—i.e., if no attenuation is applied, full echo will be heard. A value of 65 dB is the minimum requirement with the non-linear processor enabled, based on an input level of -10 dBm white noise electrical and 6 dB of echo return loss (ERL).
2.4.2 CONTROLLING COMPLEX ECHO IN A WIRELESS DIGITAL NETWORK
Although acoustic echo is present in every hands-free mobile call, the amount of echo depends on the particular handset design and model that the mobile user has. On the market are a few excellent handsets that limit the echo present, but, due to strong price pressures, most handsets do not control the echo very well at all—in fact, some phones on the market have been determined to have a terminal compiling loss of 24 dB. Echo becomes a problem when the processing inherent to the digital wireless network adds an additional delay (typically in excess of 180 ms round-trip). This combination makes for totally unacceptable call quality for the fixed network customer, as shown in Figure 2.10
This back-to-back configuration ensures a high audio quality for both PSTN and mobile customers. In addition, the echo canceller's software configuration is designed to provide a detailed analysis of background noises, including acoustic echo from the mobile user's end. Some echo cancellers incorporate a user-settable network delay, which enables network operators to fine-tune the echo control to suit their parameters via a menu option on the canceller's hand-held terminal or on the network management system (NMS).