Perceptual Echo Control and Delay Estimation

Abstract

Echo phenomenon has been always existed in telecommunications networks. Generally it has been noticed on long international telephone calls. As technology advances and the data transmission methods tend more to packet-switching concepts, the traditional echo problem remained up-to-date. An important issue in echo analysis is a round-trip delay of the network. This is a time interval required for a signal from speaker’s mouth, across the communication network through the transmit path to the potential source of the echo, and then back across the network again on the receive path to the speaker’s ear. The main problem associated with IPbased networks is that the round-trip delay can be never reduced below its fundamental limit. There is always a delay of at least two to three packet sizes (50 to 80 ms) (Choi et al., 2004) that can make the existing network echo more audible (Gordy & Goubran, 2006). Therefore, all Voice over IP (VoIP) network terminals should employ echo cancellers to reduce the amplitude of returning echoes. A main parameter of each echo canceller is a length of its coverage. The coverage means the length of time that the echo canceller stores its approximation in memory. The adaptive filter should be long enough to model an unknown system properly, especially in case of VoIP applications (Nisar et al., 2009; Youhong et al., 2005). On the other hand, it is known that an active part of the network echo path is usually much smaller compared to the whole echo path that has to be covered by the adaptive filtering algorithm. That is why the knowledge of the echo delay is important for using echo cancellers in packet-switching networks. Today, there is a wide family of adaptive filtering algorithms that can exploit sparseness of the echo path to reduce high computational complexity associated with long echo paths (Dyba, 2008; Hongyang & Dyba, 2008; Khong & Naylor, 2006; Hongyang & Dyba, 2009). In this chapter, we discuss numerous methods used for estimation of echo delay. Algorithms based on cross-correlation function and adaptive filters are used in the art. We will consider both types of them, discuss their advantages and drawbacks. Afterwards, we will pay our attention to the adaptive filtering techniques. We provide a study on different partial, proportionate, sparseness-controlled timeand frequency-domain adaptive filters. The readers will get closer to an issue of echo cancellation, which is relevant in nowadays telecommunications networks. Ones will able to recognize important features and particular areas of implementation of various adaptive algorithms. Further, we are giving a short introduction to the issue of echo control for telecommunications networks. This description emphasises on two most important aspects of perceptual echo control, which are echo loudness and echo delay.