Artificial Bandwidth Extension of Speech & Its Applications in Wireless Communication Systems: A Review

In the recent scenario of wired and wireless communication systems, amongst many reasons for the overall degradation of recovered speech quality at receiver, one of the major reasons to be considered is utilization of Narrow Band (NB) end devices and NB transmission medium supporting bandwidth of 300 Hz-3400Hz. The inherent drawback of such NB speech signal is it sounds muffled and thin because of absence of High Band (HB) spectral components. With state of the art development of communication technologies and with increased availability of end terminals capable of transmitting and receiving Wide Band (WB) signals (having bandwidth from 50 Hz to 7000 Hz), end users prefer to listen to WB speech. In order to offer a fully WB communication over wired and wireless media, both end devices and network need to be made WB compatible. A long transition period has been elapsed for upgrading existing NB systems (both end terminals and network) to fully WB compatible systems. In-between new methods have been developed to artificially extend the bandwidth of NB telephonic speech at receiver for improving the quality of recovered speech. A major task of Artificial Bandwidth Extension (ABE) is to reconstruct missing WB spectral components at receiver with the use of available NB speech. This paper discusses motivation for developing ABE algorithm along with exhaustive comparative studies of implementing it with various approaches. Issues and limitations related to real time implementation of ABE algorithm are also addressed. Alternative approaches like usage of ABE with side information transmission along with coded NB speech are also demonstrated. Main objective of ABE with side information is to extract WB spectral components from WB input speech and to embed these derived spectral components into coded NB speech signal and finally transmit them onto a NB channel. Reverse procedure can be carried out at receiver to artificially produce WB speech. Here, it is to be noted that transmission channel is NB whereas end terminals are made WB compatible so this method provides alternative solution to coexisting state of the art WB coders (which require WB channels) while offering comparable speech quality and giving natural sounding in terms of intelligibility and naturalness.