Coding of speech and wideband audio

Advances in coding algorithms and digital signal processing have led to sophisticated technologies for speech communication for a variety of applications, as well as to greater flexibilities in the design of ISDN terminals for integrated communication of speech, images, and data. For traditional telephony with a signal bandwidth of 3.2 kHz, the transmission rate for network-quality speech is now down to 16 kb/s. Robust communications-quality speech appropriate for cellular radio has been realized at 8 kb/s. Research attention is shifting toward 4 kb/s, focused on improving speaker identification and the naturalness of coded speech. For wideband audio with a signal bandwidth of 7 kHz, high-quality coding is now possible at 32 kb/s, which implies stereo teleconferencing or dual-language programming over a 64-kb/s channel. Transparent coding of 20-kHz audio has been demonstrated at 128 kb/s, with near-transparent performance at rates as low as 64 kb/s for some classes of signals.