High quality coding of telephone speech and wideband audio

Advances in signal processing which have created several new technologies for high-quality digital audio are discussed. For traditional telephony, characterized by a signal bandwidth of about 3.2 kHz, the transmission rate for network quality speech is now down to 16 kb/s, with the prospect of a new standard from the CCITT. Robust communications quality appropriate for cellular radio has been realized at 8 kb/s. The current focus is on 4 kb/s, with the aim of improving naturalness and speaker identification. In the coding of wideband audio, an important point of reference is the CCITT standard for 7 kHz speech at a rate of 64 kb/s. Results of recent research are pointing to better capabilities-higher signal bandwidth at 64 kb/s, and 7 kHz bandwidth at lower bit rates such as 32 kb/s. The coding of audio with a signal bandwidth of 20 kHz is receiving significant attention due to recent activity in the ISO (International Standards Organization), with a goal of storing a CD-grade monophonic audio channel at a bit rate not exceeding 128 kb/s. Prospects for accomplishing this are found to be very good. As a side result, emerging algorithms will offer very attractive options at lower rates such as 96 and 64 kb/s.<
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