TAILIEUCHUNG - Thực hiện tiếng nói qua IP (P2)

TECHNOLOGIES SUPPORTING VoIP1 In this chapter, we discuss and review various standard and emerging coding, packetization, and transmission technologies that are needed to support voice transmission using the IP technologies. Limitations of the current technologies and some possible extensions or modifications to support high-quality—that is, near-PSTN grade—real-time voice communications services using IP are then presented. VOICE SIGNAL PROCESSING For traditional telephony or voice communications services, the base-band signal between and KHz is considered the telephone-band voice or speech signal. This band exhibits a wide dynamic amplitude range of at least 40 dB | Implementing Voice over IP. Bhumip Khasnabish Copyright 2003 John Wiley Sons Inc. ISBN 0-471-21666-6 2 TECHNOLOGIES SUPPORTING VoIP1 In this chapter we discuss and review various standard and emerging coding packetization and transmission technologies that are needed to support voice transmission using the IP technologies. Limitations of the current technologies and some possible extensions or modifications to support high-quality that is near-PSTN grade real-time voice communications services using IP are then presented. VOICE SIGNAL PROCESSING For traditional telephony or voice communications services the base-band signal between and KHz is considered the telephone-band voice or speech signal. This band exhibits a wide dynamic amplitude range of at least 40 dB. In order to achieve nearly perfect reproduction after switching and transmission this voice-band signal needs to be sampled as per the Nyquist sampling criteria at more than or equal to twice the maximum frequency of the signal. Usually an 8 KHz or 8000 samples per second sampling rate is used. Each of these samples can now be quantized uniformly or nonuniformly using a predetermined number of quantization levels for example 8 bits are needed to support 28 or 256 quantization levels. Accordingly a bit stream of 8000 x 8 or 64 000 bits sec 64 Kbps is generated. This mechanism is known as the pulse code modulation PCM encoding of voice signal as defined in ITU-T s standard 1 and it is widely used in the traditional PSTN networks. i The ideas and viewpoints presented here belong solely to Bhumip Khasnabish Massachusetts USA. 15 16 TECHNOLOGIES SUPPORTING VoIP Low-Bit-Rate Voice Signal Encoding With the advancement of processor memory and DSP technologies researchers have developed a large number of low-bit-rate voice signal encoding algorithms or schemes. Many of these coding techniques have been standardized by the ITU-T. The most popular frame-based vocoders that utilize linear prediction with

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