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45 Speech Coding

Digital speech coding is used in a wide variety of everyday applications that the ordinary person takes for granted, such as network telephony or telephone answering machines. | Richard V.Cox. Speech Coding. 2000 CRC Press LlC. http www.engnetbase.com . Speech Coding Richard V. Cox AT T Labs Research 45.1 Introduction Examples of Applications Speech Coder Attributes 45.2 Useful Models for Speech and Hearing The LPC Speech Production Model Models of Human Perception for Speech Coding 45.3 Types of Speech Coders Model-Based Speech Coders Time Domain WaveformFollowing Speech Coders Frequency Domain WaveformFollowing Speech Coders 45.4 Current Standards Current ITU Waveform Signal Coders ITU Linear Prediction Analysis-by-Synthesis Speech Coders Digital Cellular Speech Coding Standards Secure Voice Standards Performance References 45.1 Introduction Digital speech coding is used in a wide variety of everyday applications that the ordinary person takes for granted such as network telephony or telephone answering machines. By speech coding we mean a method for reducing the amount of information needed to represent a speech signal for transmission or storage applications. For most applications this means using a lossy compression algorithm because a small amount of perceptible degradation is acceptable. This section reviews some of the applications the basic attributes of speech coders methods currently used for coding and some of the most important speech coding standards. 45.1.1 Examples of Applications Digital speech transmission is used in network telephony. The speech coding used is just sample-bysample quantization. The transmission rate for most calls is fixed at 64 kilobits per second kb s . The speech is sampled at 8000 Hz 8 kHz and a logarithmic 8-bit quantizer is used to represent each sample as one of 256 possible output values. International calls over transoceanic cables or satellites are often reduced in bit rate to 32 kb s in order to boost the capacity of this relatively expensive equipment. Digital wireless transmission has already begun. In North America Europe and Japan there are digital cellular phone systems already in .