SPEECH CODING IN GSM ECE 2526 MOBILE COMMUNICATION

SPEECH CODING IN GSM ECE 2526 – MOBILE COMMUNICATION Monday, 18 March 2019

PCM WAVEFORM GENERATİON PCM block diagram shows the conversion of the 8 bit digital signal into the PCM signal.

REGULAR PULSE EXCITED - LONG TERM PREDICTION (RPE-LTP) /01 1. The Regular Pulse Excited - Long Term Prediction (RPE-LTP) speech encoder of the GSM is the result of intense development work. 2. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited Linear Predictive Coder (RPE_LPC) with a Long Term Predictor loop.

REGULAR PULSE EXCITED - LONG TERM PREDICTION (RPE-LTP) /02 • Speech is divided into 20 millisecond samples, which contains 104, 000 × 20 = 2080 kbits. • Each sample is encoded using 260 bits, giving a total output bit rate of: 260 / 20 x 10 -3 = 13 kbps. • The 260 bits are composed of three groups as follows: 36 LPC bits; 36 LTP bits; 188 RPE bits. LPC Linear Prediction Coefficients – representing a set of parameters that are obtained from the human vocal tract system. RPE Regular Pulse Excited

GSM SPEECH CODİNG A-law PCM conversion PCM encoder using linear quantization (with companding) at 13 bits/sample. RPE-LTP Regular Pulse Excited - Long Term Prediction used to reduce the digital speech rate to 13 kbps

GSM SPEECH CODİNG PCM Speech-Coded Signal PCM coded speech at 64 Kbps is received from MSC. Extracted from an E 1 TRAU Transcoder Unit (TRAU) consists of the following 1. An 8 -bit A-law converter creating 13 bit uniform quantized signal at 104 Kbps. 2. REP/LTP block which does code conversion from PCM to RPE/LTP protocol. 8 -bit A-law to uniform conversion

WHAT IS TRANSMITTED FOR VOICE IN GSM? 20 ms of 104 Kbps stream = 2080 bits Best coefficients are selected get the best approximation of the 20 m. S sample Low order coefficients are coded as 6 -bit while Higher order are coded as 3 -bit 7