| TETRA - the new digital trunked communication system,specifies Algebraic Codebook Excited Linear Prediction(ACELP),which is improved Codebook Excited Linear Prediction(CELP),as its speech codec . The bit rate of it is 4.567kbit/s. It adopts the structure of algebraic codebook and focused search of algebraic codebook to increase agility and randomicity,which decreases the complexity of this algorithm greatly. And its conversion of Line Spectral Pair (LSP) and the production of adaptive codebook are different from that of traditional CELP. So this codec was optimized to represent speech with a high quality at the above rates using a limited amount of complexity.The paper explains the speech codec implemented in real-time by using a single fixed-point DSP,TMS320C5402 and the testing of the performance of real-time TETRA Codec through many kinds of voice signal,including DTMF signal,English speech,Chinese speech,speech with noise and other sound. The results showed reconstructed speech represents relatively high quality in both objective analysis and subjective audio test. On the basis of the performance testing of our real-time codec is optimum algorithm introduced into pre-processing of the speech encoder of TETRA. It can advance the veracity of pitch detection. The results of simulationindicated speech signal processed by the optimum algorithm presentsobvious periodicity in time domain,and effect of the formant is removed orrestrained effectively in frequency domain.And the paper expatiates on the further research on the optimum algorithmfor solving the problem of noise disturbance,and experiments to examine itseffect to remove noise. Thus it discusses how to select the width ofnumerical value filter on the basis of a compromise in pitch detection andreconstructed speech quality.In order to be compatible with digital trunked communication,the paperintroduces to use CPLD to design the module of synchronous serialcommunication,which upgraded the trunked logic controller to expand itsfunctions.The real-time TETRA codec can be applied in practical environment.Success in real-time implementation of it in a single DSP makes contributionto production of TETRA mobile terminals and depreciation of them.Consequently it will advance digital trunked communication,especiallyTETRA,in China. Furthermore it can be used in other narrowbandcommunications. |
