音频编解码技术介绍.ppt

* ADPCM: Adaptive Pulse Coded Modulation 自适应脉码调制 LPC: Linear Predictive Coding线性预测编 ACELP: Adaptive Code Excited Linear Prediction MP-MLQ: Multipulse Maximum Likelihood Quantization MOS: Mean Opinion Scores 语音的平均意见得分 * PCM: Pulse Code Modulation ACELP: Algebraic-Code-Excited linear-Prediction MP-MLQ: Multipulse Maximum Likelihood Quantization LD-CELP: Low Delay Code Excited Linear Prediction CSA-CELP: Conjugate-Structure Algebraic-Code-Excited Linear-Prediction * 52 * 54 * 55 The TI chip was chosen because it has 64Kx32 SRAM, FPGA Dynamic Gate Array, and a bus driver/transceiver (channelized interface at up to 2.048 Mbps, to support T1 or E1 interfaces). The CELP functions are performed on the TI chip. Also, the TI chip is used for echo cancellation and fax detection. It is capable of 4 channels of PCM w/ echo cancellation/fax detection, 2 channels of ADPCM with echo cancellation/fax detection, or one channel of LD-CELP or CSA-CELP with echo cancellation/fax detection. * 56 * DTMF:Dual Tone Multi-Frequency * EC: Echo cancellation AGC: Automatic Gain Control PLC:Packet Loss Concealment * * QoS: Quality of Service * QoS: Quality of Service * * * * 59 * Packet Loss Concealment (PLC) algorithms, also known as frame erasure concealment algorithms, hide transmission losses in an audio system where the input signal is encoded and packetized at a transmitter, sent over a network, and received at a receiver that decodes the packet and plays out the output. Many of the standard CELP-based speech coders have PLC algorithms built into their standards. The objective of PLC is to generate a synthetic speech signal to cover missing data (erasures) in a received bit stream. Ideally, the synthesized signal will have the same timbre and spectral characteristics as the missing signal, and will not create unnatural artifacts. Since speech signals are often locally stationary, it is possible to use the signals past history to generate a reasonable approximation to th