Digitizing and Packetizing Voice参考.ppt

Describe Cisco VoIP Implementations Digitizing and Packetizing Voice Basic Voice Encoding:Converting Analog to Digital Analog-to-Digital Conversion Steps Sample the analog signal.采样语音信号. Quantize the samples.量化采样信号数据,创建PAM信号数据. Encode the value into a binary expression.对量化的信号数据进行编码. (Optional) Compress the samples to reduce bandwidth.可选的实施VOIP的数据压缩. Basic Voice Encoding:Converting Digital to Analog Digital-to-Analog Conversion Steps Decompress the samples, if compressed.如果数据被压缩,则解压缩采样数据. Decode the samples into voltage amplitudes, rebuilding the PAM signal.解码采样数据并滤波,重建PAM信号. Reconstruct the analog signal from PAM signals.从PAM信号数据重建模拟信号. The Nyquist Theorem Sampling rate affects the quality of the digitized signal.采样比率影响数字信号的质量. Nyquist theorem determines the minimum sampling rate of analog signals.Nyquist定理定义了针对语音信号最小采样的比率. Nyquist theorem states that the sampling rate has to be at least twice the maximum frequency.Nyquist定理规定了采样比率最少是最大频率的二倍. Example: Sampling of Voice Human speech uses 200–9,000 Hz.人类讲话的频率在200-9000Hz之间. Human ear can sense 20–20,000 Hz.人类听觉频率维持在20-20000Hz之间. Traditional telephony systems were designed for 300–3,400 Hz.传统的电话系统设计频率为300-3400Hz. Sampling rate for digitizing voice was set to 8,000 samples per second, allowing frequencies up to 4,000 Hz.数字语音技术的采样比率为8000次每秒,可以达到4000Hz频率. Quantization Quantization is the representation of amplitudes by a certain value (step).量化通过数值(步长)来描述振幅. Scale with 256 steps is used for quantization.量化使用256步长刻度. Samples are rounded up or down to closer step.采量数据被舍入到最接近的刻度. Rounding introduces inexactness (quantization noise).舍入规则会导致不精确的数据产生.(量化噪音和量化错误). Quantization Techniques Linear quantization: Lower signal-to-noise ratio (SNR) on small signals Higher SNR on large signals Logarithmic quantization provides uniform SNR for all signals Provides higher granularity for lower signals Corresponds to the logarithmic behavior of the human ear Example: Quantization of Voice There are two methods of quant