What is vocoding | What is vocoder
This page explains vocoding basics.It mentions vocoder device and basic function of it.
Earlier days voice was transmitted without applying any compression method to the voice samples. This used to occupy more bandwidth on the channel. This was prevalent in first generation analog systems.
In current generation wireless standards such as 2G, 3G and 4G voice us transformed into digital 1's and 0's. This digital data is modulated on transmitted carrier using digital modulation techniques such as ASK, PSK , FSK etc. Now-a-days BPSK, QPSK, 16QAM, 64QAM modulations are widely used.
In order to use digital modulation, analog voice signal is converted into digital. This is done using A to D converter. ADC converter samples level of voice signal and transmits bit stream of ones and zeros in order to indicate either relative (differential step up or down) or absolure level. The two parameters viz. A to D sampling rate and number of bits needed to represent steps are important for A to D conversion.
Based on the above techniques,single voice channel data rate is about 64 Kbps in PCM (Pulse Code Modulation) standard. This takes up more bandwidth in wireless channel transmission. As we know bandwidth is very scarse resource in wireless system based communication. Techniques such as ADPCM represents samples differentially and hence data rate is reduced to 32 kbps from 64 kbps. However this is still higher.
In order to further reduce the data rate complex vocoders have been developed. The function performed by vocoder is referred as vocoding. Vocoders represent voice signal in the form of bits using the technique known as codebook. Using vocoders data rates below 13 kbps can be achieved.
Figure depicts vocoding process. Before vocoding process, analog signal is converted into digital signal. This process compresses amount of bits required to represent analog voice via prediction.
Examples of Vocoders:
CDMA-One uses QCELP (Qualcomm Code Excited Linear Prediction)
NA-TDMA uses VSELP (Vector Sum Excitation Linear Prediction) and ACELP (algerbraic code excited linear prediction)
Refer Speech Coding Basic Techniques➤➤
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