Low Bit Rate Speech Coding

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Coding algorithms are intended to minimize the bit rate in the digitized version of a signal. It does so without any major loss in signal quality during encoding process. Speech coding became one of the integral part of digitized speech signal processing. High quality is attained at low bit rates by exploiting signal redundancy as well as the knowledge that certain types of coding distortion are difficult because they are masked by the signal. With the recent advancements in the microelectronics, cheap programmable processors and chips are available. This helps various players to make complex coding algorithms for signal redundancy and distortion masking which in turn help us in improving the quality of low bit rate signals.

A typical speech coding system have a Speech Source followed by  Sampler, A/D Converter, Source encoder, Channel encoder at the encoding end. At the decoding end, system like Channel decoder,  Source decoder and D/A Converter is used. Filter is used eliminate aliasing. Sampler is used for discrete time conversion.Speech coding algorithms are used in telecommunications in the range of 300 to 3400 Hz. In order to avoid aliasing, sampling frequency should be more than twice the continuous time signal bandwidth. Normally 8 kHz is chosen as standard sampling frequency. For high quality speech signal 16 Bits/Sample is used. To provide error protection to the bit-stream during transmission channel encoder is used.

Most important features of a good Speech Encoder are

  • Low Bit-Rate.
  • Flexibility across Different Speakers / Languages
  • Flexibility in the Presence of Channel Errors.
  • High Speech Quality
  • Good Performance on Non speech Signals
  • Low Coding Delay

Speech coders are classified into different categories with respect to their bit rate. For High end coders bit rate should be greater than 15 kbps. For medium speech coders, bit rate should be in the range of 5 to 15 kbps. For Low end coders bit-rate should be in the range of 2 to 5 kbps.

Different Coding Techniques are categorized into following

  • Parametric Coders
  • Waveform Coders
  • Hybrid Coders
  • Single Mode and Multi mode Coders

In order to preserve signal waveform's original shape, Wave form coders are applied on a signal source. Since the performance drops sharply when there is a reduction is bit rate, Wave form coder is best suited for high bit rate coding. In parametric coders, it is assumed that signal's are generated from a model which are controlled by certain parameters. Using the parameters transmitted as encoded bit stream, parameters are calculated from the input signal during encoding. Hybrid coder uses best features of Wave form coder and parametric coder. 

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