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Speech Coder Types

There are two types of speech coders:

 - Waveform Coders
 - Source Coders

The operating ranges (data rates) of these two coder types are depicted in the following diagram:

Waveform Coders

These types of coders digitize speech on a sample-by-sample basis. The goal is to have the output waveform closely match the input waveform. There are two types of waveform coders:

  - Time-Domain Waveform Coders
  - Spectral Waveform Coders

Time-Domain Waveform Coders

Time domain waveform coders utilize digitization schemes based upon the time- Domain properties of speech. Some examples:

            PCM        Pulse Code Modulation
            log PCM    Mu-Law PCM, A-Law PCM
            ADPCM      Adaptive Differential Pulse Code Modulation
            SB-ADPCM   Sub-Band ADPCM (Broadcast Quality)
            CVSD       Continuously Variable Slope Delta
            APC        Adaptive Predictive Coding
            DPCM       Differential Pulse Code Modulation
            VQL        Variable Quantizing Level

Standards:  64K log PCM     CCITT G.711 - G.714,  AT&T Pub 43801
            32K ADPCM       CCITT G.721, ANSI T1.301
            64K SB-ADPCM    CCITT G.722

Spectral Waveform Coders

Spectral waveform coders use digitization schemes based upon the frequency- domain properties of speech. Some examples:

            SBC        Sub-Band Coding
            ASET       Adaptive Sub-band Excitation Transform
            ATC        Adaptive Transform Coding
            HC         Harmonic Coding
            TDHS       Time Domain Harmonic Scaling
            ATC-HS     Adaptive Transform Coding Harmonic Scaling
            SBC-HS     Sub-Band Coding Harmonic Scaling

Source Coders (Vocoders)

Source coders use digitization schemes that attempt to describe the input signal in terms of speech model characteristics. The goal is to obtain a high level of perceptual quality rather than waveform accuracy. Some examples:

            LPC        Linear Predictive Coding
            RELP       Residual Excited Linear Prediction
            RPE-LTP    Residual Pulse Excited-Long Term Prediction
            CELP       Codebook Excited Linear Prediction
            LDCELP     Low Delay Codebook Excited Linear Prediction

Standards:  16K LDCELP      CCITT G.728
            13K RPE-LTP     European Digital Mobile Radio Standard