Vocoders

Human voice is made up of a combination of voiced and unvoiced sounds

Vocoders exploit these properties of speech production mechanism

Vocoders do not respond to music, non-human sounds and tones from voice band modems

PCM vs VOCODERS

CDMA VOCODERS

8 kbps variable rate coder

  • Rate 1
  • Rate 1/2
  • Rate 1/4
  • Rate 1/8
Cdma development group 13.3 kbps VOCODER
8 kbps enhanced rate variable coder (evrc)

EVRC ( Enhanced Variable Rate Coder )

It is an 8 kbps vocoder that’s supposed to sound about as good as the current 13 kbps vocoder. Thus, you can have the same voice quality while improving the capacity of the system

Evrc uses a adaptive noise cancellation filter prior to encoding which ensures that noise never reaches the encoder which is tightly optimized for speech giving high quality voice at low bit rates.