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.