Vocoders Convert Voice to/from Analog Using Data Compression
There are Three CDMA Vocoders:
- IS-96A Variable Rate (8 kbps maximum)
- CDG Variable Rate (13 kbps maximum)
- EVRC Variable Rate (improved 8 kbps)
- IS-96A – moderate quality
- EVCR – near toll quality
- CDG – toll quality
All digital communication systems use various processes to convert analog voice signals to and from digital form. Long distance telephone systems have used 8-bit PCM (Pulse Code Modulation) for many years to provide high quality voice transmission. Most PCM systems sample with eight bit resolution to convert voice into a digital data stream of 64 kbps.
In recent years, ADPCM (Adaptive Delta PCM) has become a popular alternative to straight PCM since it provides essentially the same voice quality as PCM while using only 32 kbps. This allows more voice channels to be sent on the digital network with quality loss.
CDMA now has three vocoder standards for converting voice to digital form while providing a high degree of data compression. The original vocoder as defined in IS-96A is a variable rate vocoder with a maximum rate of approximately 8 kbps.
This is quite an improvement over PCM or ADPCM encoders (four to eight times more efficient). However, because of the variable rate nature of this vocoder, the average bit rate is under 4 kbps!
The new CDG (CDMA Development Group) vocoder is also a variable rate vocoder but uses a maximum data rate of 13 kbps to provide essentially toll quality voice. The most recent addition is the EVRC (Enhanced Variable Rate Coder) that retains the maximum data rate of 8 kbps but yields voice quality just slight less than the CGD 13 kbps vocoder.