Service Options 3, 68, 70, and 73 provide two-way voice communications between the base station and the mobile station using the dynamically variable data rate speech codec algorithm described in this standard. The transmitting speech codec takes voice samples and generates an encoded speech packet for every Traffic Channel frame.^† The receiving station generates a speech packet from every Traffic Channel frame and supplies it to the speech codec for decoding into voice samples.

Speech codecs communicate at one of four rates corresponding to the 9600 bps, 4800 bps, 2400 bps, and 1200 bps frame rates. However, Service Options 3 and 70 do not use 2400 bps frame rate.

The specifications defined in Sections 4 and 5 of this document provide the detailed algorithmic description of the EVRC. In the case of a discrepancy between the floating point and algorithmic descriptions, the reference floating point c-code will prevail. The specifications defined in Input Signal Preprocessing (see 4.4), Determining the Data Rate (see 4.7), and Adaptive Postfilter (see 5.8 and 5.9) are optional for implementations intended for varying operational environments (such as in-vehicle hands-free). Any implementations which deviate from the algorithms specified in this standard, shall meet the minimum performance requirements defined in the associated Minimum Performance Specifications.

^† Common speech codec terminology also uses the term frame to represent a quantum of processing. For Service Options 3, 68, 70, and 73, the speech codec frame corresponds to speech sampled over 20 ms. The speech samples are processed into a packet. This packet is transmitted in a Traffic Channel frame.