3.6 Vocoder

As briefly mentioned in the previous sections, the CDMA phone uses a speech processor to process the voice signal more efficiently into the digital bit stream. This processor is known as the Vocoder ("voice-coder"), and it takes the voice signal that has been converted into a digital string by the A-to-D and refines the output into a more efficient form. Since the average conversation is idle 40 to 50 percent of the time, and since these gaps convey little or no meaningful content, simply shoving the analog voice through an A-to-D and sending the output down the line with little or no intelligence in the processing would represent a great deal of waste in the system with no useful benefit. Since idle channel time corresponds to wasted system capacity, a more ideal approach is essential in a system where capacity is one of the primary objectives. The Vocoder offers such a mechanism. There have been several versions of Vocoders developed for CDMA since cellular CDMA was first launched in the mid 1990s (IS-95A). Most typically rely on some form of a phoneme matching and synthesizer algorithm, to parse through the voice content and intelligently analyze and replace each sound segment, or phoneme (e.g. "Hh," "Rr," "Ki," "Te," "ee," "Ki," "Te," "ee," etc.) in the voice data by the corresponding address in a phoneme table which represents each vocal sound. The Vocoder then sends the table address on through the system in place of the actual 50 to...