e station to the mobile telephone, the forward or downlink di- rection. The lower frequency, around 830 MHz, goes the other way, the reverse or up- link direction. The audio bandwidth supported on the AMPS channel goes from 300 to 3300 Hz, the standard analog telephone bandwidth. The voice signal on an FM channel modulates the frequency up and down from the center of the band. The AMPS channel allows maximum frequency deviation of 12 kHz, and the level is set SO that the nominal talker averages 2 kHz from the center of the band. 3 The voice transmission quality is improved by a two-to-one compandor system. The voice signal is compressed by two to one (in decibels) before it is sent out over the ra- dio airwaves. The receiver expands the voice signal by the same two to one to restore the original voice signal. This effectively doubles the loud-to-soft dynamic range in the conversation. The Dolby systems used in compact cassette recording and playback are similar to the AMPS compandor: They compress the high frequencies during record and expand them again in playback to suppress audible hiss. The AMPS compandor is used across the entire voice band rather than just high frequencies by compressing and expanding the entire AMPS audio bandwidth. When the voice signal gets 10 dB louder, for exam- ple, the compressed signal sent over the radio is only 5 dB higher. The receiver expands the 5-dB increase back to 10 dB so that the listener hears the voice as it was spoken. The effect of this compression and expansion is to reduce the perceived background noise dramatically. Speech coding algorithms do the same kind of processing in digital telephony. The two FM carriers in the two directions form one full-duplex AMPS voice channel. The channel is continuously on in both directions. While some portable telephones can pause their transmission (sleep) for short time intervals to save battery life, the chan- nel is not available for anybody else to use during the call. An AMPS mobile telephone has two radios, one for transmitti