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• In Digital Frequency Modulation or Digital Even though there is only one telephone wire Phase Modulation, the carrier frequency does between the two cities,each couple gets to speak not sweep smoothly in step with the information for a total of 20 minutes each hour. But after one signal. Rather, the c-trier frequency or phase minute of conversation,each couple must wait (almost the same thing) jumps in fixed incre- another two minutes for their next time slot.All ments based on the instantaneous sample of six callers must wear synchronized watches,and the input signal. TDMA is a particular digital plugging and unplugging the telephones is tedious. signal format that some cellular networks have To avoid such problems,TDM systems automate begun to use. TDMA takes advantage of a and speed up the process.Switching occurs hun- technique called Time Division Multiplexing dreds or thousands of times per second,so fast that (TDM) which breaks up each conversation two people cannot tell that they are sharing one into samples, and then interleaves samples of telephone line with many others. the different conversations into one signal "pipeline" in a particular sequence,using specific Cellular networks combine TDM and digital modulation in the format called TDMA. A time slots for each sample. TDMA cellular phone continuously samples its Using wired telephones, let us explore the core user's voice and converts those samples into concept of TDM.Suppose geography separates strings of digital words.The phone then transmits three couples who want CO speak to each other. the digital words via FM or PM in a specific time There are three telephones in each city,but only slot assigned by the base station handling the call. one wire spans the distance between the two The TDMA format used in North America allows cities involved.The members of Couple A plug three phone conversations to share one 30-kHz their telephones into the long-distance circuit and channel and the same base-station equipment speak for the first minute of each hour.They then without confusion by sampling each conversation unplug their phones,and the members of Couple twice during one basic TDMA time frame.The • B plug theirs in.Then they speak for the next sequence of samples is A1/B1/C1/A2/B2/C2(sam- minute and disconnect.The members of Couple ple 1 of conversation A,followed by sample 1 of C plug in and speak until three past the hour.The conversation B, then sample 1 of conversation C, entire process then repeats. followed by sample 2 of conversation A,etc.) ANALOG vs.DIGITAL user's voice into an analog AC electrical current Electronic circuits fall into two categories:analog An electronic circuit continuously samples the and digital.,Analog signal levels vary over a con- microphone's output signal and converts it into a tinuous range.A phonograph is analog.Its mag- sequence of digital words,made up of binary netic cartridge generates a smoothly varying signal numbers,that describe the voice's instantaneous as the stylus moves up and down and side to side volume and pitch,just as each movie frame in the record groove.But the simplest digital circuit captures the actors'instantaneous positions and has only two output levels(1 and 0),generated by expressions. The more bits in each word,and the turning on or turning off an electronic switch.By more digital words per second,the greater the sequentially keying the switch on and off like a system's ability to distinguish among subtle differ- telegraph,one can construct strings of digital bits ences in signal values. and digital words(bytes). The digital telephone then transmits the sequence Both analog and digital circuits can be used for of digital words encoded in radio form. The base communications. Traditional home telephones,as station receives the radio signal,extracts the digital well as AM/FM broadcast and television receivers, words, converts them back into analog form,and are analog.Certain state-of-the-art office telephones; introduces the regenerated analog signal into the cordless telephones,and satellite transmitters are telephone network.It all happens so quickly that a digital. listener is unaware the speaker's voice travelled Digital telephone systems(both wired and wireless) as a series of discrete samples. This is like an audience remaining unaware(so long as the are actually part digital and part analog.Human projector works properly)that a movie is really a voices vary continuously over a range of volumes sequence of still images. and frequencies in analog fashion.So the micro- phone in the telephone handset converts the El