Multiplexing and Demultiplexing Transmitting two or more signals simultaneously can be accomplished by running multiple cables or setting up one transmitter-receiver pair for each channel, but this is an expensive approach. A single cable or radio link can handle multiple signals simultaneously using a technique known as multiplexing. Multiplexing permits hundreds or even thousands of signals to be combined and transmitted over a single medium.
Topics Covered in Chapter 10 Multiplexing Principles Frequency Division Multiplexing Time Division Multiplexing Pulse-Code Modulation
Multiplexing Principles Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel. It increases the number of communication channels so more information can be transmitted. An application may require multiple signals. Cost savings can be gained by using a single channel to send multiple information signals.
Multiplexing Applications Three communication applications that would be prohibitively expensive or impossible without multiplexing are: Telephone systems Telemetry Broadcasting (Radio and TV)
Frequency Division Multiplexing In frequency division multiplexing (FDM) multiple signals share the bandwidth of a common communication channel. All channels have specific bandwidths A wide bandwidth can be shared for the purpose of transmitting many signals at the same time.
Transmitter-Multiplexers In an FDM system each signal to be transmitted feeds a modulator circuit. The carrier for each modulator is on a different frequency. The carriers are equally spaced from one another. These carriers are referred to as subcarriers. Each input signal is given a portion of the bandwidth.
Transmitter-Multiplexers (Continued) The modulator outputs containing the sideband information are added algebraically in a linear mixer. The resulting output signal is a composite of all the modulated subcarriers. This signal can be used to modulate a radio transmitter, or can itself be transmitted over a single channel The composite signal can alternatively become one input to another multiplexed system.
Receiver-Demultiplexer In an FDM system a receiver picks up the signal and demodulates it, recovering the composite signal. The composite signal is sent to a group of bandpass filters, each centered on one of the carrier frequencies. Each filter passes only its channel and rejects all others. A channel demodulator then recovers each original input signal.
Telemetry Sensors in telemetry systems generate electrical signals which change in some way in response to changes in physical characteristics. An example of a sensor is a thermistor, a device used to measure temperature. A thermistors resistance varies inversely with temperature.
Telemetry (Continued) The thermistor is usually connected into a resistive network, such as a voltage divider or bridge. The thermistor is also connected to a DC voltage source. The result is a DC output voltage which varies in accordance with temperature. This voltage is transmitted to a remote receiver for measurement, readout, and recording. The thermistor becomes one channel of an FDM system.
By Definition… The varying direct or alternating current changes the frequency of an oscillator operating at the carrier frequency. Such a circuit is called a voltage- controlled oscillator (VCO) or subcarrier oscillator (SCO). Most VCOs are astable multivibrators whose frequency is controlled by the input from the signal conditioning circuits. A system that uses FM of the VCO subcarriers as well as FM of the final carrier is called FM/FM.
Telephone System For decades, telephone companies used FDM to send multiple telephone conversations over a minimum number of cables. The original voice signal, in the 300- to 3000-Hz range is used to modulate a subcarrier. Lower sideband (LSB) SSB AM was used. Each subcarrier is on a different frequency, and those subcarriers are then added together to form a single channel. The FDM system has been replaced by an all-digital time multiplexing (TDM) system.
Cable TV In a cable TV system, TV signals, each in its own 6- MHz channel, are multiplexed on a common coaxial or fiber-optic cable and sent to homes. Each 6-MHz channel carries the video and voice of the TV signal. Coaxial and fiber-optic cables have an enormous bandwidth and can carry more than one hundred TV channels. Many cable TV companies also use their cable system for Internet access.
FM Stereo Broadcasting In recording original stereo, two microphones are used to generate two separate audio signals. Two microphones pick up sound from a common source, such as voice, but from different directions. The separation of the two microphones provides sufficient differences in the two audio signals to provide a realistic reproduction of the original sound. FDM techniques are used to transmit these independent signals by a single transmitter.
Time Division Multiplexing In FDM, multiple signals are transmitted over a single channel, each signal being allocated a portion of the spectrum within that bandwidth. In time division multiplexing (TDM), each signal can occupy the entire bandwidth of the channel. Each signal, however, is transmitted for only a brief period of time. TDM can be used with both digital and analog signals.
PAM Multiplexer The simplest time multiplexer operates like a single- pole multiple-position mechanical or electronic switch that sequentially samples the multiple analog inputs at a high rate of speed. The switch arm dwells momentarily on each contact. This allows the input signal to be passed to the output. It then switches quickly to the next channel, allowing that channel to pass for a fixed duration. The remaining channels are sampled in the same way.
Four Channel PAM Sampling Four different analog signals can be sampled by a PAM multiplexer. Signals such as: Signals A and C are continuously varying analog signals. Signal B is a positive-going linear ramp. Signal D is a constant DC voltage.
Electronic Multiplexer In practical TDM/PAM systems, electronic circuits are used instead of mechanical switches or commutators. The multiplexer itself is usually implemented with FETs. FETs are nearly ideal ON-OFF switches and can turn off and on at very high speeds.
PAM Demultiplexer Once the composite signal is received, it must be demodulated and demultiplexed. The signal is picked up by the receiver. The signal is sent to an FM demodulator that recovers the original PAM data. The PAM signal is then demultiplexed into the original analog signals.
Demultiplexer Circuit Once the composite PAM signal is recovered, it is applied to a demultiplexer (DEMUX). The demultiplexer is the reverse of a multiplexer. It has a single input and multiple outputs. Most demultiplexers use FETs driven by a counter-decoder. Individual PAM signals are sent to op amps, where they are buffered and amplified. They are then sent to low-pass filters, where they are smoothed into the original analog signals. The main problem encountered in demultiplexing is synchronization.
By Definition… Clock recovery circuits used to generate the demultiplexer clock pulses are used to remedy the synchronization problem. After clock pulses of the proper frequency have been obtained, it is necessary to synchronize the multiplexed channels. This synchronization is achieved by using a special synchronizing (sync) pulse applied to one of the input channels at the transmitter.
Pulse-Code Modulation The most popular form of TDM uses pulse-code modulation (PCM). With pulse-code modulation, multiple channels of digital data are transmitted in serial form. Each channel is assigned a time slot in which to transmit one binary word of data. The data streams from the various channels are interleaved and transmitted sequentially.
PCM Multiplexer When PCM is used to transmit analog signals, the signals are sampled with a multiplexer. The signals are then converted by an A/D converter into a series of binary numbers. Each number is proportional to the amplitude of the analog signal at various sampling points. These binary words are converted from parallel to serial format and then transmitted.
PCM Demultiplexer At the receiving end of the communication link, the PCM signal is demultiplexed and converted back into the original data. The PCM baseband signal may come over a cable. If the PCM signal has modulated a carrier and is being transmitted by radio, the RF signal will be picked up by a receiver and then demodulated. The original serial PCM binary waveform is recovered and fed to a shaping circuit to clean up and rejuvenate the binary pulses. The original signal is then demultiplexed by means of a digital demultiplexer using AND or NAND gates.
Benefits of PCM PCM is reliable, inexpensive, and highly resistant to noise. In PCM, the transmitted binary pulses all have the same amplitude and can be clipped to reduce noise. Even when signals have been degraded because of noise, attenuation, or distortion, all the receiver has to do is determine whether a pulse was transmitted or not. PCM signals are easily recovered and rejuvenated.
Digital Carrier Systems The most widespread use of TDM is in the telephone system. Years ago, the telephone companies developed a complete digital transmission system called the T- carrier system. The T-carrier system defines the range of PCM TDM systems with progressively faster data rates. The physical implementations of these systems are referred to as T-1, T-2, T-3, and T-4.
T-Carrier Systems T-1 systems transmit each voice signal at a 64-kbp/s rate. They are also used to transmit fewer than 24 inputs at a faster rate. T-2 systems are not widely used except as a steppingstone to form DS3 signals. T-1 and T-3 lines are widely used by business and industry for telephone service as well as for digital data transmission. T-2 and T-4 lines are rarely used by subscribers, but they are used within the telephone system itself.