CHAPTER 8
Multiplexing
1. Multiplexing is the process of transmitting multiple signals over a single communications channel.
2. The primary benefit of multiplexing is economic since multiple communications can take place for the cost of a single link plus the multiplexing equipment.
3. The two major types of multiplexing are frequency division and time division multiplexing.
4. Some, examples of multiplexing include telemetry, telephone systems, satellite communications, and radio/TV stereo broadcasting.
5. In frequency division multiplexing (FDM), multiple signals share the common bandwidth of a single communications channel, each occupying a separate portion of the bandwidth.
6. In FDM, each signal modulates a subcarrier on a different frequency. The subcarriers are then linearly mixed to form a composite signal that is usually used to modulate a final carrier for transmission.
7. In FDM telemetry systems, transducers frequency modulate subcarrier oscillators whose outputs are combined and used to frequency modulate a final carrier (FM/FM).
8. A subcarrier oscillator is a VCO whose frequency varies linearly in proportion to the amplitude of a modulating dc or ac signal.
9. Recovering the individual signals at the receiver is done with a demultiplexer whose main components are bandpass filters tuned to the individual subcarrier frequencies.
10. In FM/FM telemetry systems, PLL and pulse-averaging discriminators are used to demodulate the subcarriers; PLL discriminators are preferred because of their superior noise performance.
11. The telephone system routinely uses FDM systems to allow many conversations to be carried on a single cable.
12. Single-sideband suppressed-carrier modulation is used in telephone FDM systems to minimize bandwidth requirements.
13. Stereo broadcasts from FM stations use FDM techniques. Two channels of audio L and R are combined to form L + R and L - R signals. The L - R signal modulates a 38-kHz subcarrier by DSB. The L + R signal, DSB signal, and a 19-kHz pilot carrier are combined and used to frequency modulate the final transmitter carrier.
14. In some FM broadcast systems, music is broadcast on a separate 67-kHz subcarrier which is frequency modulated. This is referred to as Subsidiary Communication Authorization (SCA) signal or channel.
15. In time division multiplexing (TDM), each channel is assigned a time slot and may transmit for a brief period using the entire bandwidth of the medium. Signal sources take turns transmitting.
16. Both digital and analog signals may be transmitted by TDM.
17. Analog signals are transmitted by converting them into a series of pulses whose amplitudes approximate the shape of the analog signal. This process is called pulse amplitude modulation (PAM).
18. Pulse-amplitude modulation is produced by sampling the analog signal. This is done by periodically opening a gate for a brief period, allowing a narrow portion of the analog signal to pass through.
19. Samples must be taken fast enough in order for high-frequency components to be recognized and adequately represented.
20. A minimum sampling rate is 2 times the highest frequency component or upper bandwidth limit of the analog signal. This is called the sampling theorem.
21. Pulse-amplitude modulation signals may be multiplexed by allowing samples of several signals to be interleaved into adjacent time slots.
22. Field-Effect transistor switches are commonly used in sampling gates and PAM multiplexers. They are controlled by digital circuits that set the sampling intervals and pulse rates.
23. The period of time during which each channel in a PAM system is sampled once is called a frame.
24. Pulse-amplitude modulation signals are normally used to frequency modulate another carrier creating PAM/FM.
25. Demultiplexing PAM signals requires some means of synchronization to ensure matching clock frequencies and channel timing at the receiver.
26. Special clock recovery circuits use the PAM signal itself to derive the clock signal at the receiver rather than generating it independently. This ensures perfect frequency and phase relationships.
27. A special sync pulse with a unique shape to distinguish it from the PAM pulses is used to keep the demultiplexer in synchronization with the multiplexer. The sync pulse usually occurs as the last pulse in a frame.
28. Most TDM systems in use today use pulse code modulation (PCM) to transmit analog signals.
29. Pulse-code modulation uses A/D conversion techniques to translate analog signal into binary form for serial transmission.
30. In a multichannel PCM system, each signal is furnished with an analog-to-digital converter (ADC) and parallel-to-serial converter. The resulting binary outputs are digitally multiplexed using TOM techniques.
31. Voice, telemetry data, video, and other analog signals may be transmitted via PCM.
32. Pulse-code modulation signals are recovered at the receiver by demultiplexing and D/A conversion plus appropriate filtering.
33. Pulse-code modulation is generated by periodically sampling the analog signal as in PAM systems. Then the varying amplitude pulses are converted into proportional binary words by an ADC. This process is called quantizing.
34. Quantizing means dividing a given signal voltage range into a number of discrete increments each represented by a binary code. Each analog sample is matched to the nearest binary level.
35. Sampling in a PCM system is done by a sample/hold (S/H) amplifier that stores the analog value on a capacitor at the instant of measurement.
36. Most ADCs have an 8-bit word providing a quantizing resolution of 1 in 256.
37. Most PCM systems, especially for voice transmission, use companding to compress the voice signal dynamic range by emphasizing lower-level signals and de-emphasizing higher-level signals. Companding minimizes quantization error and improves noise immunity.
38. Companding may be done by analog or digital techniques. Analog companders use diode circuits in amplifiers for compression and expansion. Digital companders use special nonlinear ADCs and DACs.
39. A common PCM system using TDM techniques is the Bell T-1 used in telephone work.
40. A T-1 system multiplexes 24 voice channels, each represented by an 8-bit word. The sampling interval is 125 ms. A frame consists of twenty-four 8-bit words and a single sync pulse. The frame data rate is 1.544 MHz.
41. Most PCM systems like T-l use a special LSI IC called a codec to handle conversion and companding. A codec contains the ADC, DAC, serial-to-paraIIel conversion circuits, companders, and related timing circuits.
42. The sampling rate of a codec is usually 8 kHz.
43. Pulse-width moduldtion (PWM) and pulse position modulation (PPM) systems are no longer used.
44. Most TDM systems use PCM though some use PAM.
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