Page 367 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

“Transmitted”
Transmitted signal
Amplitude modulation
Frequency modulation
Phase modulation
011001
011001
13.2 Transmission 359
FIGURE 13.33
Comparison of the three methods of modulation.
of varying a high-frequency waveform (or “carrier signal”) with a modulated signal containing the information to be sent (as opposed to the baseband signal centering its frequency on some zero value). On the transmitting side, a radio carrier is generated with the carrier described by a central frequency. The central frequency carries no useful information until such time as a time varying signal is impressed upon the central carrier frequency. Modulation is the process of putting that useful information upon the carrier frequency while demodulation reverses the process to bring the signal over the carrier back to baseband (or some other discernible format) for dissemination to the end user. Either a sinusoidal or a square pattern signal can be used for transmission.
The three methods of modulation (depicted graphically in Figure 13.33) are:
1. Amplitude modulation (AM) carrier is varied in amplitude according to the information in the baseband signal.
2. Frequency modulation (FM) carrier is varied in frequency with the baseband signal.
3. Phase modulation (PM) carrier is varied in phase in accordance with the information in the
baseband signal.
13.2.10 Multiplexing
Multiplexing is a communication technique used so that multiple communications channels can fit onto a common line. In the early days of trans-Atlantic telegraphing, only a single message could be tapped out at a time—that was an extremely expensive message! Later, the messages were mod- ulated to different carrier frequencies and multiplexing was born.