Modems and Modulation

Modulation is the technique of varying aspects of a single, pure frequency called a carrier frequency, to represent different numbers. The aspects of the carrier frequency: its amplitude (volume), frequency (pitch), and/or phase (relative position in time), are changed in fixed steps, called keying in days long past, shifting between fixed choices.

We'll begin with the simplest method, Amplitude Shift Keying, see how FSK is an improvement, PSK is better and QAM used in most modems today combines phase and amplitude shifting to create multiple possible combinations. Each combination is called a signal.

You'll learn that each possible combination can represent a group of bits, the number of bits determined by the number of combinations QAM-16 has sixteen signals, so each signal can represent a different group of 4 bits.

And, no discussion of modems would be complete without pointing out that the baud rate is the number of times the signal is changed per second. The bit rate is the baud rate times the number of bits indicated by each signal. In a QAM-16 system, the bit rate is four times the baud rate.

Telecom 101 Module 16

Detailed Outline

16 Modems and Modulation

.... 16.1 Modulation of Carrier Frequencies

.... 16.2 Amplitude Shift Keying (ASK)

.... 16.3 Frequency Shift Keying (FSK)

.... 16.4 Phase Shift Keying (PSK)

........... 16.4.1 Baud Rate vs. Bit Rate

.... 16.5 Quadrature PSK (QPSK)

.... 16.6 Quadrature Amplitude Modulation (QAM)

.... 16.7 Constraints on Achievable Bit Rate