The Modulator converts an input signal into a form suitable for RF transmission. The Demodulator recovers the original signal. Since the RF signal is a based sine wave oscillating around the transmission frequency, the Modulator has to modulate this sine wave in a way so that all information is transmitted and well-recoverable but only as little Bandwidth as allowed is allocated.
|BPSK||Binary Phase Shift Keying|
|QPSK||Quadrature Phase Shift Keying|
|QAM-16||16-state Quadrature Amplitude Modulation|
|QAM-32||32-state Quadrature Amplitude Modulation|
|QAM-64||64-state Quadrature Amplitude Modulation|
|QAM-128||128-state Quadrature Amplitude Modulation|
|QAM-256||256-state Quadrature Amplitude Modulation|
|8VSB||8-state Vestigial Side Band Modulation|
|Orthogonal frequency division modulation|
Discrete multitone modulation
If you interested in the inner workings and the theory behind demodulators for digital TV, there are some signal processing tutorials at http://www.complextoreal.com/.