Introduction

Digital television signals are mainly transmitted over the air, which is also called terrestrial transmission, through cable systems, by geostationary satellites, using a microwave carrier, and over the Internet. For each transmission channel, a specific modulation scheme must be selected as the most appropriate, depending on the type of noise, power limitation, fading characteristics, transmission rate or cost.

Carrier waves are employed to allow efficient radiation of radio signals, because it is important to match the wavelength and the principal dimension of the transmitting and receiving antennas. The information is represented by an electrical signal, called a modulating signal, used to modify one or more parameters of the carrier. Appendix B presents a review of signal analysis and the use of the frequency spectrum by several services.

Modulation is the variation of one or more characteristics of the carrier waveform as a function of the modulating signal. The sinusoidal waveform is traditionally used as the carrier, and the modulation can be performed in three distinct ways:

• Amplitude modulation (AM), if amplitude is the carrier parameter that is varied. This is the oldest modulation scheme; its fatherhood was disputed by Lee DeForest and Howard Armstrong.

• Quadrature modulation (QUAM), if both the amplitude and the phase of the carrier are varied simultaneously. Single sideband (SSB) was the first QUAM scheme.

• Angle modulation, if either the phase (PM) or the frequency (FM) is the carrier parameter that changes. The first FM modulator was designed by Howard Armstrong in 1933.