Complete Communications Engineering

All of the three major analog TV standards are interlaced standards. A TV frame consists of two fields, top and bottom, following sequentially one after another. For NTSC the framerate is 29.97(30) frame per second (fps) with 59.94 (60) fields per sec, PAL and SECAM are 25 fps or 50 field per sec. Despite the fact that the major difference of NTSC, PAL and SECAM is the algorithm for chroma signal encoding, the modulation principles are essentially the same for all of them.

Digital AM/VSB Modulator
Fig.1 Digital implementation of AM/VSB modulator

For each of three, the standard chroma signal is encoded by the corresponding standard specific algorithm and modulated on the chroma subcarrier. This modulated signal is added to the luma baseband signal to form the composite signal.

Fig.2 Real, Imaginary and Amplitude Frequency responses of complex modulation filters and output AM/VSB spectrum

The composite signal is modulated to the carrier frequency by amplitude modulation with vestigial sideband (AM/VSB) modulation. It can be described as classical AM modulation

SM(t)  = SC(t) cos(2π fIF),

where SM(t) – is modulated signal, SC(t) – composite signal and fIF – intermediate frequency carrier frequency, and then one side of the modulated spectrum is attenuated by a passband filter. For analog implementation, a passband surface acoustic filter (SAW) may be utilized. Sound signal is an FM modulated signal (except SECAM-L that is AM modulated) on the sound carrier that is located slightly above the boundary of modulated video spectrum.

For DSP implementations, the composite signal that is a real signal passes through a complex baseband filter with spatial frequency response and then the complex signal is shifted to the intermediate frequency carrier by the digital rotator (multiplier by , wIF – digital IF, wIF = fIF/fS, fS – sampling frequency). The FM modulation sound signal may also be added to the real composite signal at baseband before complex modulation.

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