Modeling and performance of the north American broadcast teletext system with quasi-synchronous detection

In VSB systems the most critical region is the Nyquist region of the receiver response, and difficult to control in practice, so synchronous and quasi-synchronous detection are not perfect due to quadrature distortion. Difficulties with ideal carrier recovery techniques with narrowband reference channels which strip away Nyquist slope incidental phase modulation are discussed. These led to quasi-synchronous demodulators using carrier stripping, in which prefilter bandwidth is the major design variable. Problems with this are discussed which led to phaselocked loop (PLL) recovery in most television receivers. The advantages of PLL quasi-synchronous detection and the factors influencing their design are discussed. These factors include incidental phase modulation introduced by the Nyquist slope of the receiver IF filter in response to synchronization signals, audio buzz and the effects of other signal distortions, as well as other issues. Analysis, modeling and computer simulations were used to study quasi-synchronous detection in Gaussian and impulse noise and multipath environments, and used to find carrier recovery phase error and teletext system performance curves. Possible improvements to television carrier recovery systems are suggested. Several conclusions are given, the major one being that well-designed quasi-synchronous carrier recovery loops suffer relatively minor degradations (about 0.5 dB) compared to narrowband (synchronous) carrier recovery, in multipath and noise environments of interest to broadcast teletext application.