Modeling and performance of the North American Broadcast Teletext System with quasi-synchronous detection

In vestigial sideband systems the most critical region is the Nyquist region of the receiver response, which is difficult to control in practice, so that synchronous and quasi-synchronous detection are not perfect due to quadrature distortion. The advantages of phase-locked-loop 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. Analysis, modeling, and computer simulations have been used to study quasi-synchronous detection in Gaussian and impulse noise and multipath environments and to find carrier recovery phase error and teletext system performance curves. Possible improvements to television carrier recovery systems are suggested. It is concluded 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