A new approach to a wide screen signalling decoder

Abstract

Describes a new concept of a wide screen signalling decoder. Wide screen signalling (WSS) is used in 625-line television systems to transfer status information concerning the actual content of the signal to a television receiver. It is transmitted once per frame in the first half of line 23 of a composite video baseband signal (CVBS). The WSS signal contains a run-in, start-code and biphase code modulated data. The main energy of the WSS signal is in the lower video spectral domain. Although the design of the signal is based on a 5 MHz clock, the new decoder concept differs from conventional ones in that it does not use the run-in for a PLL to gain acquisition and lock, it does not use the start-code to detect WSS in line 23 and it does not establish a measure of data reliability by checking on biphase code violations. The new decoder concept does use the tail of the run-in and start-code for timing estimation, it does perform biphase code demodulation of the biphase encoded data and the data reliability is estimated by comparing the data samples with a threshold level. This threshold is determined by a measure of rms disturbance. A measure of data reliability is used to determine whether there is WSS in line 23 or not. Simulations and measurements have shown that this new concept of a WSS decoder is very robust to multiple transmission impairments which are encountered in a television channel (terrestrial, cable and satellite). This new concept WSS decoder performs quite well even when a VHS or SVHS consumer type video recorder (VCR) is used for recording and replay of the signal.