GB1481005A - Method and apparatus for effecting signalling over television signal channels - Google Patents

Abstract

1481005 Data transmission; television COMMUNICATIONS PATENTS Ltd 26 Sept 1974 [28 Sept 1973] 45420/73 Headings H4P H4F and H4M A method of signalling over a television channel comprises forming the unmodulated brightness carrier into two phase quadrature components which are modulated with separate signals and then combined for transmission over a common path. One component may be modulated with brightness signals and the other with data signals, or both components may be modulated with data signals and the resultant modulated with brightness signals. The frequency/amplitude response of quadrature modulated signal or signals may be shaped to minimize disturbance of brightness signals. A high frequency carrier is applied to input terminals 14 and passes through quadrature shift networks 15, 16 and applied to phase shift modulators 19, 20 each providing 0-180 degrees modulation. The modulated carriers are combined at 23 then passed through an amplitude modulator 24 receiving brightness signals 25. Data signals may be synchronized with sync. signals included in the brightness signal in ways known in the art, to occur at times when they will be least visible on reproduced television pictures. Such sync. signals may be utilized as clock signals for data. If modulators 19, 20 produce an adjustable phase shift, analogue instead of digital information may be transmitted. In another arrangement (Fig. 1, not shown) the brightness signal is modulated on to one carrier, e.g. at 21 and data modulated on to the other carrier e.g. at 22, the device 24 being omitted. The quadrature channel of Fig. 1 may be used alternatively for analogue information, e.g. sound transmission or analogue characters which may be displayed either alternatively with a television programme or superimposed upon it, e.g. sub-titles. At the receiver (Fig. 3, not shown) incoming signals are applied to a high frequency amplifier (28) then passed into parallel paths comprising demodulator (29) feeding a display arrangement (30) also through a phase locked loop (31) which demodulates the or one of the phase quadrature data signals which are then applied to utilization circuits (32). Where there are two sets of data, (31, 32) may be duplicated, or (31) may be common and is enabled selectively to demodulate either set of data by appropriate phasing of the loop. Both line and sync. signals may be utilized to resolve ambiguities in data signals.