GB1213357A - Improvements relating to television systems - Google Patents

Abstract

1,213,357. Television; multiplex radio signalling. MULLARD Ltd. 24 June, 1969 [11 July, 1968], No. 33157/68. Headings H4F and H4L. In a television system as claimed in any one of claims 1 to 5 of the parent Specification, the programme identification signals are added to sound components of the programmes for transmission therewith. Instead of being as in the parent, the identification signals may be as specified in Specification 1,183,338. Alternatively, the identification signals may be all of the same form e.g. bursts of a sinewave, being identifiable from each other by their relative timings in the respective video programme sound channels which are time division multiplexed as are the video programmes, see later. In the embodiment described the television system caters for the transmission of ten (educational) programmes only three of which are shown in Fig. 1 i.e. vision and sound pickups V and S (sources 1, 2 and 3). Gates G are sequentially opened for 0À5 second to pass 25 fields of each video signal for transmission as a time division multiplexed composite video signal. Each sound signal is fed via a low pass filter (cut-off 5À5 KHz) to an individual balanced sampler B wherein they are sampled once every line period sequentially (Fig. 2(c), no t shown). The outputs of all the samplers are combined to produce a time division multiplexed composite sound signal which amplitude modulates a sub-carrier (odd-multiple of one half of the line scanning frequency which is suppressed) and is added to the video signal. To allow reinsertion of the suppressed sub-carrier at the receiver a burst of sub-carrier is added to the video waveform during line blanking. The identification signals comprise short bursts of 6À5 KHz which are added to the input of each of the balanced samplers. This burst is preferably arranged to occur during the second field transmitted from the source to be identified (Fig. 2(d), not shown) although it may occur in different fields or be repeated during successive fields so long as it occurs before the last two fields from the source. One identification pulse (or set of pulses) is transmitted during each sequence of fields from the source so that identification signals recur at intervals of 5 seconds in any one sound channel i.e. they are related to the time of transmission of the corresponding video component (Fig. 2(e), not shown). In the receiver. Fig. 3(a), the video gate is caused to open for a period of two fields (one frame) during each of the sequences of pictures corresponding to the wanted programme. These fields are stored and repeatedly read out to the television display during the intervals between wanted pictures. The sub-carrier (sound) signals are demodulated and fed to the audio gate which is used to select only those samples corresponding to the wanted sound programme (Fig. 3(b), not shown). A gate pulse generator A controls the timing of the audio gate by producing sampling pulses from the line synchronizing pulses using a precisely timed delay. By varying this delay in steps by means of a programme selector switch S a viewer can select the desired sound component. The sound samples are passed through a filter the output of which contains both the wanted sound signal and the identification burst signal. This identification signal is selected by a 6À5 KHz tuned amplifier and passed to a gate pulse generator V where it is used together with line and field sync signals to control the opening of the video gate as described above. Thus selection of the sound signal automatically ensures selection of the appropriate vision signal. Instead of using a sub-carrier for the sound signals the normal sound carrier of the television channel may be used.