Patent GB-1149051-A (GB)
Title: Improvements in or relating to television transmission systems
Inventor: JACKSON RICHARD NORMAN
IPC: H04N7/00
Filed: 1965-07-15
Abstract: 1,149,051. Television; multiplex radio signalling. MULLARD Ltd. 6 April, 1966 [15 July, 1965; 12 Nov., 1965], Nos. 30075/65 and 48119/65. Headings H4F and H4L. A method of television transmission consists of transmitting a majority of fields appertaining to a normal continuous (BTV) programme whilst interposing at intervals a minority of fields appertaining to a dis-continuous (ETV) programme constituted by a series of still pictures or other still information not conveying motion. The invention is described primarily as applied to educational television, where the still pictures comprise the educational material, but reference is made to the use of the invention for sending monitoring or warning information. In the simplest method of operation a single field of the ETV programme is inserted after every forty-nine fields of the BTV programme. An identification signal precedes the inserted field to permit its separation and utilization at a special receiver. In a normal receiver the inserted fields may be suppressed, but if displayed introduce only limited interference with the BTV programmes. In a second method, two ETV fields, which may be time adjacent or separated, are inserted, the signal in one field being inverted with respect to that of the other. At a special receiver the two fields are separated out and the one which is inverted is restored to normal polarity for use. At a normal receiver the inserted fields cancel visually. The effectiveness of the cancellation depends upon the manner of applying gamma correction at the transmitter. The specification discusses a first method which produces complete cancellation at a normal receiver but requires extra circuits at the special receiver to obtain proper gamma correction, and a second method which produces proper gamma correction at the special receiver but incomplete cancellation at a normal receiver. The ETV fields may be transmitted with reduced amplitude or with an amplitude selected or controlled automatically in accordance with the mean level of the BTV signal. The timing of the ETV fields may be controlled so that they are not inserted in the BTV signal where there is a large difference in the levels of the signals. In a third method of operation, two fields are inserted, one consisting of the sum and the other of the difference of the ETV and BTV signals. In a normal receiver the inserted fields combine visually to produce a half amplitude effect due to the BTV signal alone, whilst in a special receiver, in which the fields are separated and one inverted, a half amplitude effect due to ETV signal alone is produced. The effectiveness of the operation depends upon the manner of applying gamma correction and this is discussed in the Specification. Identification signals are inserted in the composite signal to indicate the presence of inserted fields and to distinguish one from the other. The signal may comprise a single pulse for one field and two for the other inserted in each of six successive lines in the field blanking intervals which precede the inserted fields. The pulses may comprise " bursts " of oscillation. In a special receiver, the identification signals are separated from the composite signal and employed to open a gate to separate the inserted ETV fields from the BTV signals. A switch is also controlled so as to pass one of each pair of fields through an inverter. The fields are then stored (e.g. in a cathode-ray tube store or magnetic tape or drum arrangement) so that they may be read out continuously to a cathode-ray display tube. A single store may be used, Fig. 1 (not shown), and the BTV component of the sum and difference signals is cancelled by visual integration in the display. Alternatively, where the two inserted fields form an interlaced pair the read-out for a cathode-ray tube store may be with an elongated beam so as to produce the mean of successive pairs of lines. Reference is also made to the use of four fields, two interlaced fields with sum signal and two with a difference signal. The two pairs of fields are superposed in the store so that the summation occurs in the storage target. Alternatively a pair of stores may be used. With two inserted fields, one field of each pair goes to one store and the other field to the second store. The stores are read out simultaneously to the display. This arrangement reduces the vertical resolution to one half. With four inserted fields normal resolution may be obtained. Where the stores require an appreciable time for priming and erase operations, additional control signals may be transmitted in advance of the main identification signals. In a fourth method of operation the sum and difference signals are sent alternately at line rate during two inserted interlaced fields, the sum signals being sent on the odd lines and the difference signals on the even lines of both fields. Alternatively four fields are sent and the oddeven sequence is reversed for the third and fourth fields. The phase of the sum-difference sequence is indicated by additional line back porch pulses or by pulses inserted in alternate lines of the field blanking interval. Reference is also made to a method in which an inserted field consists of alternate lines inverted in polarity.