Patent GB-1068881-A (GB)
Title: Improvements in and relating to television apparatus
Inventor: WALLER PETER JOHN
IPC: H04N7/169
Filed: 1964-04-07
Abstract: 1,068,881. Television. R. & R. RESEARCH Ltd. March 2, 1965 [April 7, 1964], No. 14258/64. Heading H4F. A television signal which is scrambled (coded) so that satisfactory reproduction is possible only by authorized receivers also bears decoy synchronizing information to cause incorrect response in conventional receivers. The required waveform is produced in the circuit of Fig. 2 (not shown) to which a standard television signal (Fig. 1, not shown) is supplied such signal being split into two paths leading to a synchronizing signal separator (21)-passing the standard synch pulses of Fig. 3A (not shown)- and a delay circuit (22) introducing a delay (t) and supplying the delay signal to separator stages (23), (24) passing, respectively, delayed synch (Fig. 3B) and delayed video (Fig. 3C). The latter is fed to a mixer (27) in which the blanking interval is increased by the addition of blanking pulses, produced in generator (25) from the undelayed synch pulses of Fig. 3A thus producing new blanking edges (Fig. 3D, not shown). This signal (Fig. 3D) is then fed to a combiner (28) in which during the stretched blanking intervals signals are inserted comprising either " bursts " of A.C. of duration (t1) derived from generator (26) synchronously with the occurrence of the undelayed synch pulses of Fig. 3A or D.C. pulses of duration (t2) or (t3) derived via switch (35) from generators (31), (32) synchronously with the occurrence of the delayed synch pulses of Fig. 3B. The order of insertion of A.C. " bursts " or D.C. pulses is controlled by gate (29) operated by a pulse generator (30)-producing the switching waveform of Fig. 4-(not shown) and the order of insertion of the (t2) or (t3) duration D.C. pulses is controlled by switch (35) operated according to a schedule or at random by pulse generator (36). The resultant scrambled signal Fig. 5 (not shown) is unusable by conventional receivers since the synchronizing circuits of such will not respond to the A.C. " burst " pulses and, in the case of flywheel synch circuits, although the leading edges of the D.C. pulses are correctly timed relative to the picture signals the trailing edges vary and so incorrect synchronization results. Additionally, trigger type synch circuits will tend to be triggered by the transient at the beginning of the extended blanking periods when the synchronizing information is conveyed by the A.C. " bursts ". For unscrambling such a signal the D.C. pulses are rejected and the A.C. " bursts " are accepted by a band-pass-filter (63, Fig. 6, not shown) and after conversion to D.C. by detector (65) are combined in (66) with the D.C. pulses derived via L.P. filter (64) and the leading edges of all pulses are then utilized in generator (68) to produce correct synchronizing pulses. In a second embodiment (Fig. 10, not shown) scrambling is effected by removing some but not all of the synchronizing signals (S, Figs. 7 and 8, not shown) and where such have been removed, extending the blanking interval and inserting synchronizing information in the form of A.C. pulses C together with, in some instances, decoy synchronizing pulses (D). For unscrambling this signal the A.C. pulses are separated out and utilized to control the production of correctly timed synchronizing pulses which are inserted in the waveform in place of the decoy pulses the A.C. pulses not being removed since their presence does not effect the synchronizing circuits.