Patent GB-987898-A (GB)
Title: Improvements in or relating to systems for switching devices for sequentially transmitted signals
IPC: H04N9/47
Filed: 1962-09-21
Abstract: 987,898. Colour television; pulse, gating and bi-stable circuits; distributers. COMPAGNIE FRANCAISE DE TELEVISION. Sept. 21, 1962 [Sept. 22, 1961], No. 36100/62. Headings H3P, H4F and H4P. General; colour television.-Line sequential colour television signals A, B, Fig. 1(a), according to the French SECAM standards are converted to simultaneous signals with the aid of a switching system, Fig. 2, in which a switching circuit 7 receives inputs al and b corresponding respectively to the original signal, Fig. 1(a), and the same signal, Fig. 1(b), delayed for the duration T of one television line in delay line 6, and is controlled by a switching signal, Fig. 1(d), from a switching pulse generator 11 to interchange the connections from the two inputs a and b to two outputs g and e whereby there are obtained continuous trains of signals A and B each comprises alternately of original and delayed signals. An identifying signal S precedes only the A signals and the switching operation takes place during the identifying signal so that output e containing the B signals consists of at least an initial part of an identifying signal following each B signalsee Fig. 1(e), and similarly the output g containing the A signal contains a final portion of an identifying signal preceding each A signal. If the switching operation is faulty, e.g. at the commencement of operations or due to false or failed operation of switch 7, an A signal will appear incorrectly in output e and will not be followed by an identifying signal. In accordance with the invention, such faulty operation is detected by a checking circuit 10 which is connected to a point M in the channel for the B signals and responds to the identifying signals. If an identifying signal fails to follow a signal B, a connection K from circuit 10 to switching pulse generator 11 prevents a further operation of the generator and switch 7 remain static until the following line when a B signal appears and is followed in the normal manner by an identifying signal. Switching pulse generator 7 is then allowed to reoperate and switching continues in the correct phase relationship to the incoming signals. If the identifying signals are distinguished from signals A and B on an amplitude basis they may be separated out by an amplitude discriminator in circuit 10. Alternatively if the identifying signals are in the form of bursts of oscillation, or if it is desired to eliminate the effects of interfering signals, the signals may be gated out by line synchronizing pulses, Fig. 1(c), applied over connection c 2  to circuit 10. Switching pulse generator 11 comprises a bistable multivibrator or a relaxation oscillator of period greater than 2T and is triggered by line synchronizing c or fly-back pulses. In an alternative arrangement, where the generator comprises a bi-stable multivibrator, it is triggered by the identifying signals separated by circuit 10. Reference is made (without giving details) to an arrangement in which signals A and B are both preceded by identifying signals, the signals comprising, e.g. bursts of oscillation of different frequency, and circuit 10 responds to the presence of a signal of incorrect frequency. Although the invention is described in detail only as employed with two television signals it may be applied generally to any plurality of signals, switch 7 having a like plurality of outputs and a like plurality of inputs to which the original signal is applied through delay circuits introducing delays equal to zero, one, two, three &c. times the duration of an individual signal. Detailed circuits. (1) Circuit triggered by line synchronizing pulses. Fig. 3.-Switching pulse generator 11 comprises a bi-stable multivibrator T 1 , T 2  which is triggered back-and-forth by line synchronizing or fly-back pulses C applied via diodes D 1 , D 2  and has the grid of T 2  returned to earth via a triode T 3  (in checking circuit 10) which is normally maintained conductive by a bias produced across circuit C 3 , R 5  in response to the identifying signals (burst of oscillation) which are rectified in diode D 3 . If a signal fails to appear, the bias disappears and triode T 3  is cut off by source - V to stop the operation of generator 11. The identifying signals appear across tuned circuit 26, 27, rectifier D 3  being gated into operation by the line synchronizing pulses (positive-going) which appear through C 4  and are clamped with their maximum positive excursion at - V by diode D4. (2) Circuit triggered by identifying signals, Fig. 5.- Switching pulse generator 11 in this case comprises a cathode-coupled bi-stable multivibrator 69, 70 which is triggered on the grid of stage 70 by the identifying signals (burst of oscillation) which are derived from a tuned winding 58 on an output transformer 54 for the B signal channel from switch 7. The signals are separated by a gate stage 60, rendered conducting by positivegoing line synchronizing pulses c applied to the anode, and are applied to stage 70 via transformer 61 with tuned secondary 63, 64.