Patent GB-1145345-A (GB)
Title: Compensator
IPC: H04N5/94
Filed: 1966-10-03
Abstract: 1,145,345. Television; transistor pulse and switching circuits. MINNESOTA MINING & MFG. CO. 3 Oct., 1966 [10 Nov., 1965], No. 44108/66. Headings H3T and H4F. Television.-A compensating circuit, Fig. 1, for suppressing defects due to " drop-outs " in a frequency modulated video signal derived, for example, from a magnetic tape, comprises a drop-out detector 14 arranged to operate a switch 16 when the undemodulated signal from pre-amplifier 12 has remained below a predetermined level for a predetermined interval of time so that the switch passes a one line period delayed signal to the output amplifier 26 instead of the direct signal from frequency demodulator 22, a delay device 18 being disposed before the frequency demodulator to compensate for delay in operation of the switch 16 having a delay equal to the delay in the drop-out detector 14 and any delay in switch 16. The invention eliminates over compensation resulting from noise or from large changes in the light amplitude by delaying (for at least 0.8 microseconds) the production of a control signal by detector 14 until it is relatively certain that a drop-out is present. Thus, instead of producing a control signal when the amplitude of the frequency modulated video signal falls below level 104, Fig. 2a, it is not produced until the video signal reaches position 108. Delay line 18 ensures that the drop-out compensation is effective over the complete period of drop-out which is represented by line 110, Fig. 2b. The period of drop-out compensation is extended past the end of the drop-out in the information signal to ensure stability in the output signal since the limiter 20 operates on a symmetrical basis. The invention relies for its effectiveness on the inherent similarities between adjacent lines of a television picture so that when a drop-out occurs replacement of a line or a portion of a line by a preceding line or portion of a line is virtually undetectable. Drop-out detector circuit, Fig. 3.-The frequency modulated video signal is applied via emitter follower 200, amplifiers 212 and 228, which suppress one-half of the frequencymodulated video signal, and emitter follower 244 to a tunnel diode 252 which utilizes the negative resistance portion of its characteristic so that when the amplitude of the frequencymodulated video signal is at normal level it is transmitted unattenuated to the base of transistor 254, but when the amplitude falls below apredetermined level it is shorted through the tunnel diode to ground. The tunnel diode cuts off the signal shortly after the amplitude falls below the level 104, Fig. 2a, the latter portions of the circuit providing the additional delay so that the control signal for switch 16, Fig. 1, is not generated until the amplitude stays below level 104 for a period equivalent to the distance 104-108. Transistor 254 operates as an emitter follower and its output is coupled to the base of transistor 266. When a normal signal with no drop-outs is present, transistor 266 is turned on during each positive cycle of the frequency modulated signal so as to discharge capacitor 274. This lowers the voltage at the emitter of transistor 272 and causes capacitor 282 to charge through diode 284. When capacitor 274 is discharged transistor 290 is non- conductive. When drop-out occurs transistor 266 is cut-off and capacitor 274 starts to charge with a charging current through transistor 272 and capacitor 282 in a direction opposite to the charging current which had previously charged capacitor 282. The reverse action of the charge in capacitor 282 provides for a delay in charging capacitor 274, so that the control signal does not occur until after a predetermined time. When the voltage across capacitor 274 builds up Zener diode 278 breaks down and switches transistor 290 to the conductive state. The voltage at the collector of transistor 266 rises in a linear fashion (ramp generator) and thus it is possible to control the point at which the Zener diode breaks down to switch transistor 290 conductive. When transistor 290 is turned on, transistor 300 becomes conductive and an output appears at its collector and capacitor 304 is charged. When the drop-out is ended, transistor 300 becomes cut-off but the charge on capacitor 304 slowly discharges so as to provide a control signal for an additional period of time after the drop-out has ended.