Patent GB-530227-A (GB)
Title: Television scanning systems
IPC: H04N5/10
Filed: 1939-07-01
Abstract: 530,227. Television. HAZELTINE CORPORATION. July 1, 1939, No. 19208. Convention date, Aug. 10, 1938. [Class 40 (iii)] A television scanning system comprises scanning-wave generating means and synchronizing means by which scanning waves are developed which have their trace and retrace periods initiated at precisely predetermined times and is particularly applicable to field trace scanning in interlaced systems. A television receiving system comprises an antennaground system 10, 11, Fig. 1, a carrier-frequency translator 12, a detector 13, a videofrequency amplifier 14, and an image reproducer such as a cathode-ray tube 15. A synchronizing-signal separator 16 is connected to the output of the detector 13 and separates the synchronizing components l, f from the video-frequency components v in the received signal A, Fig. 2; so that the signal in the output circuit of the separator is of the form shown at B. The output circuit of the separator 16 is connected to a linefrequency scanning-wave generator 17 and a fieldfrequency scanning-wave generator 18 comprising a stabilizer and integrator 19, a differentiator 20, and a scanning-wave generator 21. The stabilizer and integrator 19 comprises a pentode 22 having an input circuit comprising a coupling condenser 23, a leak resistor 24 and a bias battery 25. The output circuit comprises a resistor 26 and a time-constant filter circuit including series resistors 27 and shunt condensers 28 connected across a resistor 26. The condenser 23, leak resistor 24 and bias battery 25 are of such values that the signal input to the pentode 22 is stabilized, i.e. the peaks of the synchronizing pulses in the signal applied from the separator 16 to the control grid of the valve 22 are maintained at a fixed level. Since the time constant of the anode circuit comprising the resistors 26, 27 and condensers 28 is long compared with the duration of the line-synchronizing pulses and line-blanking intervals, a voltage wave is produced across the condensers 28 having pulses occurring at the field-scanning frequency and of the wave form shown at C, Fig. 2. The limiter and differentiator 20 comprises a pentode 29 having an input circuit including a coupling condenser 30 and resistor 30a in series and a leak resistor 31 and biasing battery 31a, and an output circuit including an inductance 32. The characteristic of the valve 29 is such that it operates beyond both cut-off limits and thereby limits the voltage wave applied to it, so that a current wave of rectangular wave form, as shown at D, Fig. 2, flows through the inductance 32. Since the voltage across the inductance is the differential of the current, it is of spaced doublepulse wave form as shown at E, Fig. 2. The scanning-wave generator 21 comprises a pentode 33 having sharp upper and lower cut-off characteristics. The input circuit to the first grid includes a coupling condenser 36, leak resistor 34 and bias battery 35. The circuit of the second grid includes a load resistor 37 and a source of positive potential 38. The third grid is coupled to the second by a condenser 39, directly between the grids, and a resistor 40 and bias battery 41 connected between the third grid and the cathode by way of a resistor 44. The values of these elements are such that the network has a predetermined time constant related to that of the saw-tooth wave to be generated. The second grid is maintained at a higher potential than the third by the battery 38. In the anode circuit of the valve 33 is a load resistor 42 and condenser 43 having a time constant of the same order of magnitude as the period of the generated wave. A buffer amplifier is provided and comprises a pentode 45 having its input circuit coupled to the output circuit of the valve 33 by means of a coupling condenser 46 and leak resistor 47. The anode circuit is coupled to the field-scanning elements of the reproducer 15 by means of a condenser 48. The wave E, Fig. 2, is applied to the generator 21 so that a negative pulse initiates each retrace period and a separate positive pulse initiates each trace-scanning period.