Patent GB-976146-A (GB)
Title: Television standards converter apparatus
IPC: H04N7/01
Filed: 1961-08-08
Abstract: 976,146. Television. FERNSEH G.m.b.H. Aug. 8, 1961 [Aug. 6, 1960], No. 28581/61. Heading H4F. In a television standards converter apparatus in which television signals at a first field repetition rate are applied to a storage tube Ro1 Fig. 1b as indicated by the solid line and are read off at a second higher field repetition rate as indicated by the dotted line, distortion which would be produced in fields when the reading and writing beams traverse a portion of the storage target coincidently, i.e. the fourth whole field as illustrated, is prevented by the provision of a second storage tube Ro2, the signals at the first field rate being also applied to this tube during the field preceding the one during which distortion would be produced and the signal at the increased rate being derived during the next field period from this tube instead of the first. Since the second tube is used only occasionally, the signal mainly coming from the first tube, precise geometrical registration of the rasters is not essential, but any distortion due to this cause may be limited by restricting the frequency response of the second tube. Apparatus embodying the invention is shown in Fig. 2. Where the two field frequencies are locked together, control of tubes Ro1, Ro2 may be effected by a frequency divider 8 receiving an input of field synchronizing pulses V 1  at the first field rate. For converting from 50 to 60 fields per second the divider divides by 5. The divider produces pulses equal in duration to one field which gate on the writing beam of tube Ro2, and by means of circuit 10 (the details of which are given in Fig. 3, not shown), which is also controlled by field synchronizing pulses V 2  of the second field rate, control the reading beams of tubes Ro1 and Ro2 during the next following field at the second field rate, gating on tube Ro1 and gating off tube Ro2. A polarity inverter is included between circuit 10 and tube Ro2 to produce a signal of correct polarity for blanking. Where the two field rates are not locked, the circuit arrangement shown above the dotted line in Fig. 2 is used. Field synchronizing pulses V 1  at the first field rate are employed by means of circuits 1 and 2 to produce pulses which vary in amplitude in accordance with the instantaneous field frequency, being zero when the frequency equals that of the second field rate and becoming increasingly positive and negative as the frequency decreases and increases respectively with respect to the second field rate. The pulses are combined at 5 with saw-tooth signals at the second field rate which are derived from field synchronizing pulses V 2  which are first converted to a sinewave at 18 and then applied through a variable phase-shifter 4. The combined signal is applied to a clipping stage and associated switching stage 3 which, when the combined signal exceeds a predetermined level, produces an output pulse. This pulse is then employed to control tubes Ro1 and Ro2 in the same manner as described above for the pulse from divider 8. The apparatus may also be employed when conversion is effected to a reduced field-scanning rate. In this case the second tube Ro2 is not utilized and it is merely necessary to blank the writing beam of tube Ro1 during the fields when the writing and reading beams will scan the target coincidently. In the case where the two field rates are locked, the signal from the output of circuit 10 is utilized by closing switch 12. An inverter 13 is included in the circuit to provide a signal of correct polarity for beam blanking. Where the two fields are not locked, the control signal is derived from a limiter and switch 9 to which is applied a signal similar to that obtained from circuit 5 but obtained by combining in circuit 7 a saw-tooth at the second field frequency with a pulse from circuit 2 of opposite polarity to that applied to circuit 5. The saw-tooth is derived in a similar manner to that in circuit 5 from a sinewave applied through a phaseshifter 6. The apparatus may also operate when the two field rates are equal and the line rates differ. Only one of the storage tubes is arranged to be operative and the vertical flybacks of the input and output scans are displaced in time to prevent coincidence of writing and reading.