GB1007123A - Improvements relating to television standards conversion - Google Patents

Abstract

1,007,123. Television. BRITISH BROADCASTING CORPORATION. April 11, 1962 [April 12, 1961], No. 13104/61. Heading H4F. The invention relates to apparatus for converting from one field frequency to another comprising means for producing a picture display (i.e. a visible display or charge storage pattern) in writing scans at one field frequency and means for scanning the display in reading scans at the other field frequency. In such apparatus, in the case of a reduction in field frequency, an undesired signal amplitude change is introduced in the signal since, as illustrated in Fig. 1A (for the case of converting from 60 to 50 fields per second), periodically two writing scans (i.e. W 6 and W 1 ) occur between reading scans so that the second reading scan (R 1 ) results in a signal of enhanced amplitude as illustrated in Fig. 1B. Similarly, for the case of an increase in field frequency, as illustrated in Fig. 3A (for the case of converting from 50 to 60 fields per second), periodically two reading scans (i.e. R 6 and R 1 ) occur between two writing scans so that the second reading scan (R 1 ) results in a signal of reduced amplitude as illustrated in Fig. 3B. In accordance with the invention, this defect is eliminated by arranging to suppress the second of the two writing scans (i.e. W 6 ) in the case of a decrease in field frequency and to insert an additional writing scan in the case of an increase in field frequency, so that all the writing and reading scans occur alternately. In apparatus for carrying out the invention where the field frequency is to be decreased from 60 to 50 fields per second, Fig. 5 (not shown), a gate is included in the input channel and is controlled by a gate pulse generator so as to suppress the signals during the writing scan to be eliminated. Where the field frequency is to be increased from 50 to 60 fields per second, the apparatus may be arranged as shown in Fig. 6 with two channels 12A, 15A and 12B, 15B which are used alternately, the input signals being switched by gates 11A and 11B under the control of gate pulse generator 14A and 14B, whilst the output signals are derived from the channels alternately under the control of blanking pulse generator 19. The upper channel is used for odd fields, gate pulse generator 14A producing a wave however as shown in Fig. 7 (a) so as to insert an additional writing field, i.e. field 6, between fields 5 and 7, whilst the lower channel is used for even fields but with gate pulse generator 14B producing a wave as shown in Fig. 7 (b) so as to insert an additional writing field, i.e. field 5, between fields 4 and 6. The complete sequence of operation proceeds as shown in Fig. 4. In an alternative arrangement, Fig. 8, only a single channel is utilized but the display comprises an additional cathode-ray tube 12<SP>1</SP> by which the additional writing scan is inserted. The signals for the additional scan are obtained by delaying the input signals with the aid of a magnetic signal recorder and reproducer 20-22, the signals being applied to modulate tube 12<SP>1</SP> through a gate 25 which is opened periodically for 20 milliseconds, i.e. the duration of an input field, under the control of a signal at the difference of the field frequencies, i.e. 10 c./s. As an alternative to two display tubes 12, 12<SP>1</SP>, a single tube having two guns may be used.