Patent GB-1137441-A (GB)
Title: Method for reproducing a single picture from a sequence of frames
IPC: H04N5/78
Filed: 1967-05-01
Abstract: 1,137,441. Reproducing single pictures from magnetic video tape. LOEWE OPTA G.m.b.H. 1 May, 1967 [6 Aug., 1966], No. 22045/67. Heading G5R. A method for reproducing a single picture from a sequence of television frames which are recorded by one or several rotating magnetic heads on magnetic tracks of a magnetic tape which run parallel to each other and which each contain the signals of one or more fields in a frequency-modulated form is characterized in that a switching device, after operation of a release switch, automatically stops the drive of the magnetic tape whenever a maximum arises in the blank zone of the scanned composite video signal as a result of a tape transition from one magnetic track to the neighbouring track or as a result of simultaneous scanning of two magnetic tracks separated by a gap. The FM video signal 20, Fig. 4, scanned by the head 14, is passed through a pre-amplifier 15, a limiter 16 and a demodulator 17; the output 19 at 18 may be reproduced by a monitor or television set. The signal 20 contains a blank zone 21 due to tape transition occurring during scanning of consecutive tracks. To utilize the blank zone for reproducing a single picture, a switch 22 is closed, to apply the signal to the circuitry 23 to 38. At stage 23, the signal is first rectified, and from the rectified signal the blank zone is filtered out by a low-pass filter to form a pulse which is conducted through a switching stage (e.g. a transistor) and emerges as a steep pulse 24; the width of pulse 24 depends on the duration of the blank zone. At 25 (an integration stage), a pulse is produced whose amplitude depends on the width of pulse 24; this pulse is conducted along two separate paths-one (with pulse designated 26), leading to stage 27, the other (pulse designated 30), leading to stage 31. At subtraction stage 27, pulse 26 is decreased in amplitude to form pulse 28 which in a negative peak-type rectifier circuit 29 produces a negative D.C. voltage. Pulse 30 passes directly to a positive peak-type rectifier circuit 31 where a positive D.C. voltage is produced. The two paths rejoin at an addition stage 32. When, after switch 22 is closed, the blank zone 21 reaches a maximum width, the voltage at the output of stage 32 is also a maximum, and only this maximum value is able to open a Schmitt trigger 34 (switch 33 being normally closed) which in turn excites relay 35 which serves to stop the tape. The position of the transducer relative to the tape for a maximum-width blank zone is shown at 14 in Fig. 3, where the transducer spans the region between two adjacent record tracks 9, 10, at the top, and also makes contact with the opposing edges of both tracks. If now the tape is stopped (by the circuit described), while the transducer continues to move downwards, the transducer path 13 will not be at the same angle to the tape axis as the record tracks 9, 10 &c. (because these were made with the tape moving). Thus, the transducer 14 soon leaves track 9, and at the middle of its path scans track 10 exclusively, while at the end of its path it spans the region netween tracks 10 and 11 and makes contact with both. Thus, during scanning the single picture corresponding to track 10 is reproduced.