Patent GB-898723-A (GB)
Title: Television standards converter
Inventor: URRY DENIS EDWARD
IPC: H04N7/01
Filed: 1959-06-18
Abstract: 898,723. Editing television tapes. BUSH & RANK CINTEL Ltd. June 20, 1960 [June 18, 1959], No. 20964/59. Class 40 (2). [Also in Group XL (b)] Apparatus for television standards conversion includes means for recording a television signal on tape during relative movement between said tape and recording means, a plurality of playback means arranged to engage said tape in spaced succession to derive signals therefrom during relative movement between said playback means and said tape, and means for allowing signals from each in turn of said playback means to pass to an output terminal in temporal succession, the spacing of said playback means, their speed relative to the tape and the timing of the succession of signals being such that there appears at said output terminal a continuous signal in which the number of lines per frame or the number of lines per frame and the number of frames per second differs from that in the signal recorded on the tape. The record tape may comprise a magnetic, photographic or electrographic record, although only embodiments based on magnetic tapes are described. The general principle of the invention is to reproduce the recorded signal by a playback head which is moved with respect to the record tape at a speed such as to increase or reduce the time taken to reproduce the signals contained in a line to the value appropriate to the desired conversion standard. A plurality of heads are provided spaced apart by distances such that by switching from head to head lines of the recorded signal may be omitted or repeated so as to obtain from each original frame the appropriate different number of lines. Modification of the number of frames is effected in a similar manner by omitting or repeating certain of the original frames, this being effected either by switching between heads spaced apart by the distance of one frame on the record tape, or by moving the whole record tape periodically. The recording may be in the form of a single continuous track running in the direction of the recording tape, or a succession of tracks laid down transverse the tape. Fig. 1 illustrates the principle of an arrangement for converting from 625 to 405 lines per frame without change of frame rate using a record in the form of a single continuous track 7 on a magnetic tape 1. The tape is traversed at a steady speed in the direction 2 and recording of the original signal is effected by a stationary head 4. The limits of the recorded lines are indicated at 8. Playback is effected by a plurality of heads 9-13 which are moved in the direction of tape movement at a speed such that the time taken to play back a recorded line is increased to that appropriate to 405 line standards. If only a single playback head were employed, a reproduced frame would comprise the first 405 of the original 625 lines. In accordance with the invention, it is arranged that the 405 lines are distributed uniformly throughout the 625 lines by the use of a plurality of heads 9-13 which are spaced apart a distance d equal to the length of a recorded line and which are brought into use in succession by a switch 14. In converting from 625 to 405 lines, 220 (625-405) changes are necessary in a frame, each change resulting in the omission of one line. Since a very large number of heads would be necessary, a practical embodiment, Fig. 2, is based on a scanning drum 30 carrying only a small number of heads, e.g. three, 31A- 31C, as illustrated, which are brought into use in succession. The drum is constructed, Fig. 3 (not shown), so as to rotate at an appropriate speed independent of the tape which is traversed over the drum surface by capstan 26. The circumferential spacing of the heads equals the length of a recorded line. Fig. 4 shows an arrangement for converting from 625 to 405 lines without change of frame rate using a record in the form of tracks transverse the tape. Recording of the original signal is effected by four heads 44A-44D which are carried by a scanning drum 45. The tape is traversed by a capstan 47 and shaped to contact drum 45 by conventional means. The heads are spaced apart a distance equal to 2.841 recorded lines and each head as it traverses the tape creates a new track. The tape is wider than the distance between two heads and each head is operative during the whole time it engages the tape. Each track thus comprises 2.841 lines together with initial and terminal portions with lines common to the adjacent tracks. Each frame occupies 220 tracks. The distance 2.841 lines is used since one line in every 625/(625-405) =2.842 must be omitted. Playback is effected by a similar scanning drum having heads which are spaced apart 2.841 - 1=1.841 lines. The rotational speed is chosen to produce the correct time length for each line according to the 405 line standard and the reduced head spacing causes the omission of one line each time a successive head comes into use. Each head furthermore engages only one track. This may involve tilting the axis of drum .48 or recording the original signal with wide tracks. Switching between head 49A-49D is effected by a unit 51 which operates so that change-over is effected in the first blanking interval after a head encounters its track. The effect of this is that each head always reproduces signals contained in an integral number of lines, either one or two, with an average of 1.841. Modifications are discussed suitable for converting each way from 625 to 525 lines and 525 to 405 lines. Reference is also made to conversion between 819 and 405 lines. Where it is desired to convert the frame rate as well as the line rate an arrangement may be employed, Fig. 5 (not shown), comprising two arrangements constructed as shown in Fig. 4, but each with a loop of tape between the recording and playback drums to permit the tapes to be advanced or retarded such that a frame may be omitted or repeated. The recording is effected in duplicate on the two tapes and playback is effected from the tapes alternately, the tape not in use being advanced or retarded. For example, in converting from 60 to 50 frame rates playback from one tape is effected for five frames whilst the second is advanced a distance such that when the changeover is effected, playback from the second commences at the seventh frame. Thus one frame in every six of the original is omitted. A modification, Fig. 7 (not shown), is based on the arrangement described in Specification 893,133, where the two tapes are scanned by common recording and playback drums. Another embodiment, Fig. 6, is based on the arrangement shown in Fig. 4, but utilises a plurality of playback drums 107a-107k carried by a conveyer system 108. The drums are each spaced apart a distance equal to the length of a recorded frame. The embodiment is described as applied to conversion from 60 to 50 frames per second, conveyer system 108 traversing the drums in the direction of movement of tape 101 at a speed such that each recorded frame is traversed in 1/25th second instead of 1/30th second taken in recording. The drums supply their signals through a switching unit 112 which is operated so that after the signals contained in five frames have been played back by one head the connection is transferred to the next succeeding drum so omitting one frame. In yet a further embodiment, Figs. 8 and 9 (not shown), two tapes are employed, the original signal being recorded in duplicate and the tapes being employed alternately for playback. Separate playback drums are mounted for movement with respect to each tape through a distance equal to the length of a recorded frame. The drum which at any time is used to provide the output signal is moved through the distance at a constant speed, either with or against the tape movement according as the number of output frames to be less or more than in the recorded signal, and while this one drum is thus moving at constant speed the other drum, which has just completed such a movement is returned back past its starting point and accelerated again to its correct velocity in time to be taken into use. In a modification, the reading drums may remain stationary whilst the tapes are subjected to movement.