Patent GB-997237-A (GB)
Title: Image signal recording method and apparatus
IPC: H04N3/32
Filed: 1962-03-06
Abstract: 997,237. Television; bandwidth compression; cathode-ray tube operating circuits; transistor pulse circuits. D. S. HORSLEY. March 6, 1962 [March 15, 1961], No. 8622/62. Headings H3T, H4F, H4R and H4T. [Also in Division G5] General.-The invention relates to a method of recording television signals in which the video information is represented by a first component representing the times of recurrence of significant " edges," e.g. 252, 254, 256 &c., Fig. 15, and a second component representing the tonal variations 253, 255, 257 &c. which occur between " edges." The invention relies for its effectiveness on the fact that in many naturally occurring- scenes the tonal changes alternate from dark to light, giving rise to the " edges," and rarely change rapidly between successive shades of grey. Thus by separating out the " edge " information, the remaining tonal information may be conveyed with a relatively narrow band-width, 30 kc./s.- 500 kc./s., depending on the fidelity required, being suggested. In order that sharp edges may be obtained in reproduction two recordings of the tonal information are made which correspond respectively to the video signal undelayed and delayed by a brief interval which is chosen in accordance with the fidelity desired. By this means there is made available simultaneously the tonal level before an edge, termed the " tone occurring," and the tonal level after the edge termed the " tone impeding." The " edge " information signal is employed to switch from one recording to the other and thereby reproduces correctly the rapid tonal change at the edge. The delayed signal provides the output normally and the switch is made to the undelayed signal which, despite the limited bandwidth, due to the delay has sufficient time to reach correctly the tonal level which should occur after the edge. To reduce redundancy in recording the original signal is recorded in a cathode-ray tube store for one line and then compared lineby-line with subsequent lines. The tonal information is recorded (and also entered in the store in place of the original line) only when a change between lines is detected. A similar arrangement compares the signal frameby-frame but in this case unless a change occurs and a recording is to be made, the recording medium (see below) is stopped. The arrangement has limitations in that certain non-redundant parts of the signal are not recorded and certain redundant parts are recorded. Modifications to prevent this are described. The recording is effected in lines transverse to an elongated medium with the aid of a cathode-ray tube flying-spot scanner (see below) and in accordance with a further feature of the invention a circuit is employed (see below) with the aid of which the speed of recording along the lines is determined by the maximum frequency detected in the video signal. The circuit functions so that the recording is effected as shown in Fig. 13 such that a whole line 240 is employed to record the television line when the video signal is sufficiently detailed to include a component of frequency f, whereas only one half, one quarter and one eighth of a line is employed as indicated at 243, 242 and 241 when the maximum frequency is #/2, #/4 and #/8 respectively. If successive television lines continue to exhibit the same maximum frequency each recording line position is filled up, e.g. line 241 includes eight successive television lines. However, if the maximum frequency changes, the recording is started at a new position in order that an adequate line length is always available. To permit reproduction to be effected from the tonal recordings it is necessary in addition to the " edge " information, to supply information indicating when lines have been omitted and therefore the previous line should be repeated, when frames have been omitted and therefore the previous frame should be repeated and finally which of the modes of recording shown in Fig. 13 is employed at each television line for the tonal information. Thus, in addition to recordings of the tonal and " edge " component there is provided a recording of control signals. Records.-The control signal recordings are elongated recordings arranged on a medium as shown in Fig. 2. Tracks 36 and 37 control the proper reading of the tonal recordings when successive original lines are identical and only the first line is recorded, subsequent lines being omitted. The tonal recordings are read off by a cathode-ray flying-spot scanner which scans along the transverse recordings. In response to a " start " signal from track 36 the scanning spot is given a deflection in the direction of movement of the record medium so as to retrace a given line, the operation continuing until a " stop " signal is received from track 37. The record continues to move continuously and after the stop signal the scanning spot is gradually restored to its correct vertical position. Tracks 38 and 39 are employed for " start " and " stop " control when frames are omitted and an original has to be repeated. In this case the record medium is stopped and the flying-spot scanner produces a raster to scan repeatedly the recording for a full frame. To produce the effects of instantaneous stops and starts of the medium, the scanning spot is given components of motion respectively in the direction of and against the direction of motion of the medium during slowing down to a speeding up from rest. Track 40 signals which of the modes of scanning shown in Fig. 13 is in use at any instant. One or more further tracks 34 and 35 convey monophonic or stereophonic sound associated with the picture. The " edge " information recording is as shown in Fig. 4, each transverse line corresponding to one television line. The " edge " information is derived from the original video signal by a differentiation or delay and comparison method and the resulting signal is converted (by a special transistor circuit, see below) to an onoff square-wave signal which controls the recording, the black-to-white and white-to black transitions indicating where an edgeoccurs. The " edge " information recording and the control signal recording may both be photographic and may be independent. In the arrangement described, however, the two recordings are combined, the edge information being conveyed by a photographic layer 45, Fig. 3, on one face of a base material 42, whilst the control signal recording is conveyed by a magnetizable layer 43 on the other face. The photographic layer is backed by an aluminium layer 44, the recording being read by reflected light. For the purpose of synchronizing the recordings with the tonal recording (see below) index marks 41 are provided which are cut in one edge of magnetic layer 43 with a grinding wheel during manufacture. The tonal recording comprises two recordings, Figs. 5 and 7, which are made in photographic layers 53 and 54 on opposite sides of a base material 50. The layers 53 and 54 are backed by aluminium layers, the recordings being read by reflected light. The delayed and undelayed video signals comprising the " tone occurring " and the " tone impeding " information (see above) are recorded alternately on the two layers 53 and 54 under control of the " edge " information signal. The recording may be variable area as shown or variable density. Tracks 55, 59 for producing speed indicating signals are provided down the side of each recording and are employed to control the instantaneous start and stop operation (see above). Methods of recording.-The combined " edge " information and control recording is produced by the arrangement shown in Fig. 8 where the record medium 42 is exposed on to the photographic face to light from a cathode-ray tube flying-spot scanner which is modulated by the " edge " information signal, whilst the magnetic face engages a row of magnetic recording heads 62-68 for producing tracks 34-40, Fig. 2. Head 69 produces a signal from index marks 41 for synchronizing purposes. The two tonal recordings are produced by the arrangement shown in Fig. 9. A double-beam cathode-ray tube 80 produces two light spots 83 and 84 and exposes the two faces of the record medium 50 by way of lenses 85, 88 and reflectors 86, 89. In a second arrangement, Fig. 10 (not shown) the two faces of the record are exposed by separate cathode-ray tubes. Transistor circuits. Fig. 16.-The " edge " information signal 222 obtained by differentiation or otherwise from the original video signal (see above) is converted directly to a square waveform 232 by means of a PN#N transistor of the type described by Grinich & Hass in paper TP-3 of the solid state Circuits Conference, Philadelphia, 1959. Reference is also made to the use of a PNPN transistor operated as described in " Semi-conductor Products," Vol. 4, No. 2, page 32, February, 1961. Cathode-ray tube circuits. Fig. 14.-The circuit examines the maximum frequency component in the original video signal, which is applied at terminal 166, with the aid of amplifiers 171, 172, 173, 174 tuned respectively to #, #/2, #/4 and #/8, where # is the maximum frequency likely to be encountered, and controls the production of a sawtooth output at terminal 198 of appropriate periodicity by means of sawtooth generators 181, 192, 201, 210, the frequencies of which are respectively equal to/ a ,   2/# 2 , 2/# 4  and 2/# 8 , where # 2  equals the television   line scanning frequency. The recording cathoderay tube or tubes thus scan to effect traverses of the record medium as explained with reference to Fig. 13 (see above). The signal from each amplifier is integrated over the period of one television line by circuits 176, 190, 199, 208, and controls the triggering of an associated " one-shot " circuit 180, 191, 200, 209. Each of these circuits then opens a gate 182, 194, 202, 211 in its own channel and also closes a gate 194, 204, 206, 212, 214, 216 in each of the channels of lower frequency. By th