Patent GB-1162561-A (GB)
Title: Magnetic Recording and Reproducing System of Video Tape Recorder
IPC: H04N5/921
Filed: 1966-09-28
Abstract: 1,162,561. Television. AKAI ELECTRIC CO. Ltd. 28 Sept., 1966 [3 Dec., 1965], No. 43306/66. Heading H4F. In a system for reproducing television video signals which have been directly recorded on a magnetic tape and as a result of which the reproduced synchronizing pulses are effectively differentiated and comprise a series of acute and sharp pulses corresponding to the rising and descending edges of the synchronizing pulses, these acute pulses are separated from the video signal before it is equalized to compensate for distortion caused by the direct magnetic recording process, and the separated pulses are utilized to regenerate the synchronizing and blanking signals for the television signal. To enable the acute pulses to be separated from the reproduced video signal, the level of the synchronizing pulses, e.g. the horizontal, vertical and equalizing pulses, is modified before recording to be higher than the white level of the video signal, Fig. 1 (not shown). Since the length of the front porch is shorter than the back porch of the horizontal synchronizing pulses, the pulse derived from the leading edge of the sync. pulse is considerably cancelled by the pulse derived from the leading edge of the related horizontal blanking period even with the increased level for the sync. pulse. To overcome this difficulty the synchronizing pulses are delayed so that the leading edge of each of the horizontal sync. pulses is shifted, before recording, to the midpoint of the respective blanking period when the pulse derived from the leading edge is separated for regeneration of the sync. and blanking pulse, Figs. 3 and 4 (not shown). Alternatively if the pulse derived from the trailing edge of the sync. pulse is separated for regeneration of the sync. and blanking pulses, then the trailing edge of sync. pulse is shifted before recording to the mid-point of the respective blanking period. To regenerate the vertical sync. pulses from the acute pulses derived for example from the leading edges of the reproduced sync. pulses, Fig. 5a, these pulses are fed to a monostable multivibrator having a time constant shorter than one half the horizontal scanning period to produce rectangular pulses, Fig. 5b. These pulses are integrated, Fig. 5c, amplitude separated at the level of the vertical sync. pulse, Fig. 5d, and shaped to obtain a vertical sync. pulse, Fig. 5(e). Due to the narrow band-width of magnetic tape recorders the amplitude ranges in the reproduced signal of the equalizing and vertical sync. pulse periods are slightly offset, Fig. 6a. To overcome this drawback the video signal is modified before it is recorded by positively accentuating the vertical sync. pulses, Fig. 6 (b) and then removing the vertical sync. pulses and equalizing pulses from the complete video signal, Fig. 6c. As before in advance of the equalization of the reproduced signal, Fig. 6d, the differentiated pulses are separated therefrom and used to regenerate rectangular pulses as described with reference to Fig. 5, and Fig. 10 (not shown). To overcome the difficulty of drop-outs in the television signal, wherein the reproduced and separated differentiated pulses in the video signal contain a series of periodic interruptions,. Fig. 7c (not shown), the superfluous regenerated rectangular signals derived therefrom, Fig. 7e (not shown), are eliminated by arranging for the vertical sync. signals to trigger a further multivibrator having a time constant smaller than the vertical scanning period. The resulting rectangular pulses, Fig. 7f (not shown), are then differentiated and finally shaped to produce the series of vertical sync. pulses. In a detailed embodiment of the recording and reproducing system, Fig. 8, a television signal A is fed to a summation circuit 12 via a D.C. restoration circuit 3 and the separated sync. signal B with polarity reversed is fed to circuit 12 via a circuit 6 (which delays the sync. signals and raises their levels above the white level of the video signal) and via a differentiating circuit 7, amplitude separator 8, multivibrator 9, integrator 10 and shaping circuit 11 as described with respect to Fig. 9 (not shown). The output of adder 12 comprises a video signal E with its sync. signals phase-shifted to the back porch side and with its equalizing pulses and vertical sync. pulses blanked. Signal E is recorded and later reproduced from a magnetic tape by means of magnetic head 14. After preamplification 15 of the reproduced signal the effectively differentiated acute pulses at each of the rising edges of the sync. pulses are separated therefrom 18 while the video component is equalized 17 and thus subjected to wavy distortion. These pulses are fed to horizontal sync. signal shaping and blanking circuits 21 and 22 respectively and to monostable multivibrator 23, integrator 24, amplifier 26, monostable multivibrator 28, differentiators 29, 36 and vertical sync. signal shaping and blanking circuits 30 and 27 respectively (as described with reference to Fig. 5 and Figs. 7 and 10, not shown). The equalized video signal is delayed 31, D.C. restored 32, blanked 33 and the synchronizing signals are added in circuit 34.