Patent GB-1439311-A (GB)
Title: Helical scan video tape recorder apparatus and method
IPC: H04N5/7826
Filed: 1973-08-24
Abstract: 1439311 Video tape recording INTERNATIONAL VIDEO CORP 24 Aug 1973 [1 Sept 1972] 40306/73 Heading G5R Video signals are recorded as helical tracks 416, Fig. 11, on a magnetic tape 28 by a helical scanner 42, Figs. 2, 4 having a rotating member 56 carrying two video recording heads 66, 68, each helical track recorded by said heads carrying less than a complete video field. The tape deck, Fig. 2, has a capstan 52, vacuum buffer columns 30, 32, air bearing tape guides 34, 36, 40, 44, 54, an air bearing between scanner 42 and the tape and a vacuum collar, (not shown), at the scanner 42 to separate the tape 28, when stationary, therefrom. A full tape width bulk erase head 38 together with a record/play head stack 48, or monitor head stack 50 and an erase head stack 46 are also provided. Each of stacks 48 and 50 have two audio track heads, an audio cue track head, a control track head and an address track head, stack 46 only having erase heads for the two audio tracks and the cue track, the control and address tracks being saturation recorded, and stack 46 also including a video verification head with a gap transverse to the video tracks. The recording format is as shown in Fig. 11; the tape wrap at scanner 42 is 188À57 degrees. The scanner 42, Fig. 4, has a control track head (71) on a stationary drum (58), Fig. 5 (not shown) and a motor 64 rotating a drum 56 carrying video heads 66, 68 at 180 degrees to one another and a scanning erase head 70 which erases two video tracks per revolution. Light sources on a stationary member 72 co-operate with stationary light detectors, behind corresponding windows 82 on a stationary member 80, and with a rotary disc 74 to produce control signals. An outer ring 76 of slots (94), Figs. 6A, 6B (not shown), with varying spacing provide sine and cosine signals to control the motor 64, a single slot (86) aligned with head 66 gives one pulse per drum revolution (head 1 pulses). The tape is driven at 8 ips, and drum 56 is rotated at 9000 rpm giving a 1500 ips video writing speed and giving rise to a 6 kHz signal from an inner ring 78 of forty equispaced slots on disc 74. The 6 kHz signal is processed in a circuit, Fig. 20C (not shown), producing 150 Hz and 300 Hz tachometer signals both synchronized with the head 1 pulses. The heads 66 and 68 are of hot pressed ferrite type allowing FM recording of the composite colour video signal within a 7 to 9 or 9 to 12 MHz deviation band. No head switching is used during recording so that the 180 degree separation of heads 66 and 68 combined with the 188 degree tape wrap results in some of the horizontal lines being recorded by both heads on respective adjacent tracks. Some tracks record 55 lines, the others 56 so that one frame occupies 10 tracks for 60 Hz, 525 line NTSC and 12 tracks for 50 Hz, 625 line PAL/ SECAM. Head switching during playback results in playback of 52 or 53 lines per track. Video head switching between heads 66 and 68 during playback is effected by means of a logic circuit, Fig. 17A (not shown), which provides for head switching by the 150 Hz and 300 Hz tachometer signals, derived from the scanner 42, when the off-tape horizontal sync. pulses have not yet been phase locked to a reference (studio) horizontal sync. signal and more precise <SP>11</SP> switching when locking has been established, the logic circuit having inputs for said 150 Hz and 300 Hz signals, for an advanced horizontal clock signal and for a head switching pulse signal from a line counter circuit. For PAL/ SECAM the line counter circuit, Fig. 18 (not shown), has inputs for the advanced horizontal clock signal and for a tape frame pulse signal and comprises divide-by-five and divide-byfifty-one counters, for NTSC the line counter circuit, Fig. 19 (not shown), has inputs for the advanced horizontal clock signal, a tape frame pulse signal and the 300 Hz tachometer signal and comprises two divide-by-five counters and a divide-by-fifty-two counter with a ¸51 and a ¸52 output. The inherent delay in the handling of the video signal in the record/playback process requires a compensating advance in the offtape horizontal sync. pulses to ensure correct head switching in playback and this advance is achieved by feeding the off-tape composite sync. signal to a circuit, Fig. 20A (not shown), with a phase lock loop comprising a phase detector (576), digital delay (584), and a voltage controlled oscillator (582) operating at a nominal frequency equal to the horizontal sync. frequency and giving the advanced horizontal clock signal as output. This circuit also operates on the off-tape composite sync. input to produce the tape frame pulse signal.