Patent GB-1497919-A (GB)
Title: Video cassette recording and playback apparatus
IPC: H04N5/915
Filed: 1975-10-17
Abstract: 1497919 Video tape cassette apparatus GRUNDIG EMV ELEKTRO-MECHANISCHE VERSUCHSANSTALT MAX GRUNDIG 17 Oct 1975 [17 Oct 1974] 42757/75 Heading G5R [Also in Division G3] A video magnetic tape cassette recorder/ player records/replays oblique tracks on a cassette tape with one video field per track, heads N 1 , N 2 , Fig. 1, offset by 180 degrees from one another on a rotary head wheel 1, being operative in a normal mode of operation and the apparatus being operable in an extended duration mode in which the tape runs slower than in the normal mode and only odd or even video fields are recorded, each field being replayed twice, additional heads L1, L 2  on wheel 1 being operative in the extended mode instead of heads N 1 , N 2  and being offset by about but not exactly 180 degrees angularly and offset axially from one another, signals being coupled to and from heads L 1 , L 2 , N 1 , N 2  by a single rotating transformer (2), Fig. 2 (not shown), having only one stationary and only one rotating winding, and the four heads being switched into and out of operation by polarized relay contact sets 3, 6 on wheel 1 and actuated by stationary coils 4, 5 controlled by a logic circuit having inputs from switches 11  representative of the selected mode and an input from a detector 13 sensing magnet 14 on head wheel 1. The logic circuit comprises AND gates 7,8,9,10,15,18,19, OR gate 16 and inverter 17. Gates 7 and 9 are enabled directly by a signal from detector 13, and gates 8, 10 are enabled by signals from detector 13 via delay 12 having a 20 m.sec. delay corresponding to half a turn of wheel 1. In the normal mode, relay 3 connects heads N 1 , N 2  in series across the rotary transformer; in extended mode recording relays 3 and 6 connects only head L 1  across the transformer; in extended mode playback relays 3 and 6 connect heads L 1 , L 2  in series across the transformer; and in extended mode replay of a still picture from a stationary tape, inverter 17 acts so that heads N 1 , N 2 , rather than L 1 , L 2  are operative. Phase control, of head wheel rotation is effected by a phase comparator 20 which has a reference input 21, e.g. vertical sync pulser, and an input from detector 13 and gives an error signal output via amplifier 22 to motor 23. A phase shifter 24 introduces a phase shift, corresponding to angle α between heads L 1 , N 1 , into the signal from detector 13 in dependence on the selected mode. Tape speed, is controlled so that the distance longitudinally of the tape between the oblique tracks corresponds to 11 half-lines in the normal mode and 5 whole lines in the extended mode, the tape speed ratio being 11 to 5 for the two modes. The tape is driven by a capstan 27 rotated by a motor 26 having a tacho disc 29 and pickup 30, the pulses from which form one input to a frequency comparator 32 controlling motor 26 via amplifier 41, the reference frequency input to comparator 32 being changed between modes by a switch 34. An additional control input to comparator 32 is fed from the output of a phase comparator 36 having a frame rate reference input, derived via divideby-two 37 from vertical sync pulse input 38, with which the input via frequency divider 33 from pickup 30 is compared. Divider 33 is switched between modes by a switch 35, divider 33 dividing by p in the normal mode and by q in the extended mode (p/q being 11/5) to give a frame rate output to comparator 36.