DE3215414A1 - Circuit arrangement for video recorders - Google Patents

Description

32 15AH

State of the art

The invention relates to video recorders with two video heads rotating with a head drum. Each The head in contact with the recording medium, the magnetic tape, is turned on - and during this time the other head turned off.

The signal for switching the video heads is turned off a device obtained which is phase-locked to the positions of the heads. This facility exists usually made up of two magnets attached to the head drum by 180 Staggered degrees are arranged, the magnets are attached with opposite polarity. The magnets will When the head drum rotates, it moves past an induction coil and induces two anti-phase coils in it Pulse voltages and thereby give a signal to determine the respective head position. The impulses are with Processed separately with the help of monostable multivibrators and generated from them meandering signals that are and deactivation of the video heads can be used. The switch-on phase of the video heads must be in relation to the vertical sync pulse can be set and adhered to very precisely. It must be carried out separately for each video head the duty cycle of the individual video heads must be set precisely. More recent proposals assume that only one, e.g. the positive excitation pulse of the induction coil is used, with the help of a monostable trigger stage the time is set between this pulse and switching on the head. After the tipping time of the monostable Switching is done with the help of a counter, That means with a cycle time of e.g. 40 ms after 20 ms. The arrangement described works without problems

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in normal operation, i.e. when the belt is operated with Standard speed of e.g. 2,339 cm / s belt speed and a head speed of 1500 rpm.

Newer technology video recorders have the option of a search run in both directions, whereby during the The scanned video signal is visible during the search. In search mode, for example, the tape speed is increased by 9 times increased. To ensure trouble-free playback, the number of revolutions of the head drum must be changed according to the search direction. When searching in the forward direction the number of revolutions is increased e.g. by 1.8% to 1527 and in the reverse direction by 2. B% lowered to 1458. The positive pulses emitted by the induction coil have one in the first case shorter and in the second case a longer period.

In the KiPzept just described, the one used is used However, the counter is not switched during search mode, so that it continues to switch the video heads after 20 ms, But that means that the duty cycle is in search mode of the switching meander signal is disturbed and no longer 0.5 amounts to. This can cause disturbances in the visible image, which can be seen as interference stripes and poor vertical synchronization make noticeable because the switchover takes place in the visible outward flow.

task

The invention is therefore based on the object of this To eliminate disadvantage, using the existing concept with simple means the duty cycle

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the switching pulse sequence is brought _{to the value 0 r 5 during search in both directions.} This object is achieved by the measures characterized in the patent claims.

description

The invention is based on an exemplary embodiment explained in more detail with the help of the drawing.

Figure 1 shows the basic structure of the circuit arrangement in a block diagram

Figures 2 to 4 show timing diagrams to explain the Mode of operation of the circuit according to FIG. 1.

In Figure 1, the head drum 1 is indicated, with a The number of revolutions U rotates, whereby the marking magnets 2 and 3 in the coil 4 induce voltages in phase. The positive voltage peaks are used in a counter module fed. The time between this positive pulse and switching on the video head must be set. For this purpose, a monostable multivibrator B is provided in the counter module 5. Switching from the beginning of tilting back of this flip-flop stage until switching, e.g. after 20 ms with a cycle time of 4G ms, with the help of a internal counter 7 made. The output signal of the counter module 5 arrives at output A, from where it Via the Vtsr logic θ to output D of the circuit got.

The negative pulse is sent to the induction coil 4 Input of a further adjustable monostable multivibrator 9 - switched, its output signal B to a gate circuit 10 is placed, which short-circuits the output D during the triggered phase of this steady-state flip-flop 9 .

32154H

The monostable multivibrator is with the help of a timing element 11 set so that at normal speed of the Videotape the falling edge of the pulse at B. the rising edge of the output signal at A in time coincides. The output signal B triggers another monostable flip-flop 12, the flip-over time is not very critical and does not have to be adjusted. she should be between 2 and 15 ms. The output pulse of the monostable multivibrator 12 is at C via the link 8 interconnected with the output signal A.

The mode of operation will be explained with the aid of the diagrams according to FIGS.

FIG. 2 shows the state in normal operation of the video recorder shown, i.e. the tape speed is e.g. 2.339 cm / s, the number of revolutions of the head drum is e.g. 1500 rpm. In this case, coil signals occur at intervals ta of 20 ms with alternating polarity. The positive pulse triggers the monostable multivibrator 6 located in the counter module, which in this example falls back after the time tb. This time can be set with the timer 13, so that the duty cycle is 0.5 can be produced.

The negative pulses trigger a monostable multivibrator which is set in normal operation in such a way that the Trailing edge after the time te with the leading edge of the counter module 5 coincides. In Figure 1 is the course of the Output voltage C of the monostable multivibrator 12 is shown. In Figure 1, the switching signal D results from the logic function (AvC) ^ B.

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The conditions change in search mode. the Belt speed is increased e.g. to 9 times the The number of revolutions is increased accordingly when searching forwards and decreased when searching backwards. To "the mode of action To make it clearer, the deviations from the normal position are exaggerated in the following FIGS. 3 and 4 shown. In practice, the deviations are much smaller, but they are sufficient to eliminate the unpleasant ones To cause interference in the picture.

In Figure 3, the state is when searching in the forward direction shown. The pulse sequence is here compared to the normal pulse sequence shown in the first line shorter, and since the counter cannot be switched, the pulse duty factor occurring at A is no longer 0.5. The insertion the components described above, however, allow that the output signal D has a duty cycle of 0.5 at an increased switching frequency.

The relationships are similar for one Search run in reverse direction, in which the speed of the head drum must be reduced. This is in figure 4 shown. The period of the coil signal is has become longer, there is again a pulse duty factor at output A with a value that deviates from the desired value Amount. Due to the interaction of the two monostable multivibrators 9 and 12, the pulse train at output D wiurier symmetrized by increasing the momentum by the same amount is shortened as the break is lengthened. As in the other case, the impulse is lengthened by the same amount becomes how the break is shortened.

The circuit arrangement described has the great advantage that with just a few additional building elements and through only one process of the coincidence of two pulse edges an optimal duty cycle in Jewish mode (iur Cch.il t voltage for the video heads is guaranteed.

Claims (2)

• «ft Γ-PA 410 Claims Circuit arrangement for a video recorder with two with a head drum rotating video heads and a device for switching the video heads in dependence from their position, the device consisting of magnets mounted on the head drum which are in communication with an induction coil emits impulses and the impulses of a certain polarity an adjustable monostable multivibrator and a counter are fed, as well as with a switch of the belt speed for a search run in both directions, characterized in that the pulses of the other polarity are connected to the input of an adjustable monostable multivibrator (9), the output of which on the one hand is connected to the input of a further monostable multivibrator (12), the output of which is connected to the output of the Counter module (5) interconnected via a gate circuit (10) to output D of the circuit arrangement is placed, the gate circuit (10) from the output of the first monostable multivibrator (9) is locked during its triggered phase. 2) Circuit arrangement according to claim 1, characterized in that that the first monostable multivibrator (9) triggered by the pulses is set in such a way that the breakdown time at the standard speed coincides with the rising edge of the counter module (5).