JPH0229529Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tape start/end detection circuit that eliminates color beats.

There are two main types of tape loading systems for cassette-type VTRs, and Figure 1 shows a system in which the tape is loaded by rotating the loading disk. A VTR that uses this method has a tape end detection sensor S ₁ and a tape start detection sensor S ₂ placed opposite each other on each running tape on the supply side and take-up side in order to electrically detect the beginning and end of the tape. ing. Each of the sensors S ₁ ,
Each of S ₂ has built-in sensor coils L ₁ and L ₂ and constitutes a separate resonant circuit together with a capacitor, and detects the passage of the conductive tape at the beginning and end of the tape as a decrease in the Q value.

However, the tape start end detection sensor is in a resonant state.
_S2 faces the vicinity of the tape winding side of the guide cylinder G, and its leakage magnetic flux is transmitted to the rotating magnetic head during reproduction scanning, causing color unevenness (chroma beat) on the reproduction screen.

Therefore, in view of the above-mentioned points, the present invention specifically provides a new and effective tape start/end detection circuit that eliminates chroma beat from the playback screen by reducing the leakage magnetic flux of the tape start detection sensor during tape playback in the forward direction. This is a proposal.

Hereinafter, the present invention will be explained according to an illustrative embodiment. FIG. 2 shows a circuit diagram of this embodiment, and the part surrounded by thick lines is the sensor IC. The feature of this embodiment is that the inductance of the winding-side sensor coil _L2 is made larger than that of the supply-side sensor coil L1, and the inductance of the winding-side sensor coil _L2 is increased by the supply-side sensor coil _L1 and the supply-side capacitor _C1 . The first resonant circuit 1 formed by the winding side sensor coil L ₂ and the winding side capacitor C ₂ has a resonance frequency of 200 kHz.
180kHz is selected. Each resonant circuit 1, 2 is
Feedback resistors R ₁ and R ₂ are selectively connected to the + terminal of the operational amplifier 3 in the IC, and are also connected to the output of the operational amplifier 3.
are connected to form an oscillation circuit. A first analog switch SW1 provided at the + terminal of the operational amplifier 3 is connected to the first resonant circuit 1 when the tape is running in the forward direction, and connected to the second resonant circuit 2 when the tape is running in the reverse direction. Therefore, this oscillation circuit oscillates at 200kHz when traveling in the forward direction and at 180kHz when traveling in the reverse direction. The detection circuit 4 to which the oscillation output is input performs envelope detection of the oscillation output, and the detection output is input to the comparison terminal of the level comparison circuit 5 at the next stage. This level comparison circuit 5 emits an output when the detection output is equal to or higher than the reference voltage 9V, and the second
The analog switch SW2 distributes the signal to the first output terminal _O1 and the second output terminal _O2 , and the output level is inverted by the inverters IN1 and IN2.

Therefore, when the tape runs in the forward direction, the first resonant circuit 1 oscillates at 200kHz and the output level increases.
400mV, but the second resonant circuit 2 has a voltage of 200kHz.
The output level drops to 100 to 150 mV because it oscillates at Therefore, when the tape is running in the forward direction, less magnetic flux leaks from the tape start edge detection sensor _S2 .
Chroma beat does not occur on the playback screen. Further, when the conductive tape at the end of the tape faces the tape end detection sensor S ₁ while the tape is running in the forward direction, the Q value of the first resonant circuit 1 decreases, oscillation stops, and the tape is connected to the first output terminal O ₁ . A termination detection output is issued. Note that when the tape runs in the opposite direction, the resonance frequency becomes 180kHz, and the tape start end detection output is generated from the second output terminal _O2 .

As described above, according to the present invention, when the tape is running in the forward direction, a frequency different from the resonant frequency is fed back to the second resonant circuit constituted by the tape start end detection sensor, so leakage magnetic flux is also reduced and chroma beat is eliminated. The effect is great.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the arrangement of sensors in a VTR employing the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of the main drawing numbers, S ₁ ... Tape end detection sensor, S ₂ ... Tape start end detection sensor, 1, 2 ... First and second resonance circuit, 5 ... Level comparison circuit.

Claims (1)

[Scope of utility model registration request] a tape end detection sensor facing the tape running path on the supply side; a tape starting edge detecting sensor facing the tape running path on the winding side; a first resonant circuit including the tape ending detection sensor; and a first resonant circuit including the tape starting edge detecting sensor. a second resonant circuit that is set at a resonant frequency such that the resonant output level is sufficiently small when resonating at the resonant frequency of the first resonant circuit; and a second resonant circuit that supplies an output in common to both resonant circuits and runs the tape in the forward direction. An oscillation circuit that receives the first resonant circuit as an input when the tape is running in the reverse direction and receives the second resonant circuit as an input when the tape is running in the reverse direction, and a level comparison circuit that detects a change in the output level of the oscillator circuit are respectively provided. 1. A tape start/end detection circuit characterized in that the oscillation output level of the tape start/end detection sensor located near a video head that is playing back during direction travel is suppressed.