JPH0326452B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tracking state detection circuit with a simplified circuit configuration.

In a video tape recorder, to obtain a clear (no noise band) special playback image, the tape winding force must be slightly changed or the video head must be displaced in a direction perpendicular to the scan. In the latter case, in order to align the scanning direction of the video head with the direction of the recording track, it is common to apply a sawtooth wave to the piezo element that mounts and fixes the video head according to the tape speed; There is no guarantee that the video head will scan the center of the track. Therefore, the video head is forced to vibrate in the width direction of the track by superimposing an alternating current signal (frequency sufficiently higher than the field frequency) on the sawtooth wave, and the playback output level at both ends of the vibration is detected to control the video head. A tracking control signal for positioning at the center of the track is obtained, and the control signal is further superimposed on the sawtooth wave. Various tracking state detection circuits for generating tracking control signals have already been proposed, but all of them require complicated circuits.

Therefore, in view of the above-mentioned points, the present invention proposes a tracking state detection circuit with a simplified circuit configuration.

First, the operating principle of the present invention will be explained below. Figure 1 shows the relationship between the position of the video head with respect to the recording track and its playback output level, and shows the playback output level when the video head H, which is equal to the width Tw of the recording track T, deviates from the center of the recording track T. E has a linear attenuation tendency. Therefore, when the video head vibrates sinusoidally, the reproduction output level E changes as shown in FIG. That is, when the video head vibrates around the center of the recording track, the output changes as shown in a, when it deviates slightly from the center on one side, b, when it deviates further, c, and when it deviates significantly, the playback output changes partially. When it is completely missing, the output changes as shown in d, and when it deviates from the center, it shows the output change as shown in e. Therefore, if only the changes in the output waveforms shown in a to e are extracted and sampled at sampling S near one of the peak points of vibration W, the section shown by hatching in Fig. 3 will be sampled. . Therefore, it is clear from FIG. 3 that when the video head vibrates around the center of the recording track, the sampling output level is a predetermined level V, and the sampling output level changes depending on the direction of deviation of the center of vibration. is the predetermined level V
become larger or smaller. Moreover, the level difference between the predetermined level V and the sampling level (near the center of the recording track) is proportional to the amount of deviation.

Therefore, in this embodiment, tracking control is performed using a circuit configuration as shown in FIG. That is, in this embodiment, fluctuations in the playback output level are sampled at one peak point of the vibration of the video head, and the tracking state is controlled based on the sampling level.

The operation of this embodiment will be explained below with reference to the circuit block diagram of FIG.

First, in this embodiment, the video head H is supported by a piezo element P, and the control voltage applied to the piezo element P is a sawtooth wave with a field period that changes the tilt angle according to the tape speed, and a forced wave. The sawtooth wave changes based on a signal obtained by superimposing an AC signal for vibration (450 Hz) and a tracking control signal, and the sawtooth wave is derived from the sawtooth wave generation circuit 2, and its period matches the scanning of the video head. Its slope is proportional to tape speed. In addition, the AC signal is generated by converting the 30Hz head switching pulse to 450Hz by frequency multiplier circuit 3.
The signal is converted into a square wave of 450 Hz and extracted through the first band pass filter 4 of 450 Hz.

On the other hand, the tracking signal is derived as follows. First, after amplifying the head output with the preamplifier 5 and adjusting the level with the AGC circuit 6,
After detection by the first detection circuit 7, only the output change due to the forced vibration of the video head is derived through the second band pass filter 8 of 450 Hz. Furthermore, the above AGC
In order not to suppress output changes due to forced vibration,
The output of the second detection circuit 9 is passed through a 450Hz frequency limiting filter 10 as an AGC control input. The above-mentioned output change is sampled by the sampling pulse S obtained by inputting the output of the frequency multiplier circuit 3 to the phase compensation circuit 11, and the output of this sampling circuit 12 is input to the discriminating circuit 13 and the above-mentioned predetermined change is performed. It is compared with level V and the tracking state is derived as a differential output, which is then derived as a tracking control signal via the low-pass filter 14. Note that the differential output is clipped near 0V, and a tracking control signal is not derived unless the tracking deviates by more than a certain level.

The respective signals derived as described above are added in the adding circuit 1 and amplified to a control voltage via the control amplifier 15, and then input to the piezo element P.
Therefore, by applying this control voltage, the video head can scan the center of the recording track, and a stable reproduced image can be obtained.

Therefore, according to the present invention, the tracking state can be detected from only one side of the vibration, and the tracking state can be detected with a simple configuration, which is highly effective.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of changes in playback output with respect to displacement of the video head in the recording track, and FIG.
FIG. 3 is an explanatory diagram of changes in reproduction output when the video head is forced to vibrate; FIG. 3 is an illustration of each output level when sampling each reproduction output waveform of FIG. 2;
FIG. 4 shows a circuit block diagram of one embodiment of the present invention. Explanation of main figure numbers, 8... (second) band pass filter, 12... sampling circuit, 13...
Identification circuit, 2... sawtooth wave generating circuit, H... video head, P... piezo element.

Claims (1)

[Scope of Claims] 1. An alternating current signal is applied to a piezoelectric element on which a video head is placed and fixed, and the video head is vibrated in the width direction of the track. The tracking control mechanism located at the center includes a bandpass filter that separates fluctuation components of the playback output level, a sampling circuit that samples the output of the bandpass filter during one half period of vibration of the video head, and a sampling output. A tracking state detection circuit comprising an identification circuit for identifying a tracking state according to a level.