Picture Stabilizing Circuit for Generating a Forced Synchronizing Signal

This invention relates to a picture stablizing circuit for a video cassette recorder (hereinafter referred to as "VCR"). In particular, a picture stablizing circuit is designed for preventing instability caused by noise when a speed change occurs during a reproduction mode of the VCR.

In general, noise is produced when a speed change occurs during the reproduction mode of the VCR. If the noise corresponds to a synchronizing signal portion of the video signal, then the noise rapidly moves the picture in a vertical direction to the picture or vibrates the picture so that an unstable picture is produced.

In order to correct the unstable picture caused by the speed change during the reproduction mode of the VCR, the picture may be manipulated by the tracking adjustment unit that is mounted on the exterior of the VCR. By manipulating the tacking adjustment unit, the scanning position of the video head is adjusted so that the noise is not superposed on the synchronizing signal portion. However, if the reproduction mode is normally repeated, or if the speed changing direction is shifted, then the picture must be stabilized by repeating the manipulation of the tracking adjustment unit, which is complex and troublesome. This manipulation causes the vertical synchronizing signal to be lost and the picture to become unstable. Previously, a stable picture had been developed when the speed change during the reproduction mode produces a forced synchronizing signal synchronized to the head switching signal when the speed change occurs during the reproduction mode of the VCR. A vertical synchronizing portion of the video signal is produced by applying the forced synchronizing signal to the video signals. According to this conventional device, problems remain when noise is produced at the beginning of the forced synchonizing signals, and noise signals are treated similar to forced or vertical synchronizing signals. Consequently, the picture will temporarily shake or collapse and a substantially stable picture is impossible to produce.

The embodiments of this invention are proposed for solving such conventional disadvantages as described above and aims to provide a stable picture by reliably detecting the forced synchronizing signal when a speed change occurs during a reproduction mode of the VCR so that noise signals do not effect the picture.

According to these embodiments, this problem is solved by producing an analogue video signal that is developed by synchronizing the head switching signal when the speed change occurs during the reproduction mode. This analogue video signal is applied to the video signals so that the noise signal is increased around the forced synchronizing signal corresponding to the white level of the picture.

The embodiments of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings. The drawings are provided for illustration purposes only and thus are not limitative of the embodiments of the invention, wherein:

FIG. 1 is a diagram of a picture stabilizing circuit for an embodiment of the invention; and

FIG. 2 (A-G) is a view of the output wave forms for elements in FIG. 1.

In FIG. 1, a picture stabilizing circuit diagram for an embodiment of the invention is shown. In the circuit, a forced synchronizing signal is generated by the forced synchronizing signal generating section 1 which synchronizes the head switching signal. The forced synchronizing signal is transmitted to the input of the head switching signal and is also transmitted to the input of the head switching signal input terminal Si when a speed change occurs during the reproduction mode. The forced synchronizing signal is transmitted to the output of the forced synchronizing signal output terminal O.sub.1. The head switching signal input terminal Si and a diode D.sub.5 that is connected to the collector of a transistor TR.sub.1 are both connected to the base of the transistor TR.sub.1 and a capacitor C.sub.1 through a resistor R.sub.7.

Similarly, the head switching signal input terminal Si and the collector of the transistor TR.sub.1 are connected jointly through diodes D.sub.1 and D.sub.2, respectively. The junction of the diodes D.sub.1 and D.sub.2 is connected to the base of a transistor TR.sub.2 through a resistor R.sub.2. The base of a transistor TR.sub.3 is connected to a ground capacitor C.sub.2 and the base emitter of the transistor TR.sub.2 is connected to the ground capacitor C.sub.w through a ground resistor R.sub.3 and a diode D.sub.3. The base of a transistor TR.sub.4 is connected to a ground capacitor C.sub.3 and the collector of the transistor TR.sub.3 is connected to the ground capacitor C.sub.3 through a variable resistance VR.sub.2. The collector of the transistor TR.sub.4 is connected to a compensating signal output terminal O.sub.2 through a ground resistor R.sub.6 and a diode D.sub.4.

Furthermore, the collectors of transistors TR.sub.1, TR.sub.2 and TR.sub.3 and the emitter of transistor TR.sub.4 are connected to a power terminal Vcc through resistors R.sub.1, VR.sub.1, R.sub.4, R.sub.5, respectively. Thereby, a head switching signal of 30 Hz is transmitted to the head switching signal input terminal Si, and a high electrical potential is applied to the power terminal Vcc when a speed change occurs in the reproduction mode.

The operation effect of the above-mentioned embodiment of the invention will now be described in more detail with reference to the waveforms shown in FIG. 2.

When a speed change occurs during the reproduction mode of the VCR, the high electrical potential is applied to the power terminal so that the forced synchronizing signal generating section 1 is in an operating condition. In coincidence, a head switching signal of 30 Hz is transmitted to the head switching signal input terminal Si, as shown in FIG. 2(A) the forced synchronizing signal generating section 1 generates a forced synchronizing signal by synchronizing the head switching signal. The forced synchronizing signal is transmitted to the output of the forced synchronizing signal output terminal O.sub.1, and produces the vertical synchronizing portion of the video signal.

At this time, the head switching signal of 30 Hz is applied to the base of transistor TR.sub.1. A delay wave, as shown in FIG. 2(B) is generated by the resistor R.sub.7 and the capacitor C.sub.1. Therefore, the delay time td.sub.1 is determined by the charage and discharge time constant of the resistor R.sub.7 and the capacitor C.sub.1.

Thus, a waveform signal as shown in FIG. 2(C) is output from the collector of the transistor TR.sub.1. This waveform signal and the head switching signal are applied to the base of the transistor TR.sub.2 through the resistor R.sub.2. After combining the signals as shown in FIG. 2(D), through diodes D.sub.2 and D.sub.1, respectively, the transistor TR.sub.2 is cut off during a time tw.sub.1 when the low electrical potential is applied to the base of the transistor TR.sub.2, and a charge to the capacitor C.sub.2 is thereby interrupted.

Conversely, when the charging voltage for the capacitor passes through the transistor TR.sub.3 before such an interruption takes place, a charging voltage is discharged through the transistor TR.sub.3, and the waveform signal as shown in FIG. 2(E) is applied to the base of the transistor TR.sub.2. The cycle of this waveform signal is regulated according to the value of the variable resistance VR.sub.1. At this time, the transistor TR.sub.3 is in an off state, and the charging current flows to the capacitor C.sub.3 through the resistor R.sub.4 and the variable resistor VR.sub.2, so that the capacitor C.sub.3 is charged. When the transistor TR.sub.e is in an on state, the charging voltage of the capacitor C.sub.e is discharged through the variable resistor VR.sub.2 and the transistor TR.sub.3 so that the waveform signal as shown in FIG. 2(F) is applied to the base of the transistor TR.sub.4. Since the transistor TR.sub.4 is only in the on state when a low potential signal is applied to the base of transistor TR.sub.4 , a waveform signal of a definite width tw.sub.2 as shown in FIG. 2(G) is output to the collector of the transistor TR.sub.4.

Thus, a waveform signal of the definite width tw.sub.2 is output to the collector of the transistor TR.sub.4 near every point in time when the potential to head switching signal is shifted, and the width tw.sub.2 of such a waveform signal is regulated by the value of variable resistors VR.sub.1 and VR.sub.2. Since the generated waveform signal is transmitted to the output of the compensating signal output terminal O.sub.2 through the diode D.sub.4 and the generated waveform signal is applied to the video signals, the levels near the forced synchronizing signal of the video signals are compensated so that the white level of the picture rises. Consequently, the detection of the forced synchronizing signal during a speed change of the reproduction mode may be accurately detected.

As described above in detail, an embodiment of the invention compensates for the level near the forced synchronizing signal of the video signals so that the white level rises when a speed change occurs during the reproduction mode. Thereby, the device is capable of reliably detecting the forced synchronizing signal without being effected by noise signals, and a stable picture condition may be obtained. Therefore, it is unnecessary to adjust a tracking unit for stabilizing the picture when a speed change occurs during the reproduction mode.

While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as claimed.