JPH0432857Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to a video tape recorder that prevents the FM reversal phenomenon during playback.

[Conventional technology]

Traditionally, consumer (home) video tape recorders have been designed to reduce the amount of tape consumed and to be smaller and lighter. At this time, high-density recording is performed on the tape, and at this time, the high frequency of the signal recorded on the tape is has reached a limit frequency based on factors such as the relative speed between the tape and the head.

By the way, the signal recorded on the video track of the tape is an FM modulated version of the luminance signal in the video signal.
FM modulated luminance signal and color signal in video signal
It consists of a low frequency converted color signal obtained by converting the FM modulated luminance signal to a low frequency signal.

Furthermore, when performing FM modulation on the luminance signal,
In order to prevent deterioration of S/N of high frequency components due to FM modulation, pre-emphasis processing is performed on the luminance signal before modulation to enhance the high frequency range of the luminance signal.

Therefore, the signal recorded on the video track of the tape is a signal in which the FM modulated luminance signal, which is FM modulated on the pre-emphasized luminance signal, is located in the high frequency range of the low frequency converted color signal, and at this time, the FM modulated luminance signal is For example, the frequency shift at a location where the brightness level suddenly changes from low to high is the largest, and the frequency at that location is the highest.

On the other hand, when playing back a tape on which signals reaching the limit frequency are recorded at high density, as mentioned above,
If the high frequency components of 6 to 7 MHz, which are higher in frequency than the carrier signal, are not sufficiently reproduced due to poor adjustment of the reproduction circuit, the level of the high frequency components of the reproduced FM luminance signal will be low.

Then, in order to FM demodulate the reproduced FM brightness signal and obtain the reproduced brightness signal, the video head is usually
The FM luminance signal is amplified in the first stage and then limiter amplified by the limiter amplifier circuit, and at this time the reproduced
If the level of the high frequency component of the FM luminance signal is low, it is output from the limiter amplifier circuit to the FM demodulation circuit.
The FM luminance signal particularly loses the high frequency part, making it easy for so-called FM inversion phenomenon (broken phenomenon) to occur.

In addition, when the FM reversal phenomenon occurs, the playback screen will display
Black noise occurs due to the FM inversion phenomenon.

Therefore, in order to prevent the FM reversal phenomenon, conventionally,
For example, depending on the video head and circuit capacity, playback
A resonant circuit is formed to increase the level of the high frequency component of the FM luminance signal, and the resonant circuit increases the level of the high frequency component of the reproduced FM luminance signal input to the limiter amplifier circuit.

Furthermore, Japanese Patent Application Laid-Open No. 57-24177 discloses a phase shifter that has a characteristic of delaying the carrier wave of frequency _D corresponding to the black level in the reproduced FM modulated wave signal than the carrier waves of other frequencies. The circuit 11 is provided at the front stage of the limiter circuit 6 which corresponds to a limiter amplifier circuit, and at the rear stage of the limiter circuit 6, the phase of the frequency characteristic of the phase in which the sum of the frequency of the phase of the amplifier 5 and the phase circuit 11 is linear is provided. Compensation circuit 1
2 to prevent the FM reversal phenomenon.

[Problem that the invention attempts to solve]

By the way, as mentioned above, when increasing the level of the high frequency component of the reproduced FM luminance signal input to the limiter amplifier circuit using a resonant circuit, it is difficult to increase the frequency to 5 MHz or higher from the viewpoint of S/N. In reality, it is difficult to prevent the FM inversion phenomenon due to a drop in the level of high-frequency components of 6 to 7 MHz.

Furthermore, as in the above-mentioned publication, if the frequency _D component corresponding to the black level, that is, the low frequency component of the reproduced FM modulation signal, is delayed and input to the limiter circuit 6, for example, a sudden change from the black level to the white level can be achieved. There is a problem in that the frequency of the reproduced FM modulation signal becomes higher in some places, making it easier for FM inversion to occur.

[Means for solving problems]

This invention provides a video tape recorder in which a reproduced FM brightness signal of a video head is input to an FM demodulation circuit via a limiter amplifier circuit, and the reproduced FM brightness signal is demodulated. This video tape recorder is provided with a delay circuit that delays the time axis of high-frequency components of higher frequencies than the carrier wave signal of the luminance signal.

[Effect]

Then, the playback input to the limiter amplifier circuit
The high frequency component higher than the carrier signal of the FM luminance signal is
The delay circuit delays the frequency in the direction of lowering, and at this time, for example, the frequency transition at a point where there is a sudden change from the black level to the white level becomes gradual, causing a drop in the reproduced FM luminance signal output from the limiter amplifier circuit. It disappears.

〔Example〕

Next, this invention will be explained in detail with reference to drawings showing one embodiment thereof.

In FIG. 1, 1 is a video head, 2 is a preamplifier circuit connected to video head 1, 3 is a delay circuit connected to amplifier circuit 2, 4 is a limiter amplifier circuit connected to delay circuit 3, and 5 is a preamplifier circuit connected to video head 1. A correction circuit is connected to the amplifier circuit 4, and 6 is an FM demodulation circuit connected to the correction circuit 5.

The FM brightness signal reproduced from the tape by video head 1, that is, the reproduced FM brightness signal is
After being pre-amplified by the pre-amplifier circuit 2, it is input to the delay circuit 3. At this time, the delay circuit 3 has a characteristic that the higher the frequency, the greater the delay in the time axis for components of about 4 MHz or higher, as shown in FIG. and based on the characteristics, the reproduction input to the delay circuit 3
The FM luminance signal is delayed in such a direction that the frequency of high-frequency components of the FM luminance signal is lower than that of the carrier wave signal containing components of 6 to 7 MHz. In addition, the delay circuit 3 reproduces
The FM luminance signal is never amplified.

By the way, the luminance signal before FM modulation is
As shown by the solid line in Figure a, in the case of a signal where the part where the black level suddenly changes to the white level is sharply emphasized by pre-emphasis processing, the FM luminance signal recorded on the tape by FM modulating the signal is The frequency becomes significantly higher at the point where there is a sudden change from black level to white level. Note that t ₁ in the figure indicates a point at which the level changes.

When the video head 1 reproduces the FM luminance signal based on the luminance signal shown in FIG. A sudden frequency shift occurs at the point where the white level changes, and the frequency at the edge of the white level becomes significantly high, for example, 6 to 7 MHz. Since this is not done, the level of high-frequency components becomes lower as the frequency increases. Note that ta, tb, tc, and td in the figure indicate the peak points of amplitude.

Therefore, the reproduced FM luminance signal in Fig. 3b is
If the signal is directly input to the limiter amplifier circuit 4 as in the conventional case, the frequency at the end of the white level where the black level changes from the black level to the white level is extremely high, so that a dropout occurs and an FM inversion phenomenon occurs.

However, in the case of FIG. 1, the reproduced FM luminance signal of FIG.
Since the time axis of high-frequency components including The frequency at the end of the white level at a certain point becomes low, and the frequency shift at that point becomes gradual. In addition, ta′, tb′, tc′,
td′ indicates the peak point of amplitude.

Therefore, the limiter amplifier circuit 4 allows the delay circuit 3 to
Even if the reproduced FM luminance signal inputted through the limiter is amplified, the edge of the white level and the like will not be lost, and the FM inversion phenomenon will be prevented. Note that since the delay circuit 3 does not amplify the reproduced FM luminance signal, the S/N ratio does not deteriorate as would occur when a conventional resonant circuit is provided.

Even if the output signal of the limiter amplifier circuit 4 is directly input to the FM demodulation circuit 6, a reproduced luminance signal without any inversion phenomenon can be obtained, and a reproduction screen based on the reproduced luminance signal has black noise due to the FM inversion phenomenon. Never.

However, since the time axis of the high frequency component is delayed by the delay circuit 3, the reproduced luminance signal obtained by directly inputting the output signal of the limiter amplifier circuit 4 to the FM demodulation circuit 6 is shown by the broken line in FIG. 3a. , the rise time becomes longer than the original luminance signal shown by the solid line, and so-called waveform rounding occurs.

Therefore, in the case of Fig. 1, the limiter amplifier circuit 4 and
As shown in FIG. 4, there is a connection between the FM demodulation circuit 6 and the
A correction circuit 5 is provided which has a characteristic of delaying the time axis of components below Hz by a fairly large fixed amount and delaying the time axis of components above about 4 MHz by an amount inversely proportional to the frequency, and is input to the FM demodulation circuit 6. The signal is returned to the signal shown in FIG. 3b again to prevent the waveform rounding described above, and the FM demodulation circuit 6 outputs a reproduced luminance signal with the same rise time as the original luminance signal.
To provide a clear playback screen without black noise caused by an inversion phenomenon.

Note that the signal input to the FM demodulation circuit is not amplified by the correction circuit 5, and the S/N ratio does not deteriorate due to the correction circuit 5.
In addition, there is no need to prevent the waveform rounding mentioned above,
To prevent only the FM reversal phenomenon, use the correction circuit 5.
There is no need to provide

By the way, the delay circuit 3 and the correction circuit 5 can be formed by respectively configuring them as shown in FIG. 5, for example, in which Q is an NPN type transistor whose base is connected to the input terminal 1, The emitter is grounded via a resistor R1, and the collector is connected to a power supply terminal +B via a resistor R2. C and L are capacitors and coils provided in series between the collector of the transistor Q and the output terminal O, and R3 is a resistor provided between the emitter of the transistor Q and the output terminal O.

In the case of delay circuit 3, the values of capacitor C, coil L, and resistor R3 are set to values that delay the time axis of the high frequency component, and in the case of correction circuit 5, the values of capacitor C, coil L, and resistor R3 are set to values that delay the time axis of the high frequency component. Of course, the value is set to a value that delays the time axis of the low frequency component.

[Effect of idea]

Therefore, according to the video tape recorder of this invention, by providing the delay circuit, the reproduction FM luminance signal input to the limiter amplifier circuit is
For example, the frequency transition at a point where there is a sudden change from the black level to the white level becomes gradual, and at this time there is no level change in the reproduced FM luminance signal.
The FM reversal phenomenon can be prevented by not causing S/N deterioration.

Furthermore, if a correction circuit is provided along with the delay circuit as in the example, it is possible to prevent waveform rounding and prevent the FM inversion phenomenon.In this case, there is no black noise caused by the FM inversion phenomenon and clear image quality A playback screen can be provided.

[Brief explanation of the drawing]

The drawings show one embodiment of the videotape recorder of this invention; FIG. 1 is a block diagram, FIG. 2 is a characteristic curve diagram of a delay circuit, and FIGS. FIG. 4 is a waveform diagram, a characteristic curve diagram of the correction circuit, and FIG. 5 is a connection diagram of the delay circuit and the correction circuit. 1...Video head, 3...Delay circuit, 4...
Limiter amplifier circuit, 6...FM demodulation circuit.

Claims (1)

[Scope of utility model registration request] The reproduced FM brightness signal of the video head is input to the FM demodulation circuit via the limiter amplifier circuit, and the reproduced
A video tape recorder that demodulates an FM luminance signal, further comprising a delay circuit that delays the time axis of a high-frequency component of a higher frequency than a carrier signal of the reproduced FM luminance signal, at a stage before the limiter amplifier circuit.