JPH0452546B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Application of the Invention The present invention relates to a magnetic recording/reproducing device, and particularly to improving the signal-to-noise ratio of a luminance signal.

(2) Prior Art FIG. 1 shows a signal processing system diagram of a luminance signal in a conventional magnetic recording/reproducing device. When recording a video signal, a luminance signal whose band is limited to about 3 MHz by a low-pass filter 1 has its high frequency component emphasized by a video emphasis circuit 2, and then is converted into a low carrier FM signal by an FM modulation circuit 3. Thereafter, the recording equalization circuit 4 applies appropriate recording equalization characteristics to increase the transmission efficiency of the FM signal in the recording system consisting of the magnetic tape 7 and the head 6, and then the signal is transferred from the magnetic head 6 to the magnetic tape via the recording amplifier 5. 7 is recorded.

On the other hand, during playback, by switching the changeover switch SW, the playback FM signal obtained from the recording system consisting of the magnetic tape 7 and the head 6 via the playback amplifier 8 is subjected to recording equalization characteristics performed on the recording side by the playback equalization circuit 9. It compensates for the transmission characteristics that emphasize the lower sideband and suppress the upper sideband of the magnetic tape head recording system.
Furthermore, a limiter circuit 10 removes amplitude fluctuation components caused by variations in the contact state between the magnetic tape 7 and the head 6, and then an FM demodulation circuit 11 performs M demodulation. We are getting the original luminance signal.

The carrier wave frequency of the FM signal corresponds to the level of the brightness signal. For example, in the case of a VHS system VTR, the carrier wave frequency of the FM signal corresponds to the level of the brightness signal.
3.4MHz, set to 4.4MHz at white peak.
Average picture level of luminance signal
Level is hereinafter abbreviated as APL. ) may be close to gray level as in the case of Figure 2, depending on the subject, or
As shown in Figure 3 a and b, the one close to black level a
and b, which is close to the white level.

As mentioned above, the reproduced FM signal has the lower sideband emphasized and the upper sideband suppressed, so
In general, in order to obtain a reproduced luminance signal with a good signal-to-noise ratio, a so-called vestigial sideband regeneration method is used in which the upper sideband is hardly used and the lower sideband is mainly used. Therefore, the frequency characteristics, that is, the reproduction equalization characteristics of the reproduction equalization circuit 9 are as shown in FIG.
The falling characteristic of the low range centered on the carrier wave frequency of the FM signal is more gradual than the falling characteristic of the high range. As mentioned above, the APL of the luminance signal changes depending on the subject, so the conventional reproduction equalization characteristic is
As shown in the figure, the flat part is widened to accommodate all of these cases. Therefore, only a reproduced luminance signal with a worse signal-to-noise ratio than originally obtained is obtained.

(3) Purpose of the Invention The present invention aims to provide optimal recording equalization characteristics in order to deal with such problems, and to obtain a reproduced luminance signal having a better signal-to-noise ratio than the conventional one.

(4) General description of the invention In order to achieve the above object, the present invention is configured to change the reproduction equalization characteristic according to the APL of the luminance signal.

(5) Embodiment Hereinafter, the present invention will be explained in detail with reference to an embodiment shown in FIG. FIG. 5 is a signal processing system diagram of a luminance signal. In the figure, the same components as in FIG. 1 are denoted by the same reference numerals, and their explanations will be omitted.

During reproduction, the DC component of the reproduced luminance signal output from the de-emphasis circuit 12 is reproduced by a clamp circuit (not shown) and applied to the low-pass filter 13 for determining the APL of the luminance signal. The APL signal thus obtained controls the reproduction equalization characteristic of the reproduction equalization circuit 9' so that it always falls on both sides of the high and low frequencies around the FM carrier frequency corresponding to the APL.

The cut-off frequency _P and order n of this low-pass filter 13 are selected so that its delay time τ is one horizontal scanning period. For example, the delay time of a non-polar Wagner type low-pass filter can be determined by the following equation in a range where the order n is small.

τ≒1+0.5(n-1)/2π× _P (1) For example, considering the case where a fifth-order Wagner type low-pass filter is used, _P≈6KHz can be obtained from equation (1).

The reason why the delay time of the low-pass filter 13 is set to one horizontal scanning period is to match the phases of the APL signal and the reproduced FM signal. Otherwise, the purpose of the present invention, which is to provide reproduction equalization characteristics according to the APL of the reproduced luminance signal, cannot be achieved.

Luminance signals generally have a strong correlation in the vertical direction, that is, there is often almost no difference between the signal level one horizontal scanning period ago and the current signal level. Therefore, as described above, there is no practical problem in controlling the reproduction equalization characteristics to be changed using the APL signal delayed by one horizontal scanning period.

FIG. 6 shows an embodiment of the reproduction equalization circuit 9' which is a component of the present invention.

The reproduced FM signal output from the reproduction amplifier 8 is connected to the terminal 14 through the DC blocking capacitor C ₁ , the power supply V _cc and the resistor R _1.
and resistor _R2 through a bias circuit consisting of
It is amplified by transistor _Q1 , which constitutes an amplifier by connecting _R3 to the emitter end and resistor _R4 to the collector end. In addition, the emitter terminal of transistor Q ₁ is connected to an inductance L ₁ that resonates at the FM carrier frequency via a capacitor C ₂ for DC blocking, and a variable capacitance diode.
D ₁ and a resistor R ₅ that adjusts the Q of this resonant circuit are connected. Therefore, a reproduced FM signal subjected to reproduction equalization as shown in b of FIG. 7A is obtained at the emitter output terminal 15 of the transistor Q ₂ which constitutes an emitter follower with the resistor R ₆ and the transistor Q ₂ . Here, the cathode end 1 of the variable capacitance diode
6, the APL signal from the low-pass filter 13 obtained as described above is applied to change the equalization characteristics of the reproduction equalization circuit 9'. Therefore, the resonant frequency of the resonant circuit composed of the inductance L ₁ and the variable capacitance diode D ₁ is the APL signal, and the capacitance of the variable capacitance diode changes according to the APL signal. For example, when the APL is close to the white level, the 7th
As the resonance characteristics change as shown in c in Figure A,
Assuming that the frequency characteristic of the circuit provided after this circuit is a, the frequency characteristic of the entire reproduction equalization circuit, that is, the reproduction equalization characteristic, becomes b' and c' in FIG. 7B. In other words, since the reproduction equalization characteristic always changes from b' to c' around the FM carrier frequency corresponding to the APL of the luminance signal, it is no longer necessary to take a broadband reproduction equalization characteristic as in the past. As a result, the signal-to-noise ratio of the reproduced luminance signal is improved to the extent that the reproduced equalization characteristic is narrower than before. This is because FM signals have upper and lower sidebands distributed around the carrier frequency, so when the reproduction equalization characteristics are always changed around the FM carrier frequency according to the APL as in the present invention, the reproduction equalization is done to the minimum necessary. This is because the characteristics can be narrowed. Furthermore, most of the noise contained in the luminance signal is noise generated in the magnetic tape head system, and its frequency characteristics are approximately flat. Therefore, noise with frequency characteristics corresponding to the reproduction equalization characteristics is added to the FM demodulator 11, and the noise after demodulation is
It increases or decreases depending on the FM equalization characteristics. Therefore, the noise after demodulation is smaller when the narrowband reproduction equalization characteristic shown in FIG. 7B of the present invention is used than in the case of the conventional wideband reproduction equalization characteristic shown in FIG. If this is adopted, the signal-to-noise ratio of the reproduced luminance signal can be improved by 2 to 3 dB compared to the current situation.

As described above, according to the present invention, the optimal reproduction equalization characteristics are always taken according to the APL of the luminance signal, so it is possible to improve the signal-to-noise ratio of the reproduced luminance signal with a simple circuit. It has the effect of being possible. Furthermore, since the present invention only operates the playback equalization circuit according to the APL of the playback luminance signal, it is easy to play back a tape recorded with a conventional magnetic recording/playback device by changing the constants shown in FIG. This feature is also excellent in terms of compatibility.

[Brief explanation of the drawing]

Figure 1 is a signal processing system diagram of the luminance signal in a conventional magnetic recording/playback device, and Figure 2 is a diagram of the signal processing system for the VHS system.
A diagram showing the frequency allocation of a luminance signal in a VTR. Figure 3 is an example of a luminance signal with an APL close to the black level and one close to the white level. Figure 4 is the conventional reproduction equalization characteristic. Figure 5 is the invention according to the present invention. FIG. 6 is a block diagram showing one embodiment of the reproduction equalization circuit according to the present invention, and FIG. 7 is a diagram illustrating reproduction equalization characteristics according to the present invention. 1...Low pass filter, 2...Video emphasis circuit, 3...FM modulation circuit, 4...Recording equalization circuit,
5... Recording amplifier, 8... Playback amplifier, 9,9'... Playback equalization circuit, 10... Limiter circuit, 11... FM demodulation circuit, 12... De-emphasis circuit, 13...
A low pass filter that extracts APL.

Claims (1)

[Claims] 1 FM modulates the carrier wave frequency according to the brightness signal, magnetically records it on the tape head system, and extracts the brightness signal from the playback signal from the tape head system.
In the magnetic recording and reproducing apparatus having demodulation means, the reproduction signal from the tape head system is input to the FM demodulation means through a reproduction equivalent circuit that limits the frequency band. Creating a control signal indicating the average image level of the luminance signal from the output of the demodulating means with a time delay,
a control loop that changes the center frequency of the passband of the reproduction equivalent circuit in a direction corresponding to the average image level according to the control signal; A magnetic recording and reproducing apparatus characterized in that an input reproduction signal has one horizontal scanning period or a time difference between scanning lines that are highly correlated with each other.