JPH07131736A - Input circuit for fm demodulation - Google Patents

Abstract

PURPOSE:To prevent inversion of white/black levels and to easily eliminate an undesired residual FM component in the processing of a post-stage with simple configuration in an inversion suppression circuit to prevent white/black level inversion in a VTR. CONSTITUTION:For example, a primary high pass filter 31 and a primary low pass filter 32 whose cut-off frequency is the same are connected in common to an input terminal 13a receiving a reproduced FM signal. After a carrier component and a low side band component whose phase difference is 90 deg.C extracted by the filters 31,32 are limited by limiters 35,36 respectively, the resulting components are mixed by a multiplication amplifier 37, then a rectangular frequency modulation wave whose level is up-converted twice with respect to the reproduced FM signal is outputted from an output terminal 13b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulation used as an inversion suppressing circuit for preventing the occurrence of a black-and-white inversion phenomenon when FM-demodulating a video signal which is FM-modulated and recorded on a recording medium. VTR (Video Tape Record) for reproducing a video signal recorded on a magnetic tape.
r) is used.

[0002]

2. Description of the Related Art Conventionally, a VTR has an FM on a magnetic tape.
An inversion suppressing circuit is provided to prevent the occurrence of the black and white inversion phenomenon when the reproduction head reads the modulated and recorded video signal and the FM demodulation circuit demodulates it.

FIG. 5 schematically shows a conventional inversion suppressing circuit. The inversion suppression circuit is provided between a reproduction head amplifier that amplifies the output of the reproduction head and an FM demodulation circuit (neither is shown), and outputs the carrier component of the reproduction FM signal that is the output of the reproduction head amplifier. A high pass filter (HPF) 1 for extracting, a low pass filter (LPF) 2 for similarly extracting the low side band component of the reproduced FM signal, a limiter 3 for limiting the output of the high pass filter 1, and an output of the limiter 3 with respect to the above A low-pass filter 4 for compensating the phase shift caused by each of the filters 1 and 2, and an addition for mixing the carrier component phase-compensated by the low-pass filter 4 and the low-side band component which is the output of the low-pass filter 2. It consists of circuit 5.

Then, the output (mixer output) of the inversion suppressing circuit is FM-transmitted via a limiter (not shown).
It is supplied to the demodulation circuit. Here, in a normal VTR, it is possible to faithfully reproduce (reproduce) the low side band component and the carrier component at the level boundary portion of the recording FM signal that is modulated according to the black level and the white level of the video signal. Can not.

That is, in the reproduced FM signal, the waveform of the carrier component is reproduced as different from that of the recorded FM signal. Therefore, if the reproduced FM signal is simply limited, the carrier component that should be present will be lost, and this will appear as a black-and-white inversion phenomenon.

Therefore, in the case of the above-described inversion suppression circuit, by comparing and correcting the carrier component and the low side band component to eliminate carrier loss due to pre-emphasis, the black and white inversion phenomenon occurs without amplifying the noise component. It is designed to prevent

However, in the conventional circuit configuration, the high-pass filter 1 for extracting the carrier component, the low-pass filter 2 for extracting the low-side band component, and the phase shift caused by the filters 1 and 2 are compensated for. The low-pass filter 4 has a drawback that it is very difficult to set the cutoff frequency.

The setting of the addition ratio in the adder circuit 5 for mixing the carrier component and the low side band component is also an important factor in preventing the occurrence of the inversion phenomenon and becomes complicated. There is.

Moreover, the frequency of the mixer output, which is the output of the adder circuit 5, is the same as the frequency of the reproduced FM signal from the reproducing head amplifier, so that the residual FM component generated by the FM demodulation processing in the subsequent stage is reproduced as described above. It is twice the frequency of the FM signal. In this case, as shown in FIG. 6, the bands of the residual FM component and the demodulated video component are close to each other. Therefore, a low-pass filter having a sharp characteristic is required to remove unnecessary residual FM components. It Therefore, a high-order filter is required, and the number of elements increases, which makes it difficult to incorporate the IC.

Further, the phase bending near the cutoff frequency of the low-pass filter for removing the residual FM component is large,
It is difficult to flatten the group delay characteristic in the frequency band of the demodulated video component.

Further, the residual F on the substrate for external attachment.
If the M component leaks out, the beat with the delay CCD clock (7.2 MHz) becomes 800 KHz, for example, noise is generated on the TV screen, and the S / N ratio is apt to deteriorate.

[0012]

As described above, conventionally, it is very difficult to set the cutoff frequency of the filter and the addition ratio of the adder circuit, and it is complicated to remove the residual FM component in the subsequent processing. There were drawbacks such as making it

Therefore, it is an object of the present invention to provide an FM demodulation input circuit capable of preventing the occurrence of a black-and-white inversion phenomenon with a simple structure and simplifying the processing in the subsequent stage.

[0014]

In order to achieve the above object, the FM demodulation input circuit of the present invention is
In an input circuit of a demodulation circuit, an extraction unit for extracting a carrier component and a low side band component of a modulated FM signal reproduced from a recording medium with the same cutoff frequency, and the carrier extracted by this extraction unit. Component and the low side band component, and an amplifier that amplifies each output of the low side band component, a limiter that limits each output of the carrier component and the low side band component amplified by the amplifier, and a limiter that limits the output. And an arithmetic unit that multiplies the outputs of the carrier component and the low sideband component that have been generated.

Further, in the FM demodulation input circuit of the present invention, in the one used as the input circuit of the FM demodulation circuit, the primary high-pass filter for extracting the carrier component of the modulated FM signal reproduced from the recording medium. And this one
A first amplifier that adjusts the amplitude of the carrier component extracted by the second-order high-pass filter, a first limiter that limits the carrier component amplified by the first amplifier, and the same as the first-order high-pass filter A first-order low-pass filter that has a cut-off frequency of, and extracts a low-side band component of the modulated FM signal reproduced from the recording medium, and a first-order low-pass filter that adjusts the amplitude of the low-side band component extracted by the first-order low-pass filter. Two amplifiers,
A second limiter for limiting the low side band component amplified by the second amplifier, a low side band component limited by the second limiter and the carrier component limited by the first limiter are included. It is composed of a multiplier for multiplying.

[0016]

According to the present invention, the phase difference is 9 by the above means.
Since it becomes possible to output a signal that is up-converted to twice the reproduced FM signal based on the carrier component and the low side band component that are set to 0 °, it is easy to remove the residual FM component that occurs after demodulation. It is possible to do.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 4 shows a schematic configuration of a reproduction processing circuit system of a VTR in which the inversion suppressing circuit of the present invention is used.

That is, the reproducing processing circuit system of this VTR is a reproducing head amplifier for amplifying the output from the reproducing head 11 for reproducing the video signal which is FM-modulated and recorded on the magnetic tape (not shown). 12, an FM that outputs a signal that is up-converted to twice the reproduced FM signal that is the output of the reproducing head amplifier 12.
Inversion suppression circuit 13 as a demodulation input circuit, and an FM that performs FM demodulation processing on the output from this inversion suppression circuit 13
The demodulation circuit 14 and the like are included.

The inversion suppression circuit 13 is for preventing the occurrence of the black-and-white inversion phenomenon, and its details will be described below. FIG. 1 shows an outline of the inversion suppression circuit 13 described above.

The inversion suppression circuit 13 has a primary high-pass filter (H) whose input terminals are commonly connected to the reproduction FM signal input terminal 13a as shown in FIG.
PF) 31 and first-order low-pass filter (LPF) 3
2, amplifiers 33 and 34 respectively connected to the output terminals of the filters 31 and 32, limiters 35 and 36 respectively connected to the amplifiers 33 and 34, and the output terminal 13b are commonly connected, Limiter 3
It is composed of a multiplication amplifier 37 which receives the outputs from 5, and 36.

The primary high-pass filter 31 extracts the carrier component on the reproduced FM signal ((e) in the same figure) from the reproducing head amplifier 12. The primary low-pass filter 32 is a reproducing FM from the reproducing head amplifier 12.
The low side band component on the signal is extracted.

In this case, the first-order high-pass filter 31
The cutoff frequencies of the first-order low-pass filter 32 are the same, and the phase difference between both outputs is 90 ° in the entire band.

The amplifiers 33 and 34 are the filters 31 and 3 described above.
It is for adjusting the amplitude of each output from No. 2, and these amplifiers 33, 34 set the input conversion limiter level of each limiter 35, 36.

The limiter 35 obtains a rectangular limiter output ((f) in the figure) corresponding to the carrier component on the reproduced FM signal by limiting the output from the amplifier 33.

The limiter 36 obtains a rectangular limiter output ((g) in the figure) corresponding to the low side band component on the reproduced FM signal by limiting the output from the amplifier 34.

In this case, the limiter output corresponding to the low side band component is a signal whose phase is delayed by 90 ° from the limiter output corresponding to the carrier component, for example.

The outputs of the limiters 35 and 36 are set to have sufficiently large amplitude so that the input of the multiplication amplifier 37 can be reliably switched. The multiplication amplifier 37 mixes the respective outputs from the limiters 35 and 36 by multiplication, and generates a mixer output having a frequency doubled ((h) in the figure).

In this case, since the phase difference between the respective limiter outputs inputted to the multiplication amplifier 37 is always 90 °, the output terminal 13b has a frequency twice up-converted with respect to the reproduced FM signal. A signal will appear.

As described above, the inversion suppressing circuit 13 of the present embodiment.
Then, the reproduced FM signal is passed through the primary high-pass filter 31 and the primary low-pass filter 32, respectively, and the phase difference is 9
Obtain the carrier component of 0 ° and the low side band component,
Further, after passing through the limiters 35 and 36, the respective signals are mixed by the multiplication amplifier 37 to obtain a modulated FM wave having a frequency twice that of the reproduced FM signal.

As a result, it is possible to prevent the occurrence of the black-and-white inversion phenomenon with a relatively simple structure, and it is possible to easily remove the residual FM component generated after demodulation in the subsequent processing.

That is, the first-order high-pass filter 31
By setting the cutoff frequencies of the first-order low-pass filter 32 to be the same, the frequency characteristic at the output terminal 13b becomes as shown in FIG.

Therefore, the cutoff frequency Wo of the primary high-pass filter 31 and the primary low-pass filter 32 is
Need only be set in the frequency band of the above-mentioned reproduced FM signal, does not require complicated calculation, can easily set the cutoff frequency, and does not require a low-pass filter for phase compensation.

Further, since the outputs from the limiters 35 and 36 are mixed by the multiplication amplifier 37, it is not necessary to set a complicated addition ratio as in the case of using an addition circuit.

In the circuit of this embodiment, the limiter 35,
Each output of 36 need only be set to a sufficiently large amplitude to reliably switch the input of the multiplication amplifier 37. As a result, the required rectangular modulated FM wave can be obtained without requiring complicated settings and adjustments and without causing the carrier component that should be on the reproduced FM signal to be lost. is there.

Moreover, since the mixer output up-converted by a factor of 2 with respect to the input reproduced FM signal is generated, the residual FM component generated after the FM demodulation which is the subsequent process is converted into the reproduced FM signal. On the other hand, the signal has four times the frequency component.

Therefore, the relationship between the demodulated video component and the residual FM component is such that the respective frequency bands are completely separated and independent as shown in FIG. This allows
The low-pass filter for removing the residual FM component does not need to have a steep characteristic, and therefore the unnecessary residual FM component can be removed by a low-order filter. As a result, the circuit can be realized with a small number of elements and can be easily incorporated in the IC.

Further, since the cutoff frequency of the low-pass filter for removing the residual FM component can be set to a high value which is sufficiently distant from the frequency band of the demodulated video component, it does not have to be a high-order filter and therefore the video signal. It is possible to flatten the group delay characteristic in the frequency band of.

Further, since the center frequency of the residual FM component is about 16 MHz, even if it leaks to the external substrate, for example, a CCD clock for delay (7.
2MHz) and the beat becomes 8.8MHz, and does not appear as noise on the TV screen.

As described above, the reproduction F is performed based on the carrier component and the low side band component having a phase difference of 90 °.
It is possible to output a signal that is up-converted to twice the M signal.

That is, the reproduced FM signal is passed through a first-order high-pass filter and a first-order low-pass filter having the same cutoff frequency, and the carrier component and the low-side band component, each of which has a phase difference of 90 °, are limited and then multiplied. I try to mix them with an amplifier. As a result, the circuit can be simplified and the carrier component that should be present in the reproduced FM signal is lost without the need to set the cutoff frequency of the complicated filter and the addition ratio of the adder circuit. Without, it is possible to obtain the required rectangular shaped FM wave. Therefore,
With a simple circuit configuration, it is possible to prevent the black-and-white inversion phenomenon in the VTR.

Moreover, in the subsequent processing, the frequency band of the demodulated video component and the residual FM component can be completely separated, so that the residual FM component generated after demodulation can be easily removed. Is. The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made without departing from the spirit of the invention.

[0042]

As described above in detail, according to the present invention, an FM demodulation input circuit capable of preventing the occurrence of the black-and-white inversion phenomenon with a simple structure and simplifying the processing in the subsequent stage is provided. it can.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an outline of an inversion suppressing circuit according to an embodiment of the present invention.

FIG. 2 is a diagram similarly shown for explaining frequency characteristics at an output terminal.

FIG. 3 is a diagram showing frequency bands of demodulated video components and residual FM components after FM demodulation processing.

FIG. 4 is a configuration diagram schematically showing a reproduction processing circuit system of a VTR in which an inversion suppressing circuit is used.

FIG. 5 is a schematic diagram of an inversion suppressing circuit shown for explaining the related art and its problems.

FIG. 6 is a diagram showing frequency bands of demodulated video components and residual FM components after FM demodulation processing.

[Explanation of symbols]

13 ... Inversion suppression circuit, 13a ... Reproduction FM signal input terminal,
13b ... Output terminal, 31 ... First-order high-pass filter, 32
… First-order low-pass filter, 33,34… Amplifier, 35,
36 ... Limiter, 37 ... Multiplier amplifier.

Claims (2)

[Claims] 1. An FM demodulation input circuit used as an input circuit of an FM demodulation circuit, comprising: an extraction unit for extracting a carrier component and a low side band component of a modulated FM signal reproduced from a recording medium by using the same cutoff frequency. An amplifying section for amplifying each output of the carrier component and the low side band component extracted by the extracting section, and a limitation for each output of the carrier component and the low side band component amplified by this amplifying section. An FM demodulation input circuit comprising: a limiting unit for adding and an arithmetic unit for multiplying each output of the carrier component and the low sideband component limited by the limiting unit. 2. An FM demodulation input circuit used as an input circuit of an FM demodulation circuit, wherein a first-order high-pass filter for extracting a carrier component of a modulated FM signal reproduced from a recording medium, and this first-order high-pass filter are used for extraction. A first amplifier that adjusts the amplitude of the carrier component; a first limiter that limits the carrier component amplified by the first amplifier; and a cutoff frequency that is the same as that of the first-order high-pass filter. And a first-order low-pass filter for extracting the low-side band component of the modulated FM signal reproduced from the recording medium, and a second amplifier for adjusting the amplitude of the low-side band component extracted by the first-order low-pass filter, A second limiter for limiting the low side band component amplified by the second amplifier; The low side band component and the first FM demodulation input circuit, characterized by comprising a multiplier for multiplying the limit has been the carrier component by the limiter, which is limited at the limiter.