JPH03209668A - Fm signal processing circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of an inversion phenomenon while preventing a demodulation signal from being deteriorated by processing an FM reproducing signal obtained from a recording medium by an adaptive equalizer, demodulating the processed signal, and additionally demodulating the processed output to detect a zerocrossing lack and to control the equalizer. CONSTITUTION:An FM video signal reproduced from the recording medium 1 is demodulated by the 1st FM demodulator 5 through the adaptive equalizer 70 and outputted. The lower side wave of an output from the equalizer 70 is emphasized by an LPF 15, demodulated by the 2nd FM modulator 16, the zerocrossing is detected by an inversion detector 17, a voltage control signal corresponding to the zerocrossing frequency is outputted from a control signal generator 18, and the frequency characteristics of the equalizer 70 is controlled so that the zerocrossing is not lacked. Thereby, it is unnecessary to provide the equalizer 70 with a characteristic for emphasizing the lower side wave, and while preventing the deterioration of the demodulated signal, the generation of the inversion phenomenon can be prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a video tape recorder (hereinafter referred to as rVTRJ).
The present invention relates to an FM signal processing circuit used in the following applications.

[Conventional technology]

In a VTR, when an FM signal with a high degree of modulation is recorded and reproduced, a problem called an inversion phenomenon occurs.

The playback system of a VTR is configured as shown in Figure 10.
Amplifies the weak reproduced FM signal and compensates for the frequency characteristic fl +'F
, FM demodulates and reproduces the video signal.

The problem with such a system is that when the video signal changes from the black level to the white level, as shown in FIG. 11 (al), the demodulated video signal changes from
), a so-called inversion phenomenon occurs in which the light falls toward the black side.

This means that the recording and reproducing characteristics of the electromagnetic conversion system are
This is due to the fact that it has the property of lower side wave emphasis J.

In Fig. 10, the reproduced FM signal extracted from the tape (1) by the head (2) is amplified to a desired level by the amplifier (3), and the frequency which relatively suppresses the F side wave is set by the equalizer (4). After characteristic correction, l-'M
It is restored to a video signal by a demodulator (5) and taken out to an output terminal (6). At this time, if the characteristics of the electromagnetic conversion system fluctuate due to various causes, or if the degree of modulation becomes extremely high and the degree of emphasis on the lower side waves increases, the output of the equalizer (4) will change as shown in Figure 12 (b). ), it no longer crosses the AC zero potential. In other words, a zero cross is missing. When such a signal is subjected to FM demodulation, a steep level drop occurs, resulting in a waveform as shown in FIG. 11 (bl).

In other words, the lack of a zero cross causes an inversion phenomenon.

As a conventional method for preventing this reversal phenomenon, there is a proposal, for example, disclosed in Japanese Patent Application Laid-open No. 133358/1983. This method uses the reproduced FM reproduced from the recording medium.
(The % sign is manually input to the adaptive equalizer, and the output of the adaptive equalizer is demodulated by the FM demodulator, and at the same time, the output of the adaptive equalizer is input to the level comparison circuit that compares the FM carrier level and the lower side wave level. The characteristics of the adaptive equalizer are controlled by the output of the level comparison circuit so that the lower side wave level of the output of the adaptive equalizer is always lower than the FM carrier level.

Figure 13 is a block circuit diagram of this system.
The reproduced signal is amplified by an amplifier (3) and input to an adaptive equalizer (7). Adaptive equalizer (7
) output is passed through a bandpass filter +91. (II), a level comparison circuit (8) consisting of a detector (10), (121), a volume (13), and a comparator (14).
), and the output of the level comparison circuit (8) controls the adaptive equalizer (7).
) is sent to the F'M demodulator (5) for demodulation.

Next, the operation will be explained using FIG. 14, which shows the spectrum of each part in FIG. 13. Now adaptive equalizer (7)
The output spectrum of the adaptive equalizer (7) when not operating is shown in Figure 14 (at).
The lower side wave f when the carrier frequency fc is modulated by the signal [,
, indicates that the level of fe is reproduced higher than the level of fe. In the case of single-sided transmission as described above, if the signal is input to the FM demodulator (5) as it is, an inversion phenomenon will occur.

Therefore, a bandpass filter (
11) and the output of the bandpass filter (9) that passes the lower side wave frequency f are detected by the detectors (12+, (101), respectively, and compared by the comparator (14). Since the output level of the detector (10) is higher than the output level, the output of the comparator (14) is +E
becomes. The adaptive equalizer (7) is, for example, the circuit shown in FIG. 15, and when receiving a positive control signal, T R
2 becomes conductive and 1. It is configured such that the peaking amount near the resonance frequency of x and Cx (the resonance frequency is set near the FM carrier, generally near the high frequency) increases.

As a result, the output of the adaptive equalizer (7) is always A (fc)>A (f,
), and the reversal phenomenon is prevented.

However, this method has the following drawbacks.

(2) Since the level comparison of the detection outputs of the FM carrier fe and the lower wave f is used as a criterion for determining whether or not an inversion phenomenon has occurred, the control for preventing inversion is indirect, and a sufficient effect cannot be obtained.

In other words, the explanation of the effect of this method assumes single-side wave transmission, but in actual VTR transmission signals, upper side waves are also transmitted, although the level is relatively small, resulting in so-called residual side wave transmission. ing.

Therefore, there is a fundamental problem that the occurrence of an inversion phenomenon cannot be determined simply by the magnitude of the levels of fe and fl.

(2) Since the edge portion of a video signal where an inversion phenomenon is likely to occur generally has a wide frequency spectrum, it is actually impossible to simply compare the levels of the FM carrier fe and the child side wave f.

In other words, even if detection is performed using a bandpass filter,
It can only have an effect on modulated signals of specific frequencies.

[Problem to be solved by the invention]

As described above, the conventional device uses an indirect means of determining the occurrence of an inversion phenomenon by comparing the levels of the FM carrier and the lower side wave, so it is difficult to reliably prevent the inversion phenomenon. However, the equalizer characteristics must be set to suppress the lower side waves more than necessary, and as a result, there is a problem in that the frequency characteristics of the demodulated video signal are significantly degraded.

The inventions of claims 1 and 2 have been made to solve the above-mentioned problems, and provide an FM signal processing circuit that can reliably prevent the inversion phenomenon and suppress deterioration of the frequency characteristics of the demodulated signal. The purpose is to obtain.

[Means for Solving the Problem] The FM signal processing circuit according to the invention of claim 1 includes an adaptive equalizer into which a reproduced FM signal is input, and FM demodulation by emphasizing the lower side wave of the output of the adaptive equalizer. means for detecting the frequency of occurrence of zero-cross loss from the demodulated output; means for generating a control signal from the detected signal; It is provided with means for controlling, and means for FM demodulating and outputting the output signal of the adaptive equalizer.

The FM signal processing circuit in the invention of claim 1 performs FM demodulation by emphasizing the lower side wave of the output signal of the adaptive equalizer, detects zero-cross loss from this demodulated output, and adjusts the control voltage according to the frequency of occurrence of this zero-cross loss. The control voltage signal is used to control the frequency characteristics of the adaptive equalizer in such a way that no zero-cross drop occurs, thereby preventing the occurrence of the inversion phenomenon.

The FM signal processing circuit according to the invention of claim 2 is a reproduction FM signal processing circuit.
An adaptive equalizer to which a signal is input, means for emphasizing the lower side wave of the output of the adaptive equalizer and detecting zero-cross loss thereof, means for generating a control signal from this detection signal, and a means for generating a control signal based on the control signal. The present invention includes means for controlling the frequency characteristics of the adaptive equalizer so that zero-crossing loss does not occur, and means for FM demodulating and outputting the output signal of the adaptive equalizer.

The FM signal processing circuit in the invention of claim 2 emphasizes the lower side wave of the output signal of the adaptive equalizer, detects zero-cross loss of this signal, and obtains a control voltage signal according to the frequency of occurrence of this zero-cross loss, This control voltage signal controls the frequency characteristics of the adaptive equalizer so that zero-crossing points do not occur, thereby preventing the occurrence of an inversion phenomenon.

[Embodiment of the Invention] An embodiment of the invention of claim 1 will be described below with reference to FIGS. 1 to 5.

In FIG. 1, (5) is the first FM demodulator, (15
) is a lower side wave emphasizing filter, and a low-pass filter (!, P
F). (16) is a second FM demodulator, (17) is an inversion detector, (18) is a control signal generator, and (70) is an adaptive equalizer.

Fig. 2 is a circuit diagram showing an example of the configuration of the adaptive equalizer (70), Fig. 3 is a circuit diagram showing an example of the configuration of the inversion detector (17), and Fig. 4 is a circuit diagram of the control signal generator (18). - It is a circuit diagram showing a configuration example.

Next, the operation will be explained.

The reproduced FM signal extracted by the head (2) is amplified to a desired level by the amplifier (3) and input to the adaptive equalizer (70). This output signal is input to the first FM demodulator (5) and is FM demodulated. The adaptive equalizer (70) is configured so that the degree of lower side wave suppression is controlled by a control signal input from a control signal generator (18), which will be described later, and the frequency characteristics are corrected so that zero-crossing loss does not occur. has been done.

On the other hand, the output of the adaptive equalizer (70) is demodulated by the second [-'M demodulator (16) via the low-pass filter (I5) and input to the inversion detector (17).

Now, as described above, when an inversion phenomenon occurs, the demodulated output has a waveform as shown by the solid line in FIG. 5(a). The reversal detector (17) is configured as shown in Fig. 3, and with the reference potential VLll as the center, if the human power is lower than that, it is converted to F level, and if it is higher, it is converted to [7 level and binarized] It is a comparator.

Therefore, when a demodulated signal in which an inversion phenomenon occurs as shown by the solid line in FIG. 5 (al) is input, an inversion detection signal shown in FIG. 5 (bl) is output.

This inversion detection signal is input to a control signal generator (18). The control signal generator (18) is composed of, for example, a smoothing circuit as shown in FIG. 4, and the inversion detection signal is a signal with a number of pulses depending on the frequency of occurrence of zero-cross omissions, so a control signal can be generated! -) The output of the device (18), No. 7, is obtained as a weight change proportional to the number of pulses per fixed time. This is used as the control voltage (g) to the adaptive equalizer (70).
The configuration is as shown in the figure, and as the frequency of occurrence of missing zero cross increases, the number of pulses per fixed time of reversal detection No. 13 increases, so the control voltage signal rises, and the output resistance of transistor 1' F < 2 becomes smaller, C,, I-,
The amount of resonance peaking near the 1·゛M carrier frequency fe increases, and as a result, the occurrence of zero-cross loss is suppressed.

According to this embodiment, zero-crossing omissions are directly detected without using indirect determination as in the conventional example, so that occurrence of zero-crossing omissions can be reliably prevented.

In addition, since the peaking amount of the adaptive equalizer (70) changes continuously according to the frequency of occurrence of zero-cross loss, a necessary and sufficient amount of frequency characteristic correction can be ensured, and deterioration of the frequency characteristic of the demodulated video signal can be kept to a minimum. be able to.

Furthermore, the inversion detector (17) is equipped with an adaptive equalizer (
The output signal of 70) is once passed through a low-pass filter (15).
Since the demodulated signal is input through the filter, it is possible to increase the frequency of occurrence of zero-cross loss in advance and detect it. It also has the effect of being able to be controlled freely by setting it.

In the above embodiment, the control signal generator (1B) is constructed of an analog circuit, but it may be constructed of a circuit that performs digital processing such as pulse counting.

Further, the inversion detector (17) is not limited to the above embodiment, and may be any type as long as it can detect a steep drop in the demodulation level.

Further, the adaptive equalizer (70) is not limited to the above-mentioned embodiments, but may be one that relatively emphasizes the FM carrier, or in other words, relatively suppresses lower side waves.

Next, an embodiment of the invention according to claim 2 will be described with reference to FIGS. 6 to 8.

In FIG. 6, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted. (30) is an inversion detector.

FIG. 7 is a block circuit diagram of an embodiment of the inversion detector (30), in which (19) is a delay device, (201, (221, (25)
) is a comparator, (21) is a bandpass filter (B
P F ) (23) is an exclusive OR circuit, and (24) is a low-pass filter.

Next, among the operations of this embodiment, claim 1 shown in FIG.
The points that are different from the embodiment of the invention will be explained.

The output signal of the adaptive equalizer (70) is input to the inversion detector (30) via the low-pass filter (15).

Next, the operation of the inversion detector (30) will be explained using the waveform diagrams of FIGS. 7 and 8.

FIG. 8(a) shows the recorded FM signal. The output signal of the adaptive equalizer (70) has its lower side waves relatively emphasized by the low-pass filter (15), and is input to the delay device (19) and the pandobus filter (21). Note that the delay device (19) is for obtaining approximately the same delay as the bandpass filter (2I). The output signal of the delay device (19) is used to emphasize the lower side wave of the low-pass filter (15).
The waveform is shown in .

The comparator (20) converts the input signal S to the signal C (see Fig.
C)) is output. As is clear from the figure, zero crossing points ((), f) disappear in the binarization code C. In other words, it is missing. This is a zero-cross loss that occurs because the lower side waves of the output signal of the adaptive equalizer (70) are emphasized by the low-pass filter (15).

On the other hand, the output of the low-pass filter (15) is input to a band-pass filter (21). Bandpass filter (2
1) is configured to suppress the lower side waves relative to the FM carrier, so the bandpass filter (19)
The output signal d has the waveform of dl in Figure 8.
is input to the comparator (22), and the binarized signal e shown in FIG. 8 (el) is output.

Binarization signals C and e are input to an exclusive OR circuit (23), and a 24cJ signal f shown in FIG. 8 () is output.

Here, the wide pulse between zero cross (a) and zero cross (b) is a pulse that occurs due to the lack of zero cross, and the other narrow pulses are not related to the lack of zero cross. Therefore, when this binarized signal r is passed through a low-pass filter (24), a signal g having a waveform shown in FIG. 8(g) is obtained. In this signal g, only the pulse corresponding to the missing zero cross becomes a signal with a large peak value. This signal g is input to the comparator (25) and is
) is output. This binary code is
This is a zero-crossing missing detection pulse, that is, an inverted detection signal.

This reversal detection signal is input to the control signal generator (18), and thereafter processed in the same manner as in the embodiment of FIG. 1 to obtain the same reversal prevention effect.

Note that the inversion detector (30) is not limited to the above-mentioned embodiments, and may be any type as long as it can detect missing zero crosses. For example, as shown in FIG. 20), the same operation can be obtained.

[Effects of the Invention] As described in L below, the invention of claim 1 provides a method for manually inputting the reproduction)-M signal reproduced from the recording medium to the adaptive equalizer, FM demodulating this output signal, and performing FM demodulation on the output signal. Separately demodulates the output of The configuration is such that a voltage signal is obtained, and this control voltage signal is used to control the frequency characteristics of the adaptive equalizer so that zero-crossing loss does not occur, thereby reliably preventing inversion phenomena and improving the frequency characteristics of demodulated video. In addition, by controlling the characteristics of the lower side wave emphasizing filter, it is possible to freely control the criteria for performing the inversion phenomenon prevention operation.

Further, the invention according to claim 2 inputs the reproduced FM signal reproduced from the recording medium to the adaptive equalizer.

This output signal is FM demodulated, and the output of the adaptive equalizer is input to an inversion detector via a lower side wave emphasizing filter to detect zero-cross loss, and the detection signal is input to a control signal generator to control voltage. This control voltage signal is used to control the frequency characteristics of the adaptive equalizer so that zero-crossing loss does not occur, thereby reliably preventing the inversion phenomenon and preventing deterioration of the frequency characteristics of the demodulated video signal. can be kept small, and
By manipulating the characteristics of the lower side wave emphasizing filter, it is possible to freely control the criteria for performing the inversion phenomenon prevention operation.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram of an embodiment of the invention according to claim 1,
FIG. 2 is a circuit diagram of an example of the configuration of the adaptive equalizer, FIG. 3 is a circuit diagram of an example of the configuration of the inversion detector, FIG. 4 is a circuit diagram of an example of the configuration of the control output generator, and FIG. FIG. 5 is a signal waveform diagram for explaining the operation of this inversion detector, FIG. 6 is a block circuit diagram of an embodiment of the invention of claim 2, and FIG. 7 is a block diagram of an example of the configuration of the inversion detector. The circuit diagram, Fig. 8 is a signal waveform diagram of each part of this inversion detector, Fig. 9 is a block circuit diagram of another configuration example of the inversion detector, and Fig. 1Q shows the basic configuration of the FM signal processing circuit. Block circuit diagram, 11th
Figure 12 is a signal waveform diagram to explain the inversion phenomenon, Figure 12 is a signal waveform diagram to explain zero-cross loss, Figure 13 is a block circuit diagram of a conventional FM signal processing circuit, and Figure 14 is a diagram of the FM signal. FIG. 15 is a circuit diagram of a conventional adaptive equalizer. (5), (161...FM demodulator, (15)...
・Lower side wave emphasis filter (1, P F ), (17)
, (301...inversion detector, (I8)...control signal generator, (19)...delay device, (zo), (2
2). (25)... Comparator, (21)... Band pass filter, (23)... Exclusive OR circuit, (24
)...Low pass filter, (70)...Adaptive filter. Note that in each figure, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] (1) An adaptive equalizer into which a reproduced FM signal reproduced from a recording medium is input, a first FM demodulator that demodulates the output signal of the adaptive equalizer, and a lower side wave of the output signal of the adaptive equalizer. a lower side wave emphasizing filter that relatively emphasizes the lower side wave emphasizing filter, a second FM demodulator that demodulates the output signal of the lower side wave emphasizing filter, and an inverter that detects the zero crossing point of the output signal of the second FM demodulator. A detector, a control signal generator that generates a voltage signal corresponding to the number of signals detected by the inverting detector, and an output voltage signal of the control signal generator that adjusts the frequency characteristics of the adaptive equalizer so that no zero crossings occur. An FM signal processing circuit comprising means for controlling. (2) An adaptive equalizer to which a reproduced FM signal reproduced from a recording medium is input, an FM demodulator that demodulates the output signal of this adaptive equalizer, and a lower wave of the output signal of the adaptive equalizer relative to each other. a lower side wave emphasizing filter that emphasizes the lower side wave, an inverting detector that detects zero-crossing omissions in the output signal of this lower side wave emphasizing filter, and a control signal generator that generates a voltage signal corresponding to the number of signals detected by this inverting detector. and means for controlling the frequency characteristics of the adaptive equalizer using the output voltage signal of the control signal generator so that zero-crossing omission does not occur.