JPH077570B2 - Limiter circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a limiter circuit that removes a DC fluctuation of an FM signal reproduced from a recording medium and prevents an inversion phenomenon.

[Conventional technology]

A conventional technique will be described by taking a video tape recorder (hereinafter, referred to as "VTR") as an example of an apparatus for recording and reproducing a video signal on a recording medium by frequency modulation (hereinafter, referred to as "FM").

FIG. 7 is a block circuit diagram showing a schematic configuration of the reproduction signal processing system (luminance signal) of the VTR. In the figure, (11) is a magnetic tape as a recording medium, (12) is a reproducing magnetic head,
(13) is a rotary transformer, (14) is a reproduction preamplifier, (15) is a limiter circuit, (16) is an FM demodulation circuit, (1
7) is a low pass filter (LPF) that removes the carrier component, (1
8) is an output terminal for the reproduction luminance signal.

FIG. 8 (a) is a spectrum diagram of the FM signal at the time of recording, where fc is the carrier frequency and fm is the modulation signal frequency.
Further, FIG. 3B is an electromagnetic conversion frequency characteristic diagram in the tape / head system, and shows almost a low-pass characteristic. Also, the same figure
(c) is a diagram showing a change in spectrum when an FM signal having a spectrum as shown in (a) of the figure passes through an electromagnetic conversion system as shown in (b) of the figure.

FIG. 9 (a) is a diagram showing an example of an input video signal (luminance signal) waveform in a VTR, FIG. 9 (b) is a signal waveform diagram obtained by applying emphasis to this input signal, and FIG. 9 (c) shows this signal. A frequency-modulated signal waveform diagram, FIG. 3D is a waveform diagram of a reproduced FM signal obtained when this signal is recorded on a magnetic tape and reproduced.

FIG. 10 is a diagram showing the relationship between the input / output characteristics of the limiter circuit (15), the waveform of the input FM signal, and the waveform of the output FM signal.

Next, the operation will be described. In VTR, Fig. 9 (a)
The input video signal is recorded with emphasis to reduce triangular noise during FM modulation. (See FIG. 9 (b)) The carrier is modulated by the video signal subjected to this emphasis to produce an FM signal as shown in FIG. 9 (c), which is recorded on the magnetic tape (11). This was recorded
As shown in Fig. 8 (a), the spectrum of the FM signal has upper and lower sidebands S ₊₁ , S ₊₂ , ..., with respect to the carrier C.
The levels of S _{+ n} , S- ₁ , S- ₂ , ..., S- _n are even.

At the time of reproduction, the FM signal recorded by the reproduction head (12) is detected from the magnetic tape (11), and the rotary transformer (13) is detected.
To the playback preamplifier (14). The electromagnetic conversion characteristics of this magnetic tape head system are low-pass characteristics as shown in Fig. 8 (b), so the spectrum of the FM signal in Fig. (A) is as shown in Fig. 8 (c). Change. That is, the lower sideband component _{S -1, S -2, ......,} S -n are relatively emphasized, the upper sideband component _{S +1, S +2, ......,} S + n is the attenuation in the opposite To be done. Therefore, the input FM of the limiter circuit (15)
In the signal, the balance between the upper and lower sidebands is lost, and the level of the first lower sideband component S _-1 becomes equal to that of the carrier component C. Especially, in the high frequency part where the emphasis is applied during recording, the modulation index β (= maximum frequency deviation / modulation signal frequency) of FM becomes small, and most of the information (energy) of FM is concentrated in the first sideband. The difference between the carrier C and the first lower side band S _-1 has disappeared (the level sometimes reverses). Fig. 9
As shown in (d), a phenomenon called "zero crossing loss" of the reproduced FM signal occurs. This is the so-called inversion phenomenon. When demodulating such an FM signal,
Originally, the zero-cross point is missing even in the high brightness level, which is equivalent to a low frequency in the demodulator (16), and the demodulated video signal goes to the black side.

The limiter circuit (15) originally removes low-frequency level fluctuations and corrects the imbalance of the upper and lower sidebands, but the input that lacks zero cross as shown in FIG. In the case of an FM signal, the limiter circuit having the input / output characteristics shown in the figure does not restore the zero-cross point in principle, and an FM signal with a missing zero-cross shown in the figure is output. Also, the gain of the limiter circuit is usually
It is set fairly high (20 to 40 dB) in order to improve the unbalance correction capability of the upper and lower sidebands and to be able to cope with the drop in playback FM signal level (dropout, etc.) to some extent. Since the zero crossing of an FM signal is used as information, it does not matter even if the gain of the limiter circuit is increased to make the reproduced FM signal of a sine wave shape into a rectangular wave shape. Next-order distortion may increase, and errors may occur in the zero-cross information. Furthermore, if the gain is increased, the attenuated upper sideband can be restored as described above, but at the same time, noise in the FM band is also amplified, and the S / N of the demodulated video signal is also increased.
It leads to ratio deterioration. Therefore, it is practically difficult to unnecessarily increase the gain of the limiter circuit.

From the above, in the conventional limiter circuit, the zero cross is missing and is input to the demodulation circuit (16). Therefore, the output waveform (reproduction luminance signal) of the low pass wave filter (17) shows an inversion phenomenon at the zero cross missing point. It is understood that it will occur.

In order to solve such a problem, some home-use VTRs currently on the market are provided with a circuit called a double limiter, instead of the mere limiter circuit described above. This circuit configuration is shown in FIG. 11, and the spectrum of the signal of each part thereof is shown in FIG. 12, and the operation thereof will be described with these. In the following description, for simplicity, only the upper and lower first sidebands are targeted.
The principle is the same even if higher-order sidebands are included.

Playback FM with missing zero cross input to input terminal (1)
Signal a (shown in Figure 12 (a)) is the high-pass filter (HPF).
(19) and the low pass filter (LPF) (24). Signal b passing through the high-frequency filter (19) (Fig. 12 (b))
(Shown) is a signal d which has passed through the first limiter (21) and the phase correction circuit (23) and has passed through the low-pass filter (24).
(Shown in FIG. 12 (d)) is added by the adder (26). The output e (shown in FIG. 12 (e)) of the adder (26) passes through the second limiter (28) and is output from the output terminal (10) as a signal f shown in FIG. 12 (f). It Looking at the spectrum change of each part in this limiter circuit in FIG. 12, first, at the input terminal (1), the level of the carrier C (frequency fc) is smaller than the lower sideband S _-1 (frequency fc-fm), In fact, it is assumed that the carrier is missing due to low frequency fluctuations. By passing this spectrum through the high-pass filter (19), the level of the lower sideband S _-1 is shown in Fig. 12 (b).
Lower than Carrier C, then the first
As shown in FIG. 12 (c), the limiter (21) averages the upper and lower side bands S _-1 , S ₊₁ . By doing so, the lower sideband S _-1 is attenuated to restore the zero cross (the lack of zero cross is considered to be the low frequency swell due to the lower sideband). However, in this state, the energy of the lower sideband S ₋₁ with good S / N is attenuated from the input, which causes deterioration of resolution and S / N.
The above problem is solved by adding it to the lower sideband component d passing through the low-pass filter (24). That is,
The spectrum of the output e of the adder (26) is as shown in FIG. 12 (e), and the levels of the upper and lower side bands are adjusted by the second limiter (28). ((F) in the same figure) In this way, the double limiter circuit restores the zero cross, but in the actual VTR video signal, the modulation signal frequency fm changes instantaneously and instantaneously.
Since the frequency fc and the upper and lower sideband frequencies change accordingly, in the double limiter circuit where the cutoff frequency of the high-pass filter (19) is fixed, the amount of the lower sideband that is attenuated also changes. Resulting in.

For example, when fm is at a high frequency, fc−fm becomes a low frequency when fc is low, and the amount of attenuation at the high-pass filter (19) becomes large, and zero cross restoration is possible.
When is high (corresponding to high brightness level), fc-fm is high, and the attenuation may be small so that the zero cross cannot be restored. Conversely, even if fc−fm is high, if the cutoff frequency of the high-pass filter (19) is increased to increase the amount of attenuation, when fc−fm becomes low, the amount of attenuation in the lower sideband becomes too large. This leads to deterioration of resolution. Also, the high pass filter (19) in the adder (26)
Since the addition ratio of the signal passing through the low pass filter (24) and the signal passing through the low pass filter is fixed in the conventional circuit, the amount of the lower sideband changes depending on fc and fm as in the case described above. , If the amount of addition of the lower sideband that has passed through the low-pass filter (24) is large, the zero crossing will occur again in the adder (26), and if the amount of addition is small, the resolution will deteriorate and the addition ratio Adjustment is difficult.

Further, in such a circuit, nothing is compensated for variations in the carrier level, and when the carrier level decreases, the situation in which zero-cross loss is more likely to occur due to variations in low frequencies than usual does not improve at all.

[Problems to be solved by the invention]

Since the conventional limiter circuit is configured as described above, there are problems that the inversion phenomenon cannot be prevented depending on the spectrum of the FM signal and that the resolution and the S / N ratio deteriorate.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a limiter circuit that can reliably prevent an inversion phenomenon without deterioration of resolution and S / N ratio.

[Means for solving problems]

The limiter circuit according to the present invention is characterized in that it comprises means for dynamically controlling the cutoff frequency and the gain of the high-pass characteristic according to the level of the input reproduction FM signal.

[Action]

The control means of the high-pass characteristic according to the present invention controls the gains of the two limiters according to the input signal level so that the cutoff frequency and the gain become low when the input level is high, and conversely when the input level is low, the cutoff frequency and The gain is controlled to be high.

Example of Invention

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block circuit diagram of an embodiment of the present invention. In the figure, (1) is an input terminal, (2) is a first subtractor, (3) is a low-pass filter (hereinafter referred to as "LPF"), and (4) is a second
Subtractor, (5) is the first adder, (6) is the second limiter,
(7) is the third subtractor, (8) is the first limiter, and (9) is the second
(10) is an output terminal.

FIG. 2 is a diagram showing a spectrum of a reproduced FM signal applied to the input terminal of FIG. 1 and a characteristic example of LPF (3).

FIG. 3 is an input / output characteristic of a general limiter, FIG. 4 is a configuration example of a low-pass filter (3), FIG. 5 is a linearized block circuit diagram of this embodiment, and FIG. It is an example frequency characteristic diagram.

Next, the operation will be described. 1, the input terminal
A mathematical expression will be used to explain how to restore the zero cross when a reproduced FM signal having a zero cross is missing and having a spectrum distribution as shown in FIG. 2 is input to (1). Therefore, the input level of the limiter is considered to be constant so that the nonlinear portion (first and second limiters (8) and (6)) in FIG. 1 can be handled linearly, and as shown in FIG. ,
It is expressed by a gain K for the input level. here,
If K = E ₀ / Ei and the input level Ei is constant, the output level E
₀ is uniquely determined, and K can be treated as a constant. If LPF (3) is the R, C first order filter shown in FIG. 4 and its time constant is T = RC, the transfer function G (S) of LPF is Becomes

Assuming that the first and second limiters (8) and (6) have a flat frequency characteristic within the band, from the above assumption, the block circuit diagram of FIG. 1 is linear as shown in FIG. Since this is a circuit, the transfer function between input and output is H (S) = V ₀ (S)
/ Vi (S) can be calculated.

When the gains of the first and second limiters at the constant input level Ei are K ₁ and K ₂ , respectively, H (S) is here, Then, H (jω) is calculated from Eq. (2). Therefore, the amplitude characteristic is As can be seen from FIG. 3, the gains K ₁ and K ₂ of the limiters (8) and (6)
Becomes smaller as the input signal level increases, so (3)
From the formula, ω ₁ and ω ₂ also become smaller. K ₁ and K ₂ are positive numbers and K ₂
If <1 is satisfied, then ω ₁ <ω ₂ , so the approximate expression of equation (5) is | H (jω) | ≈1 (ii) ω ₁ <ω <ω when (i) ω <ω _{1 2} | H (jω) | ≈ω / ω ₁ (iii) ω> ω ₂ Becomes Therefore, the amplitude characteristic | H (jω) | is shown in the graph as shown in FIG.

In FIG. 6, for example, when the input level of the input reproduction FM signal is high, K ₁ and K ₂ have small values, and therefore,
Since ω ₁ , ω ₂ and 1 / (1-K ₂ ) are also small,
It has a frequency characteristic like a. Here, when ω ₂ is set to be sufficiently lower than the carrier frequency, the high-pass characteristic is weak, the lower sideband is not attenuated, and the high-frequency noise is not amplified, so an FM signal with good S / N can be obtained. It can be supplied to the FM demodulator of the next stage (see FIG. 7). That is, when the input level is high, the carrier component is also large, so zero-cross loss is unlikely to occur, and it is also strong against low frequency fluctuations.
It is not necessary to drop the lower sideband of the FM signal so much. On the other hand, when the input level is low or there is a zero-cross loss, K ₁ and K ₂ have large values, so ω ₁ and ω ₂ and 1 / (1-K ₂ ) also increase. In the figure, the frequency characteristic is d. Here, when ω ₂ is set to be approximately the same as the lowest frequency of the carrier, the high side is emphasized and the low side is relatively attenuated, so the lower sideband is attenuated and the zero cross is restored. Heading for. In addition, raise the carrier level enough for small signals at high frequencies where zero crossing is likely to occur.
(1-K ₂ ) is also made larger.

〔The invention's effect〕

As described above, according to the present invention, since the cutoff frequency and the attenuation amount of the high-pass filter are dynamically controlled according to the level of the input reproduction FM signal,
There is an effect that a limiter circuit can be obtained that can prevent the inversion phenomenon without deteriorating the resolution and the S / N ratio.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of an embodiment of the present invention, FIG. 2 is a diagram showing a spectrum of an input reproduction FM signal and characteristics of a low-pass filter, and FIG. 3 is an input / output characteristic diagram of a general limiter. FIG. 4 is a circuit diagram of a configuration example of a low-pass filter, FIG. 5 is a block circuit diagram in which the embodiment of FIG. 1 is linearized, and FIG. 6 is a frequency characteristic diagram of this linearized embodiment. FIG. 7 is a block circuit diagram of a conventional VTR reproducing system, FIGS. 8 (a) to 8 (c) are diagrams showing changes in the spectrum of the FM signal in the electromagnetic conversion system, and FIGS. 9 (a) to 9 (d) are Fig. 10 shows the waveform change during recording / reproduction in the conventional VTR, Fig. 10 shows the input / output characteristics of the conventional limiter circuit and the change in the input / output signal waveform, and Fig. 11 shows the block circuit diagram of the conventional double limiter circuit. , Fig. 12
(a)-(f) is a figure which showed the spectrum in each part of this double limiter circuit. (2) …… First subtractor, (3) …… Low-pass filter, (4)
...... Second subtractor, (5) ...... First adder, (6) ...... Second
Limiter, (7) ... third subtractor, (8) ... first limiter, (9) ... second adder. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A first subtractor to which an FM signal reproduced from a recording medium is supplied to a positive input terminal, a low-pass filter to which an output signal of the first subtractor is input, and the reproduction. FM
A second subtractor for subtracting the output signal of the low-pass filter from the signal, a first adder to which the output signal of the second subtractor is supplied to one input terminal, and the first addition And a first limiter to which the output signal of the adder is input, and the output signal of the first adder is also input to output the output signal from the first limiter.
Second limiter input to the other input terminal of the adder and the output signals of the first and second limiters are added, and the output signal is input to the negative input terminal of the first subtractor. A limiter circuit comprising a second adder and a third subtraction circuit for subtracting the reproduced FM signal from the output signal of the second limiter.