JPH077572B2 - Video signal recording / reproducing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention prevents waveform omission during recording of video information,
The present invention relates to a video signal recording / reproducing apparatus capable of reducing noise and smear immediately after a large amplitude edge during reproduction.

[Prior Art] Low carrier frequency modulation (hereinafter referred to as "FM") recording used in a video tape recorder (hereinafter referred to as "VTR") is performed in a high frequency range to improve so-called SN ratio against so-called triangular noise. There is a recording / reproducing apparatus having a structure as shown in FIG. 6 which is disclosed in Japanese Patent Application Laid-Open No. 58-159213 and which uses an emphasized video emphasis system.

In the figure, (25) shows a recording system, (26) shows a reproducing system,
The recording system (25) has a non-linear pre-emphasis circuit (27) having a non-linear amplification circuit configuration, a filter (28) having a predetermined pass band to which the signal S8 is input, and the filter (28).
Of the nonlinear pre-emphasis circuit (27) is composed of a limiter circuit (29) for receiving the output S10 of the above, and an adder circuit (30) for adding the output S11 of the limiter circuit (29) to the input video signal S7. Linear pre-emphasis circuit (3
1), and the output signal S9 is input to the FM modulator (3
After being FM-modulated in 2), it is recorded on the tape (34) by the recording head (33).

In the reproducing system (26), the signal S12 detected by the reproducing head (35) from the magnetic tape (hereinafter referred to as "tape") (34) is input to the FM demodulator (36) and demodulated video signal. Is input to the linear de-emphasis circuit (37), and its output signal S13 is a non-linear pre-emphasis circuit (27).
It is input to the nonlinear de-emphasis circuit (38) having the inverse characteristic of. Non-linear de-emphasis circuit (38)
A filter (39) with a predetermined pass band to which S13 is input
And a limiter circuit (40) to which the output S14 of the filter is input, and a subtracter circuit (41) for subtracting the output S15 of the limiter circuit from the output signal S13 of the linear de-emphasis circuit (37).

Next, the operation of the recording system (25) of this conventional example will be described.

In an adder (30) described later, an addition signal S8 (sixteenth signal) of an input signal S7 (shown in FIGS. 16 (a) and (b)) and a signal S11 is added.
The filter (28) to which the diagrams (a) and (b) are input has a frequency characteristic of, for example, a high-pass filter as shown in FIG. 7 so that a frequency component to be nonlinearly amplified can be extracted. The output S10 of the frequency component extracted from the filter (28) (Fig. 16 (a),
(B) is amplified by the limiter circuit (29) with an amplification degree corresponding to the gain. In the limiter circuit (29), as shown in FIG. 8, when the gain of the input signal S10 becomes larger than the lower limit value IN ₀ , the amplification degree decreases accordingly and the input signal S1
When 0 is the reference value IN ₂ of 0 [dB], the amplification operation is performed with a small amplification degree A _v0 . Therefore, if the input signal S10 has a small level, it is amplified with a large amplification degree, whereas if it is a large level, it is amplified with a small amplification degree. However, when the gain of the input signal S10 reaches the lower limit value IN ₀ , the limiter (29) is in a state of amplifying with the upper limit amplification degree A _vu determined corresponding to this, and the gain of the input signal S10 is lower than the lower limit value IN ₀ . Even if it decreases, the amplification factor is limited to the upper limit amplification factor A _vu . Effectively, this input signal S1
The lower limit value IN 0 of the gain ₀ is selected to be a value slightly larger than the noise level in consideration of the noise level, and thus it is possible to amplify even a small level signal in the level range close to the noise level. .

Therefore, the output terminal of the limiter circuit (29) is shown by the curves K0, K1, K2 in FIG. 9 corresponding to the three levels IN ₀ , IN ₁ , IN ₂ of the input signal S10 in FIG. So that the input signal S1
The smaller the level of 0 is, the higher the level (K0) becomes, and when the level of the input signal S10 becomes equal to or lower than the lower limit value IN ₀ , the input signal S11 that is maintained at the highest level (K0) is transmitted.

The output S11 thus obtained in the limiter circuit (29) is given to the adder circuit (30), thus forming the positive feedback circuit (42). As a result, the total frequency characteristic of the nonlinear pre-emphasis circuit (27) is as shown in FIG.
Limiter circuit (29) for the input video signal S7 shown by the curve LV
The output S8 (shown in FIG. 10) of the curves L0, L1, L2 obtained by adding the curves K0, K1, K2 of the output S11 (shown in FIG. 9) is obtained.

Further, the linear pre-emphasis circuit (31) always amplifies with the same amplification degree even when the level of the input signal S8 changes, and combined with the emphasis characteristic of the non-linear pre-emphasis circuit (27), the total emphasis of the recording system (25). The characteristic of the output signal S8 of the non-linear pre-emphasis circuit (27) is corrected so that the characteristic becomes a desired characteristic. In this case, the linear pre-emphasis circuit (31) greatly attenuates the low frequency part of the input signal S8 as shown by the curve M in FIG. 11, and gradually attenuates the relatively high frequency part as the frequency increases. It has a frequency characteristic that goes on and on.

As a result, the total emphasis frequency characteristic of the recording system (25) is as shown in FIG.

Further, in the case of FIG. 12, when the level of the input video signal S7 is small, the lower limit value to be emphasized is set to the nonlinear point of the limiter circuit (29) (that is, the bending point of the broken line).
P0 (shown in FIG. 8)),
It can be set to a level close to the noise level (-30 to -40 [dB]).

Next, the operation of the reproduction system (26) will be described.

The linear de-emphasis circuit (37) and the non-linear de-emphasis circuit (38) are composed of the reverse circuits of the linear pre-emphasis circuit (31) and the non-linear pre-emphasis circuit (27) of the recording system (25), respectively, and are reproduced from the tape (34). The reproduced video signal S12 reproduced by the head (35) is demodulated in the FM demodulation circuit (36), and in the linear de-emphasis circuit (37) the curve N of FIG.
After being de-emphasized as shown in (4), it is given to the non-linear de-emphasis circuit (38). Non-linear de-emphasis circuit (38) is the output of the linear de-emphasis circuit (37)
S13 (shown in FIGS. 16 (c) and (d)) is given to the filter (39), and its output S14 (shown in FIGS. 16 (c) and (d)) is given to the limiter circuit (40), and its output S15. Subtraction circuit (4
1) Give to linear de-enmphasis circuit (37) output S13
From the output video signal S16 (Fig. 16 (c),
(D) Shown).

The filter (39) and the limiter circuit (40) have the same configuration as the filter (28) and the limiter circuit (29) of the nonlinear pre-emphasis circuit (27), so that the frequency characteristic of the output S16 of the nonlinear de-emphasis circuit (38) is the first. Ten
As shown by the curves L01, L11, L21 in the figure, it exhibits a characteristic opposite to the frequency characteristic of the output S8 of the non-linear pre-emphasis circuit (27).

Therefore, as shown by the curve Q in FIG. 12 of the total de-enmphasis characteristic of the reproducing system (26), the characteristic is the opposite of the comprehensive pre-emphasis characteristic of the recording system (25), and thus the frequency characteristic of the input video signal S7 is almost the same. Output video signal
You can get S16.

FIG. 13 is a block circuit diagram showing another conventional example of the pre-emphasis circuit. This nonlinear pre-emphasis circuit (2
7) is a positive feedback circuit (42) from the filter (28) and limiter circuit (29) explained in the conventional example of FIG.
0) and a non-linear pre-emphasis circuit block (4
3) and (44) are connected in cascade, and some of the characteristics of each non-linear pre-emphasis circuit section (43) and (44) are shared so that the overall characteristics match the frequency characteristics of FIG. Is configured.

Although only the non-linear pre-emphasis circuit (27) of the recording system (25) is shown in this conventional example, the non-linear pre-emphasis circuit (27) of the reproducing system (26) has two stages corresponding to the non-linear pre-emphasis circuit (27). It is composed of cascaded subtraction loops composed of inverse circuits.

If configured as in this conventional example, the load of the amplification degree of the limiter circuit (29) at each stage is lightened, so that the positive feedback circuits () of the nonlinear pre-emphasis circuits (43) and (44) of the recording system (25) ( 42) does not have the possibility of oscillating and each gains a small level of gain in a stable state, thus increasing the SN ratio of the reproduced image. By the way, the gain required for the limiter circuit (29) is determined according to the tape characteristics, so that if the number of stages is only one, it may be necessary to bring the transfer function close to one. In such a case, the positive feedback circuit may become unstable.

FIG. 14 is a block circuit diagram showing the configuration of another conventional example of the non-linear pre-emphasis circuit (27). The low-pass filter (4) is connected in series with the filter (28) and the limiter circuit (29).
5) and attenuator (46) are inserted. The low pass filter (45) is used to correct a part of the frequency characteristic of the filter (28), and the attenuator (46) is used to adjust the level of the output of the limiter circuit (29). One of the low pass filter (45) and the attenuator (46) may be omitted. It is possible to obtain a video signal recording / reproducing apparatus having a frequency characteristic that is close to the desired frequency characteristic.

FIG. 15 is a block circuit diagram showing still another conventional example. In this example, the series circuit (49) of the filter (47) and the limiter circuit (48) is shared by the recording system (25) and the reproducing system (26). Is configured to. That is, both ends of the series circuit (49) are connected to the output end and the positive feedback input end of the adder circuit (30) of the recording system (25) via the first switching switches (50) and (51), and It is connected to the addition input terminal and the subtraction input terminal of the subtraction circuit (41) of the reproduction system (26) via the second switching switches (52) and (53). Here, the switching switches (50), (51) and (52), (53) operate in reverse to connect the series circuit (49) to the recording system (25) or the reproducing system (26).

According to this embodiment, it is not necessary to separately provide the serial circuit (49) of the filter (47) and the limiter circuit (48) in the recording system (25) and the reproducing system (26), so that the entire configuration is reduced accordingly. Can be simplified.

As described above, various conventional examples are conceivable, but the frequency characteristics thereof are the same, and the basic circuit configuration of FIG. 6 may be considered. However, in this case, the recording system and the reproducing system filter and the limiter have the same circuit.

[Problems to be Solved by the Invention] Since the conventional video signal recording / reproducing apparatus is configured as described above, it may be considered as a filter (28) or (39) (a high-pass filter in the circuit configuration of FIG. 6). ) Is set to 300 KHz to 1 MHz due to the nature of the noise component (the noise component is considered to be a high frequency component), and the limit level IN ₀ of the limiter (29) or (40) is 100
It is set to a level of several% (2% to 5%) with respect to the level of the% luminance signal. Therefore, the limiter (29) or (4
(0) limits the time width to several hundreds of ns, and smear that is determined by the limit level occurs in the limited feedback, and noise components cannot be removed during that period. As shown, the smear and noise components immediately after the edge of the large-amplitude input signal remain, resulting in a problem that the reproduced image is not good.

To solve this problem, set the cutoff frequency of the filter (28) or (39) high to reduce smear and set the limit level of the limiter (29) or (40) high. , There is a method to reduce the noise component immediately after the edge of the large amplitude input signal, but both, that is, the reduction of smear and the reduction of the noise component immediately after the edge of the large amplitude input signal, have the opposite property. Therefore, in practice, the two are balanced and the cutoff frequency of the filter (28) or (39) and the limiter (2
Since the limit level IN ₀ of (9) or (40) is set, there is a problem that the smear is not reduced and the noise component immediately after the edge of the large amplitude input signal is not sufficiently reduced.

The present invention has been made to solve such a problem, and an object thereof is to obtain a video signal recording / reproducing apparatus capable of reducing smear even at the time of a large amplitude input signal and reducing the noise component immediately after the edge. And

[Means for Solving the Problems] A video signal recording / reproducing apparatus according to the present invention comprises a non-linear circuit section composed of two limiter circuits, one filter and three subtraction circuits, and an input video signal is recorded at the time of recording. It is characterized in that the signal passing through the above-mentioned non-linear circuit section is positively fed back and the signal passing through the above-mentioned non-linear circuit section is subtracted from the reproduced video signal at the time of reproduction.

[Operation] The non-linear circuit section according to the present invention and the positive feedback circuit during recording,
The subtraction circuit during playback is composed of an emphasis circuit that changes the time constant so that the period during which the amplitude is limited becomes small when a large-amplitude video signal comes in, and a de-emphasis circuit corresponding to this circuit. It is possible to obtain a video signal with prevention of chipping and reduction of smear and residual noise immediately after the edge of a large amplitude signal during reproduction.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to the drawings. First
The figure is a block circuit diagram of this embodiment.

In the figure, (1) is a recording video signal input terminal, (2)
Is an addition circuit, (3) is a first subtraction circuit, (4) is a second subtraction circuit, and (5) is a low-pass filter (hereinafter, "LPF").
, (6) is the first limiter circuit (7) is the third subtraction circuit, (8) is the second limiter circuit, (9) is the output terminal of the non-linear circuit section (10), and (11) is Non-linear circuit section (1
(0) input terminal, (12) fourth subtraction circuit, (13) non-linear de-emphasis output terminal, (14) and (15) conductive switches during recording, (16) linear pre-emphasis circuit, (17) ) Is an FM modulation circuit, (18) is a recording head, (1
9) is a magnetic tape, (20) is a reproducing head, (21) is an FM demodulation circuit, (22) is a linear de-emphasis circuit, (23),
(24) is a switch that conducts during reproduction.

Next, the operation of this embodiment will be described.

If the level and frequency at the input terminal (11) of the non-linear circuit section (10) are the same, the output of the non-linear circuit section (10) during recording or reproduction will be exactly the same. As described in the conventional example, if the input level of the limiter circuit is determined, the gain is determined correspondingly. Therefore, if the input level is the same and constant during both recording and reproduction, it is nonlinear. The circuit (10) can be linearized, and if the transfer function at that time is H (s), the transfer functions of nonlinear pre-emphasis and nonlinear de-emphasis are respectively Therefore, it is understood that they are inverse functions of each other. Therefore, it can be understood from the above that the waveform reproducibility during recording and reproduction can be maintained if the input levels are equal.

Now, let us specifically find out how the transfer function H (s) of the above-mentioned nonlinear circuit (10) is in recording or reproducing.

When the transfer function of LPF (5) is G (s) and the time constant is T (for example, when LPF (5) is configured as shown in FIG. 2, time constant T = CR), Required.

First, when the input signal level is small, and the two limiter circuits (6) and (8) have the input / output characteristics as shown in FIG. 3, the gain becomes a constant value A _L. (The limiter circuit (6) is A _L1 and the limiter circuit (8) is A _L2 ) Therefore, the input signal is E _i (s), the input signal is E ₀ (s), and the output of the limiter circuit (6) is X (s), limiter circuit (8)
Let the output of Y (s) (= E ₀ (s) (However, s is a Laplace operator and s = jω) G (s) is from FIG. Substituting equation (2) into equation (1) and rearranging Let the transfer functions of nonlinear pre-emphasis and nonlinear de-emphasis be F _E (s) and F _D (s), respectively.
From the formula (3), when a small signal is input, (4) Dividing the numerator and denominator of (5) in 1 + A _L1 here If Ta> Tb (∵A _L1 , A _L2 > 0), then F
_It can be seen that _E (s) shows a high-pass characteristic and F _D (s) shows a low-pass characteristic, and its time constant can be controlled by the gains A _L1 and A _L2 of the limiter circuits (6) and (8).

When the input signal level is large (larger than v _{L in} FIG. 3), the gain of the limiter circuit becomes small. For example, when the input signal of 100% level is input, the gain of the limiter circuit becomes A _{If NL} is reached (the limiter circuit (6) is A _NL1 and the limiter circuit (8) is A _NL2 ), the transfer functions corresponding to equations (4a) and (5a) are Next to If the limiter circuit is configured so that A _NL1 and A _NL2 << 1 with Ta ′> Tb ′, Ta′≈T and Tb′≈T.

FIG. 4 shows a frequency characteristic diagram in which the input levels of nonlinear emphasis and de-emphasis are used as parameters. In this figure, when the input signal level is lower than v _{L in} FIG. 3, the gain is relatively high, and for example, 0 <A _L1 <
When A _L2 is set, Ta> T, Tb <T, and the characteristics a and a ′ in FIG. 4 are obtained. This is the frequency at which emphasis begins to take into consideration when using emphasis. Lower and saturated frequencies Means higher.

When the input signal is large (larger than v _{L in} FIG. 3), the gain gradually decreases, so the gain A
Assuming that the way of decreasing _NL2 is larger than that of A _NL1 , Ta> Ta 'and Tb <Tb'. It can be seen that the characteristic curve changes as b → c → d → e in FIG. 4 as the input level increases.

In the case of de-emphasis, since the characteristics are clearly opposite to the emphasis according to equations (5a) and (7), a '→ b' → c '→ in FIG. 4 as the input signal level increases.
It can be seen that the characteristics change as d '→ e'.

Further, Enfuashisu amount of saturated portions of the respective characteristic curves or Deienfuashisu amount Enfuashisu weight _g e = 1 + A _L2, or _g e = 1 + A _NL2 Since it is determined only by the gain for the input level of the limiter circuit (8), it is easy to obtain an emphasis amount or a de-emphasis amount equivalent to that of the conventional example.

Therefore, in this embodiment, it is possible to change the effective time constant of the filter dynamically according to the input signal level while keeping the noise reduction effect of the flat portion of the video signal as it is. This effect will be described with reference to the waveform chart of FIG. When a large amplitude signal S ₁ is input during recording (5th
In the figure (a), the time constant Ta 'becomes smaller, and the edge portion subjected to emphasis like S ₂ shown by the solid line in the figure (a) converges faster (in the conventional example, it is like the dotted line). Takes time to converge). Therefore, the time width of the overshoot when it is limited by the limiter circuit (8) at the subsequent stage becomes short, and as a result, the edge as shown by S ₃ in the same figure (a) is formed. Since this signal also passes through the same circuit during playback, the time constant of the edge part overshoot part becomes shorter than before due to the small time constant, and is shown by S ₆ in the same figure (c). As described above, smear and edge noise are significantly reduced as compared with the conventional example. Further, when a small amplitude is input (FIG. 5 (b)), the time constant Ta is larger than Ta 'when a large amplitude is input, but since the signal amplitude is smaller than the limiter level, the same figure (d) is used. As indicated by S ₆ in the figure, the occurrence of smear and edge noise is subtle and not a problem.

[Effects of the Invention] As described above, according to the present invention, two limiter circuits, one filter, and three subtraction circuits that can switch the non-linear pre-emphasis and the non-linear de-emphasis of a video signal recording / reproducing apparatus at the time of recording and at the time of reproducing. Since the circuit is composed of a non-linear circuit, the effective time constant of the filter becomes smaller as the input signal becomes larger, so that the S / N ratio improvement equivalent to the conventional emphasis is obtained and the smear is improved. And edge noise can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of one embodiment of the present invention, FIG. 2 is a circuit diagram showing a configuration example of a low-pass filter of this embodiment, and FIG. 3 is an input / output characteristic diagram of a limiter circuit of this embodiment. FIG. 4 is a frequency characteristic diagram of the non-linear circuit portion of this embodiment, FIG. 5 is a waveform diagram of each portion in this embodiment, and FIG. 6 is a block circuit of a recording system and a reproducing system of a conventional video signal recording / reproducing apparatus. FIG. 7, FIG. 7 is a characteristic diagram of the filter, FIG. 8 is a characteristic diagram of the limiter circuit, FIG. 9 is a characteristic diagram of the positive feedback circuit, and FIG. 10 is a characteristic diagram of the non-linear pre-emphasis circuit of this conventional example. FIG. 11 is a characteristic diagram of the linear pre-emphasis circuit, FIG. 12 is a general characteristic diagram of the recording system of this conventional example, and FIGS. 13, 14, and 15 are other conventional examples of the video signal recording / reproducing apparatus. Block schematic of the 16th
The figure is a waveform diagram of each part of the conventional example of FIG. (2) ... Adder circuit, (3), (4), (7), (12)
…… Subtraction circuit, (5) …… Low-pass filter, (6),
(8) …… Limiter circuit, (10) …… Nonlinear circuit section,
(14), (15), (23), (24) …… Switch, (16)
...... Linear pre-emphasis circuit, (17) …… FM modulation circuit, (18) …… Record head, (19) …… Magnetic tape,
(20) …… Playback head, (21) …… FM demodulation circuit, (22)
...... Linear pre-emphasis circuit. In each drawing, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. An input video signal is angle-modulated and recorded via a nonlinear emphasis circuit and a linear emphasis circuit,
What is claimed is: 1. A video signal recording / reproducing apparatus configured to angularly demodulate a reproduced video signal to obtain a reproduced video signal via a linear de-emphasis circuit and a non-linear de-emphasis circuit, wherein the non-linear emphasis circuit has one input video signal. An adder circuit input to the input terminal and an output signal of which is input to the linear emphasis circuit, a first subtractor circuit to which an output signal of the adder circuit is input to a plus terminal, and an output signal of the first subtractor circuit Is input to the plus terminal, a low-pass filter to which the output signal of the second subtraction circuit is input, and an output signal of this filter are input, and the output signal is the second subtraction circuit. The output signal of the first limiter circuit and the output signal of the filter are input to the minus terminal of the second limiter circuit, and the output signal of the second subtraction circuit is input to the minus terminal of the A third subtraction circuit input to the input terminal and an output signal of the third subtraction circuit, and the output signal is input to the negative side of the first subtraction circuit and the positive terminal of the addition circuit, respectively. Second
The video signal recording / reproducing apparatus is characterized in that the non-linear de-emphasis circuit is composed of an inverse circuit of the non-linear de-emphasis circuit.