JPH0555949B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal recording and reproducing device, and can be applied to, for example, a video tape recorder.

Low-carrier FM (frequency modulation) recording, which is used in video tape recorders (VTRs),
In order to improve the signal-to-noise ratio against so-called triangular noise, a video emphasis system for emphasizing high frequencies was used, and conventionally there was a recording system configured as shown in Figure 1. That is, the input video signal S1 is subjected to emphasis in the first stage pre-emphasis circuit 1 to improve the SN ratio in the middle and high frequencies in a band of 130 [kHz] or more, as shown in FIG. The output is given to a first stage nonlinear compressor 3 having a diode configuration via an amplifier 2, and performs compression in which attenuation increases at high levels and frequencies. The output is passed through an amplifier 4 to a second-stage pre-emphasis circuit 5 where, as shown in FIG. 3, emphasis is applied to improve the band above 43 kHz for the purpose of reducing crosstalk from adjacent tracks. Furthermore, the output is a peaking circuit in the third stage pre-emphasis circuit 6 which has a peak around 1.8 [MHz] as shown in FIG.
Perform emphasis to improve the ratio. The output is improved for low frequency, large amplitude signals in the second stage ratio linear compressor 7 having a diode configuration.

The output S2 of the second stage nonlinear compressor 7 is modulated by an FM modulator and recorded on a tape as a recording medium.

On the other hand, the playback system demodulates the playback video signal with an FM demodulator, then the second stage nonlinear compressor 7, the third stage pre-emphasis circuit 6, the second stage pre-emphasis circuit 5, the first stage non-linear compressor 3, and the first stage non-linear compressor 7. A first-stage expander, a first-stage de-emphasis circuit, each having inverse characteristics corresponding to the stage pre-emphasis circuit 1;
Processing is performed in a second stage expander, a second stage de-emphasis circuit, and a third stage de-amplification circuit, thus maintaining the linearity of the input/output characteristics.

The overall pre-emphasis characteristics of the recording system with this conventional configuration are as shown in FIG.
A characteristic is obtained in which the emphasis gain becomes maximum near , and decreases as the frequency becomes higher or lower than the maximum gain. Here, the nonlinear compressors 3 and 7 have a gain of 0 [dB] (=
A total emphasis gain characteristic curve is provided such that the emphasis gain automatically increases when changing from 1 [V _pp ] to -20 [dB]. In this way, by applying a large amount of emphasis to the low-level portion where noise is visually noticeable and a small amount of emphasis to the high-level portion where the noise is not noticeable, an inversion phenomenon occurs in the spike portion of the rising portion of the video signal from black to white. prevent the occurrence of Incidentally, this inversion phenomenon is likely to occur when a large amount of emphasis is applied when the amplitude of the input video signal S1 is large.

By the way, in actually configuring the circuit shown in FIG. 1, the first and second stage nonlinear compressors 3 and 7 in the recording system have a configuration in which a diode forming a nonlinear characteristic is connected to the collector or emitter of a transistor, and The first, second and third stage pre-emphasis circuits 1, 5 and 6 are constructed of transistor amplifier circuits having frequency characteristics as shown in FIGS. 2, 3 and 4, respectively. The expander and de-emphasis circuits in the reproduction system have similar configurations, and in the end, a large number of transistors (approximately 10 to 20 transistors) are connected in cascade for the entire recording and reproduction system. This means that the characteristics of the recording system and playback system of a video signal recording and playback device are affected by the characteristics of the transistors as a whole. For example, delays occur in the video signal processed by each transistor, and high-frequency frequency characteristics will deteriorate.

It is thought that compensation for the effect of this transistor characteristic can be done in any part of the pre-emphasis system or de-emphasis system.
In practice, it is necessary to perform compensation in the de-emphasis system by taking into consideration the characteristics of the entire system, and for this reason, designing the de-emphasis system has been extremely difficult in the past. Incidentally, in reality, the circuit configurations and circuit constants of pre-emphasis and de-emphasis systems are quite different due to the relationship with the frequency characteristics of the tape or head system, and therefore playback image quality and
This is because it is considered good to use a de-emphasis system to compensate for the characteristics of the transistor as well as the characteristics of the RF system.

In addition, in the conventional configuration shown in FIG. 1, in practice, semi-fixed resistors,
Adjustable elements such as capacitors are prepared to achieve predetermined characteristics, so when configuring it on an IC, it is necessary to be able to connect many of these adjustable elements as external components. There is an inconvenience.

Furthermore, in the conventional configuration shown in Figure 1, since diodes are used as nonlinear elements in the compressor and expander, the level of the nonlinear point (that is, the bending point of the polygonal line in the nonlinear characteristic curve) (this is the threshold of the diode). (determined by the waveform level) cannot be made too small, making it difficult to maintain the reproducibility of small-level parts that are visually noticeable on the reproduced image, and deterioration such as smearing cannot be avoided.

The present invention has been made in consideration of the above points, and
By making it possible to easily reduce the nonlinear points in the pre-emphasis and de-emphasis systems to a small level, the reproducibility of the reproduced image is further improved, and the pre-emphasis level of the low-level portion of the video signal is increased for playback. painting
Provided is a video signal recording/reproducing device that can maintain the SN ratio at the same level as before, and further simplify the design of the de-emphasis circuit by configuring the de-emphasis circuit as an inverse circuit to the pre-emphasis circuit. This is what I am trying to propose.

An embodiment of the present invention will be described below in detail with reference to the drawings. In FIG. 6, 11 is a recording system, 12 is a reproduction system, and the recording system 11 has a nonlinear pre-emphasis circuit 13 having a linear amplifier circuit configuration. A filter 14 having a predetermined passband, and this filter 1
4, and an adder circuit 16 that adds the output of the limiter circuit 15 to the input video signal S1. The output S11 of the adder circuit 16 is input to the filter 14, and is also sent to the linear pre-emphasis circuit 17 as the output of the non-linear pre-emphasis circuit 13. The output S12 of the linear pre-emphasis circuit 17 is FM modulated in the FM modulator 18, and then sent to the recording head 1.
9 is recorded on the tape 20.

Here, the filter 14 has a seventh
The output S14, which has the frequency characteristics of, for example, a high-pass filter as shown in the figure, and is made up of frequency components extracted from the filter 14 is amplified by the limiter circuit 15 at an amplification degree corresponding to its gain. As shown in FIG. 8, the limiter circuit 15 decreases the degree of amplification when the gain of the input signal S14 becomes larger than the lower limit value _IN0 , and the gain of the input signal S14 decreases accordingly.
When 14 is the reference value N ₂ of 0 [dB], the amplification is small.
Amplifies at _v0 . Therefore, if the input signal S14 is at 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 S14 reaches the lower limit value IN ₀ , the limiter 15 enters a state where it amplifies at the corresponding lower limit amplification degree A _vu , and even if the gain of the input signal S14 falls below the lower limit value IN ₀ . Limit the amplification degree to the upper limit amplification degree A _vu . In practice, the lower limit value IN ₀ of the gain of this input signal S14 is selected to be slightly larger than the nozzle level in consideration of the noise level, thus making it possible to amplify even small level signals in the level range close to the noise level. is being done.

Therefore, at the output end of the limiter circuit 15, the three levels IN ₀ , IN 1 , IN ₁ of the input signal S14 in FIG.
The curves KO, K1, K2 in Figure 9 correspond to IN ₂ .
As shown by , the smaller the level of the input signal S14, the higher the level (K0), and when the level of the input signal S14 becomes lower limit value _IN0 or less, the output signal is maintained at the highest level (K0). S15 is sent.

The output S15 thus obtained in the ritter circuit 15 is given to the adder circuit 16, thus forming a positive feedback circuit 25. As a result, the overall frequency characteristic of the nonlinear pre-emphasis circuit 13 is as shown in FIG. 10, with curves K0 and K1 of the output S15 (FIG. 9) of the limiter circuit 15 for the input video signal S1 (FIG. 9) shown by the curve LV. , K2 are added to obtain the output S11 of the curves L0, L1, and L2.

Furthermore, the linear pre-emphasis circuit 17 always amplifies the input signal S11 with the same amplification degree even if the level changes, and in combination with the emphasis characteristics of the non-linear pre-emphasis circuit 13, the overall emphasis characteristics of the recording system 11 can be adjusted to desired characteristics. The output signal S11 of the nonlinear enhancement circuit 13 is
Correct the characteristics of. In this case, the linear pre-emphasis circuit 17 greatly attenuates the low frequency portion of the input signal S11, as shown by curve M in FIG. 11, and gradually attenuates the relatively high frequency portion as the frequency increases. It has a unique frequency characteristic.

As a result, the overall emphasis frequency characteristic of the recording system 11 has a characteristic curve similar to the emphasis characteristic of the conventional recording system shown in FIG. 5, as shown in FIG. However, in the case of FIG. 12, the lower limit value to be emphasized when the level of the input video signal S1 is small can be determined by the nonlinear point of the limiter circuit 15 (i.e., the bending point P0 of the polygonal line (FIG. 8)). , a level close to the noise level (at most -20 in the conventional case shown in Figure 5)
It can be set as low as -30 to -40 [dB].

The above is the configuration of the recording system 11, but the reproduction system 12
The linear de-emphasis circuit 31 and the non-linear de-emphasis circuit 32 are respectively inverse circuits of the linear pre-emphasis circuit 17 and the non-linear pre-emphasis circuit 13 of the recording system 11, so that the reproduced video signal S21 reproduced from the tape 20 by the reproduction head 33 is is demodulated in the FM demodulation circuit 34, and the linear de-emphasis circuit 31
After being de-emphasized as shown by curve N in FIG. The nonlinear de-emphasis circuit 32 gives the output S22 of the linear de-emphasis circuit 31 to the filter 35, gives its output S23 to the limiter circuit 36, and gives its output S24 to the subtraction circuit 37 to obtain the output S2 of the linear de-emphasis circuit 31.
2 and sent as an output video signal S25.

The filter 35 and limiter circuit 36 have the same configuration as the filter 14 and limiter circuit 15 of the nonlinear pre-emphasis circuit 13, so the frequency characteristics of the output S25 of the non-linear de-emphasis circuit 32 are shown by curves L01, L11, L in FIG.
A nonlinear pre-emphasis circuit 13 as shown at 21
The frequency characteristic of the output S11 is opposite to that of the output S11.

Therefore, the overall de-emphasis characteristic of the reproduction system 32 is as shown by the curve Q in FIG.
The overall pre-emphasis characteristic shown in FIG. 1 is the opposite, and thus it is possible to obtain an output video signal S25 having almost the same frequency characteristics as the input video signal S1.

FIG. 13 shows another embodiment of the present invention,
In this case, the nonlinear pre-emphasis circuit 13
A plurality of, for example, two stages of nonlinear pre-emphasis circuit sections 41 and 42 each comprising a positive feedback circuit 25 consisting of the filter 14 and limiter circuit 15 and an adder circuit 16 described above with reference to the figure are connected in cascade. However, each nonlinear pre-emphasis circuit section 41 and 4
2 shares a part of the total characteristic so that it matches the frequency characteristic shown in FIG.
Although only the recording system 11 is shown in FIG. 13, the structure of the reproducing system is formed by cascade-connecting subtraction loops each consisting of a plurality of inverse circuits, for example two stages, corresponding to the nonlinear pre-emphasis circuit 13.

With the configuration shown in FIG. 13, the load on the amplification of the limiter circuits in each stage is reduced, so that the positive feedback circuits 25 of the nonlinear pre-emphasis circuits 41 and 42 of the recording system 11 can each operate in a stable state without the risk of oscillation. A small level of gain can be achieved, thus increasing the signal-to-noise ratio of the reproduced image. Incidentally, since the gain required of the limiter circuit 15 is determined depending on the tape characteristics, if there is only one stage, it may be necessary to make the transfer function close to 1 in some cases. In such a case, the positive feedback circuit may become unstable.

FIG. 14 shows still another embodiment of the present invention, in which the nonlinear pre-emphasis circuit 13 includes a low-pass filter 45 and an attenuator 46 inserted in series with the filter 14 and limiter circuit 15. In this case, the low-pass filter 45 is filter 1
The attenuator 46 is used to correct a part of the frequency characteristics of the limiter circuit 15, and the attenuator 46 is used to adjust the level of the output of the limiter circuit 15. Also in this case, a reproducing system having a reverse circuit configuration may be prepared in correspondence with the recording system 11.

With the configuration as shown in FIG. 14, it is possible to obtain a video signal recording/reproducing apparatus having frequency characteristics that closely approximate the desired frequency characteristics.

FIG. 15 shows still another embodiment of the present invention, in which a series circuit 53 of a filter 51 and a limiter circuit 52 is connected to a recording system 11 and a reproducing system 12.
It is designed to be shared among people. That is, both ends of the series circuit 53 are connected to the first changeover switches 55 and 5.
6 to the output end and positive feedback input end of the adder circuit 16 of the recording system 11, and connected to the adder input end and the subtraction input end of the subtracter circuit 35 of the reproduction system 12 via second changeover switches 57 and 58. It is connected to the. Here, selector switches 55, 56 and 57, 5
8 operate in reverse to each other to connect the series circuit 53 to the recording system 11.
Or connect it to the reproduction system 12.

According to the configuration shown in FIG. 15, the series circuit 53 of the filter 51 and the limiter circuit 52 does not have to be provided separately in the recording system 11 and the reproducing system 12, so the overall configuration can be simplified accordingly.

As described above, according to the present invention, the level at the bending point of the polygonal characteristic that limits the output level to a constant value can be easily lowered as needed by using the limiter circuit as a nonlinear element. It is possible to improve the SN ratio by emphasizing the low-level signal components close to the noise level during recording and playback, thereby further improving the expression of detail in the reproduced image. can. Incidentally, since conventional emphasis circuits use diodes to obtain polygonal characteristics, the level at the polygonal point cannot be lowered sufficiently.Therefore, if the pre-emphasis and de-emphasis do not match well, a large deterioration in image quality will occur. Depending on the invention, it can be lowered sufficiently (-30 to -40
(up to about [dB]), this problem can be effectively solved.

In this way, there is no need to connect a large number of transistors in cascade as in the conventional case, so the influence of transistor characteristics can be reduced, and external adjustment elements can be attached much more simply than in the conventional case. Incidentally, in the case of the present invention, only the area around the limiter circuit requires adjustment.

In addition to this, according to the present invention, the positive feedback circuit is
A feedback loop includes a high-pass filter that passes high-frequency components, a limiter circuit that limits the output of this high-pass filter, a low-pass filter that passes reduced components of the output of this limiter circuit, and an attenuator that attenuates the output of this low-pass filter. By connecting the positive feedback output in cascade, the frequency characteristics of the positive feedback output are not determined only by the frequency characteristics of the high-pass filter, but are determined by the low-pass filter provided on the output side. At the same time, the output level of the positive feedback output is not determined solely by the limiting characteristic of the limiter, but can be adjusted by the attenuator provided on its output side, thus making it possible to use a nonlinear pre-emphasis circuit or a non-linear de-emphasis circuit. By inserting the low-pass filter and the attenuator, the overall frequency characteristic can be made even closer to the desired frequency characteristic.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing a conventional recording system, Figure 2 is a block diagram showing a conventional recording system.
4 are curve diagrams showing the frequency characteristics of the pre-emphasis circuit, FIG. 5 is a curve diagram showing the overall characteristics of the recording system of FIG. 1, and FIG. 6 is an example of the video signal recording/reproducing apparatus according to the present invention. 7, 8 and 9 are curve diagrams showing the characteristics of the filter, limiter circuit and positive feedback circuit.
Figure 0 is a curve diagram showing the characteristics of the nonlinear pre-emphasis circuit shown in Figure 6, Figure 11 is a curve diagram showing the characteristics of the linear pre-emphasis circuit shown in Figure 6, and Figure 12 is a curve diagram showing the overall characteristics of the recording system shown in Figure 6. Curve diagram, 13th
15 are block diagrams showing other embodiments of the present invention. 1, 5, 6...Pre-emphasis circuit, 2, 4
...Amplifier, 3,7...Compressor, 11...Recording system, 12...Reproduction system, 13...Nonlinear pre-emphasis circuit, 14...Filter, 15...Limiter circuit, 16...Addition circuit, 17... ...Linear pre-emphasis circuit, 18...FM modulator, 19...
Recording head, 20... Tape, 25... Positive feedback circuit, 31... Linear de-emphasis circuit, 32...
...Nonlinear pre-emphasis circuit, 33...Reproduction head, 34...FM demodulation circuit, 35...Filter, 36...Limiter circuit, 37...Subtraction circuit,
41, 42...Nonlinear pre-emphasis circuit section,
45...Low pass filter, 46...Attenuator, 51...Filter, 52...Limiter circuit,
53...Series circuit, 54-58...Selector switch.

Claims (1)

[Claims] 1. An input video signal is recorded after being angularly modulated through a non-linear pre-emphasis circuit and a linear pre-emphasis circuit, and a reproduced video signal is obtained through a linear de-emphasis circuit and a non-linear de-emphasis circuit after the video signal is angularly demodulated during playback. In the video signal recording and reproducing apparatus, the nonlinear pre-emphasis circuit comprises a high-pass filter that passes high-frequency components, a limiter circuit that limits the output of the high-pass filter, and a low-pass filter that passes the low-frequency components of the output of the limiter circuit. and an attenuator that attenuates the output of the low-pass filter are cascaded in a feedback loop, and at least two stages of positive feedback circuits are connected in cascade. A video signal recording and reproducing apparatus characterized in that the circuit is constructed by cascading at least two stages of reverse circuits of the positive feedback circuit.