JPH01236786A - Frequency characteristic correcting device for video signal - Google Patents

Abstract

PURPOSE:To improve the precision of an AGC voltage by preventing a synchronizing signal part of a video signal, which is supplied to an AGC voltage generating circuit, from being subjected to characteristic correction at the time of supplying the video signal, which is subjected to frequency characteristic correction by emphasis or de-emphasis, to a frequency characteristic correcting device. CONSTITUTION:The video signal given from a variable gain amplifier 1 is separated by a synchronous separating circuit 6 and has a synchronizing signal (d) detected by a synchronous detecting circuit 7, and correction is not performed by an (f) characteristic correcting means 2 at least in the synchronous period. An output (c) is outputted from the circuit 6, and the pulse width of the detection output (d) is wider than that of a synchronizing signal HP to cover a pedestal PD. Consequently, the signal HP and the pedestal PD of a video signal (a) inputted to the means 2 are not emphasized, and therefore the AGC voltage of an AGC voltage generating circuit 5 is accurate based on the level from the pedestal to a sink tip.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency characteristic correction device for a video signal used in a video tape recorder (VTR) or the like.

For example, in a VTR, in order to obtain high image quality, video signals are recorded with emphasis, and upon playback, de-emphasis is applied to the reproduced video signals.

FIG. 4 shows in block form the configuration of a conventional circuit for correcting the frequency characteristics of a video signal (hereinafter referred to as "f-characteristic correction"), particularly during recording, together with related circuits. In the figure, (1) is a variable gain amplifier that varies the gain of the video signal to be recorded, and its output is passed through some predetermined circuits (not shown) to f-characteristic correction means (
2). At this time, the f-characteristic correction means (2) is adapted to vary the amount of correction depending on the magnitude of the input video signal. (3) is a detection means that outputs a signal depending on the magnitude of the input signal in order to control the amount of correction. Note that the f-characteristic correction means (2) applies emphasis to the video signal during recording. Here, the video signal subjected to emphasis is FM modulated by an FM modulator (4) and supplied to a recording circuit. The output of the f characteristic correction means (2) is A
It is also supplied to the GC voltage forming circuit (5). The AGC voltage forming circuit (5) supplies an AGC voltage corresponding to the level of the synchronizing signal to the gain control amplifier (1).

Next, during reproduction, the f-feature correction means (2) applies de-emphasis to the video signal input thereto.

The de-emphasized video signal is applied to a demodulation circuit (not shown) and also to an AGC voltage forming circuit (5).

■Problems to be solved As mentioned above, when recording and playing back video signals on a VTR, various types of emphasis and de-emphasis have been applied to improve the signal-to-noise ratio. It is sufficient to apply only the signal of the video section and there is no need to apply it to the synchronization signal, but in reality, emphasis or de-emphasis is applied to both the video section and the synchronization signal section without distinguishing between them. for that,
The waveform of the synchronization signal section also changes and the AGC voltage formation circuit (
An error occurs in the AGC voltage formed in step 5), and the variable gain amplifier (1) cannot perform accurate gain control. In other words, the video signal in FIG. 5(a) with emphasis applied (
When applying AGC using the synchronization signal (HP) of the video signal in b), the gain of the variable gain amplifier (1) must be adjusted so that the level (Vs) of the synchronization signal (HP) from the pedestal to the sync tip is constant. However, since the waveform of the synchronization signal is changed by emphasis, the level (Vs') from the pedestal to the sync tip is changed by the amount of the change (ΔVs) to the level (HP) of the reference synchronization signal (HP). It is determined that the synchronization signal is increasing from V s ), and the gain is controlled to be small according to the rate of increase to keep the level of the synchronization signal constant. As a result, the video signal as shown in Figure 5 (c) becomes smaller by (8V).

The present invention has been made in view of these points, and is
It is an object of the present invention to provide an f-character correction circuit for a video signal that can always form a correct AGC voltage when forming an AGC voltage using a video signal that has undergone special correction.

In order to achieve the above-mentioned object, the present invention provides a method for controlling a video signal when a video signal whose frequency characteristics have been corrected by emphasis or de-emphasis is supplied to an AGC voltage forming circuit for automatically varying the gain of the video signal. The frequency characteristic correction device is configured to include means for not performing the frequency characteristic correction on a synchronizing signal portion of the video signal supplied to the AGC voltage forming circuit.

Operation With this configuration, the synchronization signal portion of the output video signal of the f-characteristic correction means does not undergo waveform deformation due to emphasis or de-emphasis, so the level from the vedestal level to the sync tip level is accurately maintained as it is. and the AG formed based on that level.
The accuracy of C voltage also improves.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the drawings. In FIG. 1, parts that are the same as those in the conventional example shown in FIG. 4 are given the same reference numerals, and redundant explanation will be omitted.

In this embodiment, the video signal given from the variable gain amplifier (1) is separated by the synchronization separation circuit (6), and the synchronization signal is detected by the synchronization detection circuit (7), and at least during the synchronization signal period, the f-characteristic correction means The correction in (2) is not made.

In FIG. 2, (c) shows the output of the synchronization separation circuit (6), and (d) shows the detection output of the synchronization detection circuit (7). As can be seen from the figure, this detection output is a synchronization signal (
The width of the pulse is wider than that of HP) and covers the pedestal (PD).

Therefore, among the video signals inputted to the f-characteristic correction means (2) [FIG.
) is not emphasized, only the video portion (S) is emphasized. Therefore, the AGC voltage forming circuit (
The AGC voltage formed in step 5) will be accurate.

FIG. 3 shows the specific configuration of the f-characteristic correction means (2), in which (8) shows the first operational amplifier circuit that contributes to the emphasis operation, and (9) the second operational amplifier circuit that contributes to the de-emphasis operation. The circuit (10) is their feedback circuit. The first operational amplifier circuit (8) is used in video signal recording mode, and the second operational amplifier circuit (9) is used in playback mode. (11), (12), and (13) are switches for switching the supply of operating power (10 B) for these operational amplifier circuits (8) and (9), switching the connection of the feedback circuit (10), and switching the output output. , and the terminal (1
4) is switched to the first operational amplifier (8) side or the second operational amplifier (9) side. In this case, a recording/reproduction switching signal is used as the switching signal.

The detection output of the synchronization detection circuit (7) is added to the output of the detection means (3) for detecting the magnitude of the input signal in the addition circuit (15), and then applied as a control signal to the feedback circuit (10). . The feedback circuit (10) changes the amount of feedback in response to the magnitude of the control signal applied thereto. For example, when the control signal is large, the amount of feedback is made small, and as a result, the amount of emphasis or de-emphasis is made small. When the synchronization detection output (d) is superimposed on the control signal, the amount of emphasis or de-emphasis is substantially zero. Conversely, when the control signal is small, the amount of feedback is increased, and as a result, the amount of emphasis or de-emphasis is increased.

If we are now in recording mode, switch (11).

(12) and (13) are both set upward in the figure as shown, and the first operational amplifier (8) is in an operating state.

Therefore, the f-characteristic correction means (2) operates as an emphasis circuit, and the amount of emphasis is determined by the output of the detection means (3). Since the output from the synchronization detection circuit (7) is superimposed on the synchronization signal and the pedestal portion of the video signal, the amount of feedback in the feedback circuit (10) becomes zero and no emphasis is performed. In this way,
A video signal as shown in FIG. 2(b) is obtained from the r-special correction means (2).

Next, in playback mode, switches (11), (12),
(13) is set downward in the figure, and the second operational amplifier (9)
is in an operating state, and the f-characteristic correction means (2) operates as a de-emphasis circuit, but de-emphasis is canceled for the synchronization signal portion and pedestal portion of the video signal.

In this embodiment, a VTR capable of recording and playback has been described, but the present invention is not limited to this, and can also be applied to, for example, a video recording device that only has a recording function, and a video playback device that only has a playback function. Needless to say. Of course, the recording method is not limited to the magnetic one, and may be, for example, an optical one or an electrostatic one.

The same applies to the recirculation device.

According to the present invention, the synchronization signal portion of the video signal that undergoes f-characteristic correction (frequency characteristic correction) is not corrected, so that the synchronization signal of the video signal is not used. This has the effect that an accurate AGC voltage can be obtained when the AGC voltage is formed using the same method.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a frequency characteristic correction device embodying the present invention, FIG. 2 is a waveform diagram for explaining its operation, and FIG. 3 is a circuit diagram of the main part of FIG. 1. FIG. 4 is a block diagram of a conventional example, and FIG. 5 is an explanatory waveform diagram thereof. (1)...variable gain amplifier. (2) --- f characteristic correction means, (3) --- detection means. (4)...-FM modulator. (5)・-=AGC voltage formation circuit. (6)--One synchronization separation circuit, (7)--Synchronization detection circuit. (lll), (9)--1. Second operational amplifier circuit. (10) --- Feedback circuit.

Claims (1)

[Claims] (1) In a video signal frequency characteristic correction device for supplying a video signal whose frequency characteristics have been corrected by emphasis or de-emphasis to an AGC voltage forming circuit for automatically varying the gain of the video signal, the video signal is supplied to the AGC voltage forming circuit. 1. A frequency characteristic correcting device for a video signal, comprising means for not performing the frequency characteristic correction on a synchronizing signal portion of a video signal.