JPH0515108B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging device, and is suitable for application to, for example, a video camera.

[Background technology and its problems]

For example, in a video camera, the output of the imaging device that converts the subject into an electrical video signal is relatively weak, so the detected video signal is amplified to a predetermined signal level via the multi-stage amplifiers connected in cascade, and then the image is captured. It is designed to perform so-called γ correction to correct the characteristics of the device, and a configuration as shown in FIG. 1 has been used in the past.

In FIG. 1, a detected video signal S1 obtained in an imaging device 1 is sequentially amplified in multi-stage preamplifiers 3A, 3B...3N through coupling capacitors 2A, 2B...2, N-1, and then is sequentially amplified by a clamp circuit. 4 to the γ correction circuit 5, and the output video signal S2 is sent out from the γ correction circuit 5. Here, the clamp circuit 4 is the preamplifier 3
A, 3B, . . . , 3N perform a DC reproduction of a predetermined reference level signal on the amplified video signal S3, thereby making the video signal input to the γ correction circuit match the predetermined operating point of the γ correction circuit. It is made to do so. Normally, the reference level clamped in the clamp circuit 4 is configured to clamp the black level to a predetermined value, and as shown in FIG. The level of the horizontal synchronizing signal SH is clamped to a black level LB which is higher than the level _LS of the horizontal synchronizing signal _SH . Thus, the detected video signal S1 of the imaging device 1 has
A black level signal having a predetermined reference potential is inserted every 1H, and the clamp circuit 4 sets this black level signal to a predetermined operating point of the γ correction circuit.

The reason why the clamp circuit 4 is inserted before the γ correction circuit 5 is that the γ correction circuit 5 is a circuit with non-linear characteristics, so when the operating point changes, the waveform of the output signal also changes. The settings are made at the front stage of the circuit.

In the configuration shown in FIG. 1, preamplifiers 3A, 3B,
The coupling capacitors 2A, 2B, . . . , 2N inserted between . The components are preamplifiers 3A, 3
A black level signal, which is a reference signal, is time-divisionally inserted in the clamp circuit 4 into the video signal S3 which is amplified in the video signal S3 that is amplified in the video signals S3, which are amplified in the video signals S3, which are amplified in the video signals S3, which are amplified in the video signals S3 that are amplified in the video signals S3, which are amplified in the clamp circuit 4, and are then subjected to γ correction in the γ correction circuit 5, and then are subjected to γ correction in the γ correction circuit 5, resulting in the output video signal S2. Sent as . For this purpose, coupling capacitors 2A, 2B, ...2, N- are connected between the preamplifiers 3A, 3B, ...3N.
1, which has the effect of greatly simplifying the configuration of the preamplifier section. Incidentally, it is conceivable to configure all of the preamplifiers 3A, 3B, ... 3N with DC amplifiers so that they can sequentially amplify the DC level signal, but in reality the video signal, which is an AC signal from the DC level, can be amplified sequentially. Although it is practically difficult to construct a configuration that stably amplifies the range up to the signal, and the overall configuration is unavoidable, if the configuration is as shown in Figure 1, the front The configuration of the amplifier section can be significantly simplified.

However, if the coupling capacitors 2A, 2B, . . . , 2, N-1 are used as shown in FIG. That is, when the detected video signal S1 is sequentially amplified and passes through the coupling capacitors 2A, 2B, . Due to this sag, there is a possibility that the reference DC level to be clamped by the clamp circuit 4 cannot be correctly adjusted to the black level. In other words, if the time constant of the clamp circuit 4 can be set to a small value, the black level signal can be clamped in a relatively short period of time without being affected by sag.
If the time constant of the clamp circuit 4 is set too small, there is a risk that the reference level on which noise contained in the video signal S3 is superimposed will be clamped as the black level. Therefore, in practice, when selecting the time constant of the clamp circuit 4, it is necessary to set the value to be large enough to ignore the influence of noise, and at the same time to be small enough to be unaffected by sag. A value that satisfies this condition must be selected, and in practice it is very difficult to satisfy this condition. As a result, the DC level that is affected by sag and/or noise is clamped as the black level, which results in the black level changing every 1H section (1 line of video signal) of the video signal. However, there are certain limits to improving image quality.

[Purpose of the invention]

The present invention has been made in consideration of the above objectives.
The present invention attempts to prevent variations in the black level without using a coupling capacitor, which causes sag, and without using a complicated configuration such as a DC amplifier.

[Summary of the invention]

In order to achieve this purpose, the present invention includes:
A pre-clamp circuit is provided between each pre-amplifier to clamp the DC level at the timing when a reference DC level signal of the video signal is inserted.

〔Example〕

An embodiment of the present invention will be described in detail below with reference to the drawings. In FIG. 3, in which parts corresponding to those in FIG.
A pre-clamp circuit 11A is provided between each N.
11B, . . . 11, N-1 are inserted, and a configuration as shown in FIG. 4 is applied. That is, the input video signal S _IN is applied to the base of an input transistor 12 connected as an emitter-follower, and is connected from the emitter connected to the output resistor 13 to the base of an output transistor 15 connected as an emitter-follower through a clamp capacitor 14. The output video signal S _OUT is sent out from the emitter to which the output resistor 16 is connected.
A clamp power supply 18 is connected to the base of the output transistor 15 through a switching transistor 17.
is connected to the base of the transistor 17 at the timing of a predetermined phase of the screen horizontal period of the video signal, that is, the black level section P1 or P2 in the 1H section.
A clamp pulse CLP that arrives at the timing shown in FIG. 2 is applied.

In the above configuration, the video signal component of the detected video signal S1 of the imaging device 1 is an AC signal.
VD is input transistor 12 to capacitor 14
to the output transistor 15, thus providing an output video signal to the next stage preamplifier.
It is sent as is as S _OUT . When a clamp pulse arrives at the timing of the black level section P1 or P2 between the video signal components VD, the switching transistor 17 is turned on for a time corresponding to the pulse width, so that the clamp voltage of the clamp power supply 18 is applied to the output transistor. 15 base, and this is sent out as the output video signal S _OUT . Thus, each pre-clamp circuit 11A,1
1B,...11,N-1 are the video signal components of the input video signal S _IN that are output as they are.
S _OUT as well as the input video signal S _IN
The black level DC component is temporarily connected to capacitor 14.
, and the clamp voltage of the clamp power supply 18 is replaced. Therefore, the output video signal S _OUT of each pre-clamp circuit 11A, 11B, . . . 11, N-1 has a DC level signal obtained by replacing the black level contained in the input video signal S _IN .

Here, the pre-clamp circuits 11A, 11B,...
The time constant of 11,N-1 is selected to be a sufficiently large value, thereby making it possible to remove noise components contained in the detected video signal S1. Therefore, even if the input video signal S _IN contains a noise component, no noise is superimposed when the clamp voltage is applied to the base of the output transistor 15, so the black level of the output video signal S _OUT is is set to a value that is not affected by noise. In addition to this, the black level is set between each preamplifier using a clamp pulse CLP.
It is possible to prevent sag from occurring in the video signal S3 obtained at the output end of the amplifier section.

To obtain such an effect, the pre-clamp circuit 11
The configuration of A, 11B, . . . 11, N-1 is practically the same and simpler than the case where the coupling capacitors 2A, 2B, . A simple configuration is sufficient. Incidentally, in the case of the AC coupling shown in Fig. 1, the input transistor 21 is connected as an emitter follower as shown in Fig. 5.
A configuration is used in which the output of the output transistor 23 is connected via a coupling capacitor 22 to the base of an output transistor 23 connected as an emitter follower, and the output video signal S _OUT is obtained from the emitter. As is clear from a comparison with the configuration shown in FIG. 4, it has a simple configuration that is almost the same.

Although the above description deals with the case where the black level is set as the reference level of the output video signal S2, this also applies when setting other levels as the reference level, such as the falling level of the horizontal synchronization signal S _H. The effects of the invention can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, when the detected video signal of the imaging device is amplified by the cascade-connected multistage amplifiers to obtain the output video signal of the imaging device, the preamplifiers are connected between each other by the preclamp circuit. By coupling the circuit to a large value, the output video signal of the preamplifier stage can be free from noise by selecting a large circuit time constant. By clamping the level signal, it is possible to prevent sag from occurring at the clamp position, so when clamping the reference DC level of the output video signal S2 in the clamp circuit 4 in the preceding stage of the γ correction circuit 5, the nozzle and You can avoid being affected by sag.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional imaging device, and FIG. 2 is a signal waveform diagram showing a video signal to be processed.
FIG. 3 is a block diagram showing an embodiment of the imaging device according to the present invention, FIG. 4 is a connection diagram showing a specific configuration of the pre-clamp circuit, and FIG. 5 is a specific example of the AC connection circuit shown in FIG. FIG. 3 is a connection diagram showing the configuration. 1... Imaging device, 2A to 2, N-1... Coupling capacitor, 3A to 3N... Preamplifier, 4... Clamp circuit, 5... γ correction circuit, 1
1A to 11, N-1... Front clamp circuit.

Claims (1)

[Claims] 1. A detected video signal is obtained by inserting a reference level signal at a predetermined phase point in the horizontal period of the screen from an imaging device, and the detected video signal is amplified by a plurality of preamplifiers and clamped. In an imaging device configured to output the output of the clamp circuit via a non-linear circuit after DC reproducing the reference level signal of the amplified output in the circuit, the non-linear A pre-clamp circuit is provided that has a time constant large enough not to affect the operating point of the circuit and that reproduces the reference level signal as a direct current at a timing of a predetermined phase point in the horizontal period in which the reference level signal is inserted. An imaging device characterized by: