JPH074003B2 - Signal compressor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal compression device used in video equipment such as a video camera and a video tape recorder.

2. Description of the Related Art A conventional signal compression apparatus is shown in Technical Report VoL5 No. 12 P43 to P46 "High Latitude Camera" of the Television Society.

FIG. 12 shows a block diagram of an image pickup apparatus using a conventional signal compression apparatus, and 1 is an image pickup element, which performs photoelectric conversion. Reference numeral 2 denotes a signal processing circuit, which synthesizes a luminance signal before gamma correction and two color difference signals. Reference numeral 3 denotes a gamma correction circuit, which performs gamma correction on the luminance signal and simultaneously compresses the signal level. 4 is an encoder circuit,
A luminance signal and two color difference signals are converted into, for example, an NTSC signal and output. For the characteristics of the gamma correction circuit of No. 3, the conversion characteristics shown in Fig. 6 are used.
The signal up to% is compressed to 130% as the output signal.

In the conventional signal compression device configured as described above,
It is possible to convert a luminance signal with a range of up to 400% to a range of up to 130%. However, at the level exceeding 100%, the signal is output after being greatly compressed.
For example, when a signal that changes at a level of 100% or more is input as shown in FIG. 13, the characteristic conversion shown in FIG. 6 is converted into the waveform shown in FIG.
The component that changes around 0% is compressed to a 1/10 change level and output.

Problems to be Solved by the Invention However, in the above-mentioned configuration, although a signal of a large level can be output without being saturated, there is a problem that the compression rate becomes large and the gain is greatly reduced to 1/10. Was there.

In view of such a point, the present invention compresses a signal without significantly lowering the gain even when compressing a signal of a large level, or increases the gain of a signal of a small level and changes components of the signal at all levels. An object of the present invention is to provide a signal compression device that compresses a signal so that the signal has a sufficient level.

Means for Solving the Problems In order to achieve the above object, the present invention provides a band limiting circuit for band limiting an input signal, a gain control circuit for changing a signal amplitude from the band limiting circuit, and The gain control circuit comprises a subtraction circuit for subtracting the output signal of the gain control circuit and a state detection circuit for detecting the level state of the input signal, and the gain control circuit changes the gain of the circuit according to the output of the state detection circuit. ,
The state detection circuit includes a first averaging circuit for averaging the input signals, a second averaging circuit for averaging the input signals below a predetermined level, and a signal or an input signal from the two averaging circuits. A gain control circuit for controlling the gain of the gain control circuit according to the level distribution,
The change component of the high level portion of the input signal is not compressed but only the signal level thereof is compressed, and the low level portion of the input signal is not expanded or compressed relatively.

The state detection circuit divides the input signal into n (n is an arbitrary positive number) on the image and averages in the range divided by n, and a signal from the averaging circuit. A gain control circuit that determines the gain of the gain control circuit in each part on the image according to the distribution of the signal level in the range divided into n parts is provided, and signal processing suitable for each part on the image is performed.

The present invention has the above-described configuration, detects a high-level portion of the input signal, and subtracts the low-frequency portion of the detected signal to convert the high-level signal to a low level, thereby obtaining the signal level. Compression of. In addition, by changing the gain that subtracts the low-frequency part of the signal according to the state of the input signal, the signal level can be compressed more effectively, and the low-level signal can be adjusted to a low signal level so that the signal level is sufficient. The level gain is increased, and the signal is compressed so that the variation component of the signal becomes a sufficient level at all levels.

First Embodiment FIG. 1 shows a block diagram of a signal compression apparatus and an image pickup apparatus using this signal compression apparatus according to a first embodiment of the present invention. In FIG. 1, 10 is a level compression block and 11 is a distribution detection block. Blocks 1 to 4 relating to the image pickup device are the same as those of the conventional one shown in FIG. 7, and a luminance signal before gamma correction is input to the level compression block 10 and the distribution detection block 11. A detailed block diagram of the level compression block 10 and the distribution detection block 11 is shown in FIG.

In FIG. 2, 13 is a band limiting circuit that limits the signal band, 14 is a gain control circuit that controls the gain by a control signal from an external circuit, and 15 and 16 are averaging circuits that average the input signals. , 17 are gain control circuits that determine the gain of the gain control circuit 14, and 18 is a subtraction circuit.

The signal input to the level compression block 10 is, for example, the signal shown in FIG. The input signal is first band-limited by the band limiting circuit 13. The signal when the band is limited is shown in FIG. The band limitation is performed not only in the horizontal direction but also in the vertical direction, and the two-dimensional LPF structure can perform natural level compression. (This will be described later.) The level of the signal band-limited in this way is controlled by the gain control circuit 14. The signal for performing this control is calculated by the state detection block 11. State detection block
11 includes two averaging circuits 15 and 16 and a gain control circuit 17. Averaging circuit 15 calculates an average value A _t of the total level of the input signal, the averaging circuit 16 calculates an average value A _m excluding the range of high level of the input signal. The gain control circuit 17 is a ratio Rav of two average values. The gain control circuit 1 is calculated by this Rav value.
The gain of 4 is determined as shown by the solid line in FIG. When the average value ratio Rav is close to 1 (in the range of 1.0 to 0.9 in FIG. 4), the gain is zero and the output of the gain control circuit 14 is zero. Further, the gain is 0.4 at a small Rav (range of 0.6 or less in FIG. 4), and the output of the gain control circuit 14 is set to 0.4 of the output level of the band limiting circuit 13.

Now, it is assumed that the high brightness level of the input signal is 150. The band-limited signal waveform is shown in FIG. Then, the output of the gain control circuit 14 becomes 60 levels. And the subtraction circuit
By subtracting the signal having the level of 60 from the signal input by 18, the signal level can be compressed to 90 in the low frequency band.

The signal thus compressed is shown in FIG. When the gain of the gain control circuit is increased, the subtraction amount of the signal increases and the compression amount of the signal increases as described above. The gain control need not be limited to that shown by the solid line in FIG. 4, but may be a characteristic such as the one-dot chain line. In this case, the average value ratio Rav is 0.4 or less, and the signal of the brightness level 150 is 108
Compressed up to.

In FIG. 5, the portion where the signal level changes abruptly is
Waveforms of overshoot and undershoot occur before and after that, but such a level change of the signal causes an illusion when humans perceive brightness. Therefore, even if the signal level is compressed, the brightness of the image perceived by humans is hardly compressed, which is a good result for the signal compression. Especially when the two-dimensional band limitation is performed, the effect is high.

In the signal compression described above, a signal having a small amplitude change at a high luminance level is not compressed and can be output at the same amplitude level.

Gamma correction is performed by the gamma correction circuit 3 on the signal thus compressed, as in the conventional image pickup apparatus. The gamma correction performed here may be the same as the conventional one. If the gamma characteristics shown in FIG. 6 are used, the output after gamma correction is
The waveform is as shown in the figure. By performing the process of the present invention before performing the gamma process to reduce the signal level,
A signal with a small change in the level of high brightness can be output without receiving a large reduction in the amplitude level. It is also possible to allocate a portion where the high level signal is compressed and the signal level is reduced to the low level signal. That is, the low level gain of the γ correction circuit 3 (in FIG. 6, the gain is 3 to 1.4) can be made larger than the conventional one. Therefore, it is possible to perform compression so that the low-level change component is larger than before and the high-level change component can be output at a sufficient level without being compressed.

As described above, according to this embodiment, the signal level is compressed by band-limiting the input signal and subtracting it from the original signal with the gain determined by the state detection unit. By performing this compression, as compared with the compression method in which the level is simply compressed only by the slope of the gamma correction curve, the amplitude level of the changed portion is not compressed even in the signal with a small change in the high luminance level. Level compression of the entire signal can be performed.

Also, since the entire signal level is compressed, low level signals can be expanded relatively. In this way, at all signal levels, small amplitude variation components are not compressed or expanded,
At the same time, the overall signal level can be compressed.

In this embodiment, the state detection unit detects the signal state using the average value of the input signal and the average value excluding the high level range. The method is not limited to this, and the state can be detected from the distribution of the signal level, and when there are many distributions in the high level, the gain control circuit 14 can be controlled in the direction in which the compression increases. When the input image has a large amount of signal, one image is divided into multiple blocks, and the signal state is detected using the average value of the data in the block and the distribution of the signal level in the block. You can also When divided into multiple blocks, the level of compression can be performed according to the partial characteristics of the image, and even if there are various level changes in the image, the state of each partial image is supported. Appropriate processing can be performed.

Further, in the present embodiment, the signal compression amount is shown as a configuration in which the signal state is detected and is varied according to the state, but the operation can be performed with the compression amount fixed.

As a second embodiment, FIG. 8 shows the configuration of a signal compression device with a fixed amount of compression. In the figure, 13 is a band limiting circuit,
Reference numeral 18 denotes a subtraction circuit, which has the same structure as that shown in FIG. The difference from the configuration shown in FIG. 2 is that the non-linear circuit 12 is provided in front of the band limiting circuit 13, and the gain setting circuit 21 is not controlled by a signal from the outside but is operated with a predetermined constant gain.

The operation of the second embodiment configured as described above will be described below.

The basic operation is similar to that of the first embodiment, and the nonlinear circuit
The low level signal is converted to zero level by 12. The converted signal is band-limited (two-dimensional band-limited) by the band limiting circuit 13, then a constant is applied by the gain setting circuit, and this signal is subtracted from the input signal to be input. Compress the signal level. In the case of this embodiment, the constant of the gain setting circuit is fixed, and a constant compression operation is always performed. Therefore, the low-level input signal is converted to zero level by the non-linear circuit 12, and the compression operation is not performed in the range converted to zero. The signal from 0 level to 50 level is converted into 0, and the signal that has passed through the band limiting circuit 13 is
Shown in the figure. The waveform of the input signal is shown in FIG. 13 which is the same as that of the first embodiment. FIG. 10 shows an output waveform when the constant of the gain setting circuit 21 is 0.5. 150 level signal is 1
Can be compressed to 00 level.

As described above, according to this embodiment, the non-linear circuit is placed before the band limiting circuit, and the gain setting circuit and the subtraction circuit are further provided to compress the low-level input signal even in the configuration in which the state detection circuit is omitted. It is possible to compress the high level part of the input signal without depending on the level. At this time, by placing the nonlinear circuit in front of the band limiting circuit, the waveform after band limiting can be subtracted from the input signal without distortion. As a result, the signal changes smoothly before and after the transient portion of the signal, and the effect of the brightness illusion occurs when the image is observed. Therefore, the image of the compressed signal has almost no impression that the level of the signal has been compressed, and the signal can be naturally and sufficiently compressed.

Although the input signal is directly input to the band limiting circuit 13 in the first embodiment, the second embodiment has the configuration shown in FIG.
It goes without saying that even with the configuration using the non-linear circuit 12 as in the embodiment, natural and sufficient compression can be obtained.
The operation of the configuration shown in FIG. 11 is the same as that of the first and second embodiments, and the description thereof will be omitted.

Further, it is possible to perform sufficient signal compression even in the configuration excluding the non-linear circuit in the second embodiment.

Further, it is obvious that the level for converting to 0 in the non-linear circuit and the constant of the gain setting circuit may be changed as necessary.

It is also apparent that good level compression can be achieved by using the level compression device of the present invention together with the gamma circuit used in a video camera or the like.

EFFECTS OF THE INVENTION As described above, according to the present invention, even a signal with a large amplitude level can relatively expand a signal with a low level without compressing a component that slightly changes contained therein. In addition, since non-linear processing is not performed after band limitation, the effect of the brightness illusion can be sufficiently added, and the impression that the signal level is compressed hardly occurs, and natural and sufficient signal amplitude compression is performed. You can Furthermore, even if there are various level changes in the image, appropriate expansion and compression processing can be performed according to each partial image state, and the image from the low level part to the high level part Natural and easy-to-see processing according to partial characteristics is performed, and the entire signal level can be compressed, which is a great effect.

[Brief description of drawings]

FIG. 1 is a block diagram of a signal compression apparatus and an image pickup apparatus using the same according to an embodiment of the present invention, FIG. 2 is a block diagram showing a configuration example of the signal compression apparatus, and FIGS. FIG. 4 is an operation waveform diagram of an example, FIG. 4 is a gain control characteristic diagram of the gain control circuit of the present embodiment, FIG. 6 is a gamma characteristic diagram of the image pickup device, and FIG. 7 is a signal compression device of one embodiment of the present invention. FIG. 8 is a block diagram of a signal compression apparatus according to the second embodiment of the present invention, FIGS. 9 and 10 are operation waveform diagrams of the same embodiment, and FIG. Fig. 12 is a block diagram of an example, Fig. 12 is a block diagram of an image pickup device in which signal compression is performed only by the gamma correction circuit of the conventional example, Fig. 13 is a waveform diagram showing an example of an input waveform, and Fig. 14 is an output waveform of the conventional example. It is a waveform diagram showing. 10 …… Level compression block, 11 …… Status detection block,
12 ... Non-linear circuit, 13 ... Band limiting circuit, 14, 21 ... Gain setting circuit, 15, 16 ... Average circuit, 17 ... Gain control circuit.

Claims (2)

[Claims] 1. A band limiting circuit for band limiting an input signal,
A gain control circuit for changing the signal amplitude from the band limiting circuit, a subtraction circuit for subtracting the output signal of the gain control circuit from the input signal, and a state detection circuit for detecting the level state of the input signal. However, the gain control circuit changes the gain of the circuit according to the output of the state detection circuit, and the state detection circuit has a first averaging circuit that averages the input signal and a level equal to or lower than a predetermined level of the input signal. And a gain control circuit that controls the gain of the gain control circuit according to the distribution of the signals or the input signal levels from the two averaging circuits, and the high level portion of the input signal Of the input signal is not compressed but only the signal level thereof is compressed, and the low level portion of the input signal is relatively expanded or compressed. Signal compression apparatus characterized by not. 2. A band limiting circuit for band limiting an input signal,
A gain control circuit for changing the signal amplitude from the band limiting circuit, a subtraction circuit for subtracting the output signal of the gain control circuit from the input signal, and a state detection circuit for detecting the level state of the input signal. However, the gain control circuit changes the gain of the circuit according to the output of the state detection circuit, and the state detection circuit divides the input signal into n (n is an arbitrary positive number) on the image. An averaging circuit that averages in an n-divided range, and a gain control circuit that determines the gain of the gain control circuit in each part on an image by a signal from the averaging circuit or a signal level distribution in the n-divided range A signal compression device comprising: