JP3197909B2 - Electronic still camera gradation improvement circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital electronic still camera for converting an analog luminance signal into a digital luminance signal and recording the digital luminance signal on a recording medium. The present invention relates to a key adjustment circuit.

[0002]

2. Description of the Related Art As an alternative to the conventional silver halide photographic camera, a digital electronic still camera which records or reproduces an image as a still image on a semiconductor memory or a magnetic recording medium has appeared. In this digital electronic still camera, an image is captured by a light receiving sensor such as a CCD (Charge Coupled Device), and an analog luminance signal obtained by this is digitally converted by an analog-to-digital converter (hereinafter, referred to as an A / D converter). It converts to a signal. The converted digital luminance signal is recorded in a semiconductor memory or the like. On the other hand, at the time of reproduction, the recorded digital signal is converted into a digital-to-analog converter (hereinafter, referred to as a D / A converter).
To convert the signal into an analog signal and display it on a CRT display or the like.

[0003]

By the way, when shooting, A
The value of the digital signal converted and output by the / D converter is determined by the lower reference voltage and the upper reference voltage supplied to the A / D converter, but conventionally, these reference voltages are fixed. Therefore, when a dark subject without contrast is photographed, such as a dark indoor wall, CC
The level of the luminance signal is considerably lower than the upper limit reference voltage despite the exposure control of D, so that if the A / D conversion is performed as it is, the digital value as a whole will be small.

As described above, in the conventional digital electronic still camera, since the lower limit reference voltage and the upper limit reference voltage applied to the A / D converter are fixed, there are the following problems.

For example, when the A / D converter has 8 bits, the digital value that can be taken is 256 steps, but when the subject is entirely dark, several tens of steps which are only a part of these steps are used. Only one hundred steps are used. Therefore, even if the gray levels of the original adjacent pixels are slightly different, the digital values have the same value, and so-called quantization noise is generated. This quantization noise will appear in the reproduced image as a contour striped pattern that is not present in the actual subject, that is, a pseudo contour.

As described above, the conventional digital electronic still camera has a problem that when a dark subject having a low contrast is photographed, the image quality is likely to deteriorate due to quantization noise.

[0007] Therefore, there is a problem that the above problem must be solved. SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and in a digital electronic still camera, a gradation improving circuit capable of suppressing the influence of quantization noise regardless of the brightness and contrast of a subject and obtaining good image quality. The purpose is to obtain.

[0008]

A gradation improving circuit according to the present invention is a digital electronic still camera, comprising : an imaging section for outputting an analog luminance signal corresponding to the amount of received light; and an analog luminance signal input from the imaging section. Analog-to-digital conversion means for converting to a digital luminance signal based on a predetermined reference voltage; maximum value detection means for detecting the maximum value of the digital luminance signal output from the analog-digital conversion means; and detection by this maximum value detection circuit Depending on the maximum value
A reference voltage changing means for changing the reference voltage, the reference
Correction coefficient indicating the content of reference voltage change by voltage change means
And a correction coefficient storage means for storing

[0009]

In the digital electronic still camera according to the present invention, the brightness of the subject is changed by changing the upper-limit reference voltage applied to the analog-digital converter according to the maximum value of the digital luminance signal output from the analog-digital converter. Irrespective of the level, the digital luminance signal of an appropriate value is always obtained, and the quantization noise is reduced. Also,
By memorizing the changes of the reference voltage,
Enable playback.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 shows a main part of a digital electronic still camera to which a gradation improving circuit according to an embodiment of the present invention is applied. This circuit is provided with a video signal amplifier 11, which amplifies a video signal 13 input from a CCD (not shown) via an input terminal 12 to a predetermined amplitude. The output side of the video signal amplifier 11 is connected to an A / D converter 15 via a clamp circuit 14. In the clamp circuit 14, the black level of the luminance signal from the video signal amplifier 11 is clamped to a specified value. The A / D converter 15 A / D converts the luminance signal from the clamp circuit 14 in synchronization with the clock signal 16 and outputs the result to the memory control circuit 18 as an 8-bit digital luminance signal 17.

The memory control circuit 18 is connected to the video memory 22 via an 8-bit data bus 21 and is connected to the memory card 2 via an 8-bit data bus 23.
4 is connected. The video memory 22 is composed of a two-port video RAM, and writing and reading are performed in parallel. The memory card 24 is composed of a non-volatile memory capable of recording images of a predetermined number of frames.

The memory control circuit 18 is connected to a D / A converter 26 via an 8-bit data bus 25, and the maximum value detection circuit 2 via an 8-bit data bus 27.
8 is connected. Among them, the D / A converter 26 is
The 8-bit digital luminance signal output from the memory control circuit 18 is converted into an analog signal, and is output from an output terminal 49 via a variable amplifier 47 and a low-pass filter (LPF) 48 for cutting high frequencies. The variable amplifier 47 changes its amplification gain in response to a gain control signal 32 from a central processing unit (hereinafter referred to as a CPU) 31.

On the other hand, the maximum value detection circuit 28 is connected to the CPU 31 via an 8-bit data bus 34. C
The PU 31 controls the memory control circuit 18 through the control line 35 and supplies the latch circuit 36 with a digital value 37 for setting an upper limit reference voltage having an 8-bit width. In the initial state, the upper-limit reference voltage setting digital value 37 is set to a value corresponding to the upper-limit reference voltage corresponding to the dynamic range of the CCD. CPU3
The clock signal 38 serving as a reference for the latch timing is also supplied from 1 to the latch circuit 36.

The digital value 37 for setting the upper limit reference voltage once latched by the latch circuit 36 is converted into an analog upper limit reference voltage 41 by the D / A converter 39 and supplied to the A / D converter 15. ing. A / D converter 15
Is supplied with an analog lower limit reference voltage 42 from a lower limit reference voltage setting circuit (not shown).

The operation of the digital electronic still camera configured as described above will be described.

A video signal 13 input from a CCD (not shown) via an input terminal 12 is amplified to a predetermined amplitude by a video signal amplifier 11 and input to a clamp circuit 14.
In the clamp circuit 14, the black level of the analog luminance signal 19 amplified by the video signal amplifier 11 is clamped to a specified value and input to the A / D converter 15. A / D converter 15
A / D-converts the luminance signal from the clamp circuit 14 in synchronization with the clock signal 16 and outputs it to the memory control circuit 18 as an 8-bit digital luminance signal 17. FIG. 2 shows A
3 shows the / D converter 15 in detail. This A /
The D converter 15 has 256 comparators 44-1 to 44-2.
The analog luminance signal 19 is input in parallel to one input terminal. Further, the other input terminals respectively provide a voltage dividing resistor 45-1 to 45-255 between a predetermined upper reference voltage 41 and a lower reference voltage 42.
Thus, a divided voltage divided into 256 equal parts is provided. For example, the upper reference voltage 41 and the lower reference voltage 4
Assuming that the values of 2 are VT and VB respectively, the value ΔV1 of one unit of the partial pressure is expressed by the following equation (1).

ΔV 1 = (VT−VB) / 256 (1) FIG. 3 shows the relationship between the upper reference voltage VT, the lower reference voltage VB, and the analog luminance signal 19.
This figure shows a case where the range of the analog luminance signal 19 covers almost the entire range between the upper limit reference voltage value VT and the lower limit reference voltage value VB, and a subject having relatively high illuminance and good contrast is photographed. In this case, A
The value of the digital luminance signal 17 output from the / D converter 15 is distributed in the range of 0 to FF (hexadecimal).
In this figure, the signal 51 indicates a horizontal synchronizing signal. Also,
The lower-limit reference voltage VB is a black-level voltage clamped by the clamp circuit 14.

Each of the comparators 44-1 to 44-256 outputs a signal of "0" or "1" level according to the result of comparison between the analog luminance signal 19 as an input signal and the divided voltage value. Therefore, 256 signals are output, and the decoder 46 decodes the signals into 8 bits in synchronization with the clock signal 16 and outputs them as the digital luminance signal 17.

The digital luminance signal 17 output from the A / D converter 15 is sequentially recorded in the video memory 22 under the control of the memory control circuit 18. The maximum value detection circuit 28 outputs the digital luminance signal for one frame. By monitoring the signal 17, the maximum value within this period is detected,
Send to PU31. When the maximum value is equal to or less than the predetermined value, the CPU 31 determines that the contrast is insufficient, changes the upper-limit reference voltage setting digital value 37 to a preset small value, and outputs the digital value to the latch circuit 36. The latch circuit 36 latches the digital value 37 for setting the upper-limit reference voltage at the timing of the clock signal 38 and performs D / A conversion 39
Output to The D / A converter 39 performs D / A conversion on this,
The A / D converter 1 converts the upper-limit reference voltage VT 'for a low-luminance subject to
5 is output. Note that VT '<VT.

FIG. 4 shows an original upper reference voltage VT, an upper reference voltage value VT 'for a low-luminance subject, and a lower reference voltage value VB.
, And the analog luminance signal 19. In this figure, the range of the analog luminance signal 19 is less than half of the original upper limit reference voltage value VT and lower limit reference voltage value VB, and shows a case where a dark subject with poor contrast is photographed. In this case, when the A / D conversion is performed with VT as the upper limit reference voltage, the value of the output digital luminance signal 17 becomes a small value, and as described above, the contour stripes are limited due to the limitation of the quantization number. Patterns are likely to occur.

Therefore, in this case, A / D conversion is performed using VT 'as the upper limit reference voltage. As a result, the value ΔV2 of one unit of the partial pressure in FIG. 4 becomes small as shown in the following equation (2), so that the value of the output digital luminance signal 17 becomes large as a whole. That is, the A / D converter 15
Has a large conversion gain.

ΔV 2 = (VT′−VB) / 256 (2) On the other hand, when the maximum value input from the maximum value detection circuit 28 to the CPU 31 is equal to or larger than a predetermined value, the digital value 37 for setting the upper limit reference voltage is used. Is used without changing the upper limit reference voltage VT.

When photographing a low-luminance object, the upper-limit reference voltage is reset to VT 'and A / D conversion is performed.
Since 9 is not stored anywhere, retransformation is not possible. Therefore, the same picture is successively taken twice for one press of the shutter, and the second picture is converted again. That is, the digital value obtained by A / D-converting the first picture is written to the video memory 22, but when the maximum value of the digital luminance signal is equal to or more than a predetermined value, the contents of the video memory 22 are left unchanged. The data is transferred to the memory card 24 and written therein. If the value is equal to or smaller than the predetermined value, the digital value re-converted by changing the upper limit reference voltage value is directly written into the memory card 24.

When writing image data to the memory card 24, the CPU 31 performs a process of writing an amplification correction coefficient K to be given to the variable amplifier 47 during image reproduction to the memory card 24. This value is given, for example, by the following equation (3).

K = VT '/ VT (3) FIG. 5 shows the contents of the memory card 24.
As shown in this figure, data for one frame is composed of an image data area 53 and an attribute area 54. The attribute area 54 stores various attribute data relating to the image data in the image data area 53, and is referred to during reproduction. The amplification correction coefficient K described above is also stored in the attribute area 54 as one of these attribute data.

As described above, by appropriately changing the upper-limit reference voltage of the A / D converter 15 in accordance with the luminance of the subject, recording on the memory card 24 while ensuring a sufficient A / D conversion gain is achieved. Will be performed.

Next, an operation for reproducing an image recorded on the memory card 24 will be described. When there is an instruction to reproduce an image, the CPU 31 controls the memory control circuit 18 to sequentially read out the pixels of the corresponding image frame from the memory card 24 and output them to the D / A converter 26. At this time, the amplification correction coefficient K is read from the corresponding attribute area 54 (FIG. 5), and a value KA obtained by multiplying the amplification correction coefficient K by the original amplification factor A is given to the variable amplifier 47 as the gain control signal 32.

The variable amplifier 47 amplifies the analog signal converted from the 8-bit digital luminance signal by the D / A converter 26 by KA times. The amplified analog luminance signal is output from an output terminal 49 via a low-pass filter 48 and supplied to a reproduced image display unit or a print unit (not shown).

The reason why the amplification factor of the variable amplifier 47 is changed using the amplification correction coefficient K is as follows.
That is, at the time of A / D conversion in the A / D converter 15, when VT 'is used as the upper limit reference voltage, the converted digital value is generally smaller than when the original upper limit reference voltage VT is used. It has a large value. Therefore,
If the analog value obtained by D / A conversion of this digital value during reproduction is amplified at the original amplification factor A, the image becomes much brighter than the original subject, and faithful reproduction cannot be performed. Therefore, an appropriate analog luminance signal is generated by lowering the amplification rate by multiplying the amplification rate A by the amplification correction coefficient K. Of course, in the case of an image using VT as the upper limit reference voltage, K becomes 1, so that the original amplification factor A
Will be used.

In this embodiment, the A / D conversion is performed by selectively using two types of upper limit reference voltages in accordance with the illuminance of the subject. However, the present invention is not limited to this.
It is also possible to set more than kinds of upper limit reference voltages and to perform processing by further categorizing the illuminance of the subject.

[0032]

As described above, according to the present invention,
The upper reference voltage applied to the analog-to-digital converter is changed according to the maximum value of the digital luminance signal output from the analog-to-digital converter. A digital luminance signal can be obtained. Therefore, even when a low-illuminance subject is photographed, it is possible to eliminate the occurrence of false contours due to quantization noise, and to obtain a high-quality reproduced image.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a main part of a digital electronic still camera to which a gradation improving circuit according to an embodiment of the present invention is applied.

FIG. 2 is a block diagram showing in detail an A / D converter for converting an analog luminance signal into a digital value in FIG.

FIG. 3 is an explanatory diagram showing a relationship between an analog luminance signal and an upper reference voltage value and a lower reference voltage value when a subject having relatively high illuminance and good contrast is photographed.

FIG. 4 is an explanatory diagram showing a relationship between an analog luminance signal and an upper reference voltage value and a lower reference voltage value when a subject having low illuminance and poor contrast is photographed.

FIG. 5 is an explanatory diagram showing contents of a memory card in FIG. 1;

[Explanation of symbols]

 Reference Signs List 15 A / D converter 18 Memory control circuit 22 Video memory 24 Memory card 26 D / A converter 28 Maximum value detection circuit 31 CPU 39 D / A converter 41 Upper reference voltage 42 Lower reference voltage 47 Variable amplifier

Claims (1)

(57) [Claims] A digital electronic still camera for outputting an analog luminance signal corresponding to an amount of received light; and an analog converter for converting the analog luminance signal input from the imaging unit into a digital luminance signal based on a predetermined reference voltage. Digital conversion means; maximum value detection means for detecting the maximum value of the digital luminance signal output from the analog-digital conversion means; and changing the reference voltage according to the maximum value detected by the maximum value detection circuit. The reference voltage changing means and the contents of the change of the reference voltage by the reference voltage changing means are shown.
A gradation improvement circuit for an electronic still camera, comprising: a correction coefficient storage unit that stores a correction coefficient .