JPH0786944A - A/d conversion circuit for video signal - Google Patents

Abstract

PURPOSE:To provide sufficient A/D conversion accuracy without improving the resolution of an A/D converter itself. CONSTITUTION:This circuit is provided with a first A/D converter 100 for converting an analog video signal to a digital signal and a second A/D converter 102 for converting an analog video signal amplified by an amplifier 101 to a digital signal. When the digital signal outputted from the second A/D converter 102 is a maximum value, the output digital signal of the first A/D converter 100 is selectively outputted by a selector circuit 103 and when the digital signal outputted from the second A/D converter 102 is smaller than the maximum value, the output digital signal of the second A/D converter 102 is selectively outputted by the selector circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D conversion circuit for converting a video signal picked up by a camera into a digital signal.

[0002]

2. Description of the Related Art A parallel comparison type A / D converter for converting an analog video signal picked up by a camera into a digital signal is known. FIG. 4 shows the configuration of the parallel comparison type A / D converter. Here, the video signal is a signal in which two-dimensional image information is decomposed into one scanning line. Assuming that the resolution of A / D conversion is N bits, (2 ^N −1) comparators 21 are connected in parallel to the input stage, and a voltage obtained by equally dividing the input voltage range by 2 ^{N is} applied to the terminal (+ REF) − (− The reference voltage applied between (REF) is divided into the reference voltage for comparison of each comparator. Then, when an analog signal is applied to the input terminal VIN, the respective comparators operate at the same time to output H and L digital signals. This digital signal is converted into a desired code by the encoder 22 at the timing when the sampling clock signal is input to the terminal CLK, and is output to the digital output terminal 24 via the driver 23.

[0003]

By the way, it is sufficient to perform A / D conversion on a video signal of an object having normal brightness with a resolution of about 8 bits, but if the brightness of the object becomes low, it becomes 8
With a resolution of about bits, the accuracy is insufficient, and it is necessary to increase the number of bits of the A / D converter to increase the resolution.

However, in order to increase the resolution of the A / D converter, there are problems that two comparators are required for one bit and the circuit itself becomes complicated.

An object of the present invention is to provide an A / D conversion circuit for a video signal which can obtain sufficient A / D conversion accuracy without increasing the resolution of the A / D converter itself.

[0006]

The invention according to claim 1 will be described with reference to FIG. 1 which is a claim correspondence diagram.
Is applied to an A / D converter circuit for converting an analog video signal into a digital signal, and the first A / D converter 100 converts an analog video signal into a digital signal.
An amplifier 101 for amplifying the analog video signal with a predetermined amplification degree, and a second A / D converter 10 for converting the analog video signal amplified by the amplifier 101 into a digital signal.
2 and the digital signal output from the second A / D converter 102 has the maximum value, the output digital signal of the first A / D converter 100 is selected and output, A
When the digital signal output from the A / D converter 102 is smaller than the maximum value, a selection circuit 103 that selects and outputs the output digital signal of the second A / D converter 102 is provided. Achieve the purpose. The invention of claim 2 will be described with reference to FIG. 2 showing the configuration of an embodiment. The invention of claim 2 is applied to an A / D conversion circuit of a video signal for converting an analog video signal into a digital signal. Is
A first amplifier 3 for amplifying the analog video signal with a first amplification degree; a first A / D converter 5 for converting the analog video signal amplified by the first amplifier into a digital signal;
A second amplifier 4 for amplifying the analog video signal with a second amplification degree larger than the first amplification degree, and the second amplifier 4
Second A / D converter 6 for converting the analog video signal amplified by the digital signal into a digital signal, and the second A / D converter 6
When the digital signal output from is the maximum value, the digital signal output from the first A / D converter 5 is selected and output, and the digital signal output from the second A / D converter 6 is selected. Is smaller than the maximum value, the selection circuit 7 for selecting and outputting the output digital signal from the second A / D converter 6 is provided, thereby achieving the above object.

[0007]

The second A / D converter 102 performs A / D conversion on the analog video signal amplified by the amplifier 101.
When the output digital signal of the A / D converter 102 has the maximum value, it is considered that the analog video signal is at a high level and the subject brightness is sufficiently high. Therefore, in this case, the output digital signal of the first A / D converter is selected and output. On the other hand, when the output digital signal of the second A / D converter 102 is smaller than the maximum value, the analog video signal is at a low level and it is considered that the brightness of the subject is low. The output digital signal of 102 is selected and output. The output digital signal of the second A / D converter is
The resolution of A / D conversion is increased by the amount by which the analog video signal is amplified by the amplifier 101, and a highly accurate A / D conversion value for a low level video signal can be obtained.

[0008]

EXAMPLE FIG. 2 shows the configuration of an example. Preamplifier 1
Amplifies an image pickup signal input from an image pickup device such as a camera with a predetermined amplification degree. Filter 2 is preamplifier 1
A high-level amplifier 3 and a low-level amplifier 4 are provided by cutting out components of a predetermined frequency or higher from the image pickup signal amplified by
Output to. The high level amplifier 3 amplifies the image pickup signal output from the filter 2 with an amplification degree A, and outputs a high level A signal.
Output to the / D converter 5. On the other hand, low level amplifier 4
Is the amplification factor K
It is amplified by A and output to the low level A / D converter 6. In this embodiment, K = 8 and the high level amplifier 3 is used.
The low-level amplifier 4 and the low-level amplifier 4 both have a limiter for limiting the output voltage to 8V.

The high-level A / D converter 5 is an 8-bit parallel comparison type A / D converter, which converts the analog image pickup signal input from the high-level amplifier 3 into a digital signal and converts it into a signal converter 7. Output to. The low-level A / D converter 6 is an 8-bit parallel comparison type A / D converter, which converts the analog image pickup signal input from the low-level amplifier 4 into a digital signal and outputs it to the signal converter 7. Output. The signal converter 7 selects and outputs the output digital signal of the high-level A / D converter if the digital signal output of the low-level A / D converter 6 is FFH, and outputs the low-level A / D converter. If the digital signal output of the device 6 is smaller than FFH, low level A
The output digital signal of the / D converter 6 is selected and output.

In this embodiment, the high-level A / D converter 5 and the low-level A / D converter 6 both have 8 bits, but the A / D converters 5 and 6 have the same number of bits. Is not limited to this example. Further, it is not necessary to use the A / D converters having the same number of bits for the high level and the low level. Furthermore, the amplification levels of the high level amplifier 3 and the low level amplifier 4 are not limited to those in the above embodiment.

FIG. 3A shows the relationship of the digital signal output to the analog signal input of the high level A / D converter 5, and FIG. 3B shows the analog signal input of the low level A / D converter 6. Shows the relationship of the digital signal output with respect to. In the high level A / D converter 5, the analog input voltage is 0 to 8V.
Is converted to a digital value of 00 to FFH. Therefore, the analog input voltage 4V is converted into 7FH and 1V is converted into 1FH. On the other hand, the low-level A / D converter 6 converts the analog input voltage 0 to 8 V into a digital value of 00 to FFH.

By the way, since the low level amplifier 4 amplifies the video signal with an amplification factor of 8 times that of the high level amplifier 3, the maximum analog input voltage 8V of the low level A / D converter 6 is high. This corresponds to 1 V of the analog input voltage of the level A / D converter 5. That is, the low-level A / D converter 6
Is the hatch part shown in FIG. 3A, that is, A for high level.
A / D converter 5 has an analog signal input of 0 to 1 V and a digital signal output of 00 to 1 FH in a range of 8 times resolution of 00 to FFH
Convert to digital value with.

In order to make the explanation easy to understand, in this embodiment, the video signal supplied from the filter 2 to the high level amplifier 3 and the low level amplifier 4 is 0 to 8V,
The amplification level of the high level amplifier 3 is set to 1, and the amplification level of the low level amplifier 4 is set to 8. When the digital signal of the maximum value FFH is output from the low level A / D converter 6,
8 from low level amplifier 4 to low level A / D converter 6
This is when the V analog video signal is being supplied. The low level amplifier 4 is equipped with a limiter so that the output becomes 8 V or less, so the low level amplifier 4 at this time is
An analog video signal of 1 V or higher is supplied from the filter 2 to the input of. On the other hand, when the analog video signal of 1 V or more is input from the filter 2, the high level amplifier 3 has an amplification degree of 1 and therefore outputs an output signal of 1 V or more equivalent to the input level to the high level A / D converter. Supply to 5. At this time, the high level A / D converter 5 outputs a digital signal of 1 FH or higher.

That is, when 1 V of the video signal supplied from the filter 2 is used as a boundary, when the voltage exceeds 1 V, the low level amplifier 4 is saturated, the output is clamped at 8 V, and the low level A / D converter 6 is used. Outputs a digital signal of FFH. At this time, the output digital signal of the low-level A / D converter 6 is not an accurate A / D conversion value of the analog video signal. On the other hand, the video signal supplied from the filter 2 is 1V
Even above, the high level amplifier 3 is not saturated, so the output digital signal of the high level A / D converter 5 is an accurate A / D conversion value of the analog video signal. Therefore, the digital signal output from the low-level A / D converter 6 has the maximum value F
In the case of FH, the output digital signal of the high level A / D converter 5 is selected as the A / D conversion value of the video signal, and the digital signal output from the low level A / D converter 6 is the maximum value. If it is smaller than FFH, the output digital signal of the low-level A / D converter 6 is selected as the A / D conversion value of the video signal.

The video signal that has passed through the preamplifier 1 and the filter 2 is simultaneously supplied to the high level amplifier 3 and the low level amplifier 4 and is amplified with a predetermined amplification degree. The amplified video signals are respectively A / D converted by the high level A / D converter 5 and the low level A / D converter 6, and the respective digital video signals are output to the signal converter 7. When the digital signal output from the low level A / D converter 6 is the maximum value FFH, the signal converter 7 selects and outputs the output digital signal of the high level A / D converter 5. When the digital signal output from the low level A / D converter 6 is smaller than the maximum value, the output digital signal of the low level A / D converter 6 is selected and output.

In this way, the video signal of the subject having normal brightness is considered to be at least 1 V or more when converted into the output level of the filter 2, so that high-level A / D conversion having 8-bit resolution is provided. Sufficient accuracy can be maintained if A / D conversion is performed by the device 5. On the other hand, since the video signal of a subject with low brightness is considered to be smaller than 1 V when converted into the output level of the filter 2, the low level A / D converter 6 having 8 times the resolution performs A / D conversion. By doing so, a highly accurate digital signal can be obtained. In other words, two 8-bit A / D converters can be used to perform A / D conversion with optimum accuracy according to the analog video signal level.

In the above embodiment, the video signal level is set to 0.
Although it is divided into two areas of ˜1V and 1 to 8V and A / D conversion is performed with optimum accuracy, respectively, the video signal level is further finely divided, and A / D conversion provided for each area. A / D conversion may be performed by a container.

In the configuration of the above embodiment, the high-level A / D converter 5 is the first A / D converter, the low-level amplifier 4 is the amplifier, and the low-level A / D converter 6 is the The signal converter 7 constitutes a second A / D converter, and constitutes a selection circuit.

[0019]

As described above, according to the present invention, the first A / D for converting an analog video signal into a digital signal.
A converter and a second A / D converter for converting the analog video signal amplified by the amplifier into a digital signal;
When the digital signal output from the A / D converter of is the maximum value, the output digital signal of the first A / D converter is selected and output, and output from the second A / D converter. Second digital signal is smaller than the maximum value, the second A / D
Since the output digital signal of the converter is selected and output, the A / D of the video signal is sufficiently accurate for the video signal of the subject of normal brightness without increasing the resolution of the A / D converter itself. D conversion can be performed, and A / D conversion can be performed with high accuracy for a video signal of a low-luminance subject. Generally, when an A / D converter is provided immediately after an image sensor to perform A / D conversion of a video signal, high accuracy is required in a low-luminance portion rather than a high-luminance portion for gamma processing or the like. In such a case, a look-up table or the like was usually used, but it becomes possible to combine the work of converting the output data of a plurality of A / D converters with the look-up table. The work efficiency is significantly improved.

[Brief description of drawings]

FIG. 1 is a complaint correspondence diagram.

FIG. 2 is a block diagram showing the configuration of an embodiment.

FIG. 3 is a diagram showing digital signal output characteristics of an A / D converter with respect to an analog signal input.

FIG. 4 is a block diagram showing a configuration of a parallel comparison type A / D converter.

[Explanation of symbols]

 1 Preamplifier 2 Filter 3 High level amplifier 4 Low level amplifier 5 High level A / D converter 6 Low level A / D converter 7 Signal converter 21 Comparator 22 Encoder 23 Driver 24 Digital output terminal 100 First A / D converter 101 Amplifier 102 Second A / D converter 103 Selection circuit

Claims (2)

[Claims] 1. A video signal A / D conversion circuit for converting an analog video signal into a digital signal, comprising: a first A / D converter for converting the analog video signal into a digital signal; and a predetermined analog video signal. An amplifier that amplifies with the amplification degree of, a second A / D converter that converts the analog video signal amplified by this amplifier into a digital signal, and a digital signal output from this second A / D converter When it is the maximum value, the output digital signal of the first A / D converter is selected and output, and when the digital signal output from the second A / D converter is smaller than the maximum value. And a selection circuit which selects and outputs the output digital signal of the second A / D converter. 2. A video signal A / D conversion circuit for converting an analog video signal into a digital signal, comprising: a first amplifier for amplifying the analog video signal with a first amplification degree; and amplifying with the first amplifier. A first A / D converter for converting the analog video signal thus converted into a digital signal; and a second amplifier for amplifying the analog video signal with a second amplification degree larger than the first amplification degree, A second A / D converter that converts the analog video signal amplified by the second amplifier into a digital signal, and if the digital signal output from the second A / D converter has the maximum value, When the output digital signal from the first A / D converter is selected and output and the digital signal output from the second A / D converter is smaller than the maximum value, the second A / D converter outputs the second A / D converter. Output digital from D / D converter A / D conversion circuit of the video signal, characterized in that it comprises a selection circuit for selecting and outputting No..