VD6640 datasheet(82/126 Pages) STMICROELECTRONICS

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Video system

VG6640, VD6640

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DocID023597 Rev 11

7.3.3

Digital gain

To compensate for the relatively coarse analog gain steps and differences in integration

times between the short, medium and long frames, the image sensor contains digital

multipliers to increase the granularity of the gain system.

The details of the digital gain implementation are listed below.

•

One set of white balance digital gain before the merge of the medium- and long

integration frames

•

One set of digital gain multipliers at the output of the final merge of the short, medium,

and long integration frames.

Each set of digital gain multipliers consists of:

•

Four individual 16-bit digital channel gains (one per Bayer channel)

–

digital_gain_greenR

–

digital_gain_red

–

digital_gain_blue

–

digital_gain_greenB

•

The digital gain range for each channel is 0.000 to 255.9961 in steps of 0.0039 (1/256),

i.e. 8 integer bits and 8 fractional bits

–

digital_gain_min = 0x0000 (0.000)

–

digital_gain_max = 0xFFFF (255.9961)

Note:

Text refers only to RGB

7.3.4

Pixel gain

The imaging sensor has a feature whereby the readout pixels can be operated in two

different conversion gain modes. In low-light conditions, a high conversion gain can be used

to improve sensitivity. In high-light conditions, a low conversion gain is selected to improve

signal SNR. The gain modes are set in the pixel_gain register (0x212c). Typically the

highgain / lowgain ratio will be approximately 5.8

0x07

0111

5.0 dB (x1.8)

5.0 dB (x7.2)

0x08

1000

6.0 dB(x2.0)

6.0 dB(x8.0)

0x09

1001

7.2 dB (x2.3)

7.2 dB (x9.2)

0x0A

1010

8.5 dB (x2.7)

8.5 dB (x10.8)

0x0B

1011

10.1 dB (x3.2)

10.1 dB (x12.8)

0x0C

1100

12.0 dB (x4.0)

12.0 dB (x16.0)

0x0D

1011

14.5 dB (x5.3)

14.5 dB (x21.2)

0x0E

1110

18.0 dB (x8.0)

18.0 dB (x32.0)

0x0F

1111

24.0 dB (x16.0)

24.0 dB (x64.0)

Table 37. Analog gain control (continued)

Gain value

Coarse gain code [A3:A0]

Analog gain(L1)

Analog gain(L4)

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