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CS7620

CCD Imager Analog Processor

Cirrus Logic 

CS7620

using the picture content as a guide, the user can se-

lect which curve will lead to the best overall dy-

namic range in the picture. The Companding

module takes 13-bit data as input, and outputs ei-

ther 10-bit companded data, 12-bit MSB-clipped

data or it lets the original 13-bit data pass through.

By programming the compander in the way that is

shown in Figure 13, it is possible to compensate for

backlighting conditions. Details in dark areas stay

visible, even in very complex lighting conditions.

These three modes can be selected through 2 regis-

ter bits in operational control.

Bits_out register bits

0x

10

11

Output mode

10 bits companded

13 bits

12 bits (clipped)

Table 1. Companding Operational Control

In the 12-bit clipped mode, any input above 4095

gets clipped to 4095. In the 10-bit companded

mode, the input gets companded through a four

segment, three knees, fully programmable curve.

To program the curve, the placement of the three

knees in the companding curve must be deter-

mined. The next step is to determine the slope of

the four segments created by the three knees (slope

for each segment is defined as delta y / delta x). Fi-

nally, offsets must be calculated to keep the com-

panding curve continuous.

A fourth knee exists in the curve, which represents

the black level value. There are two options for the

10-bit black value. In case one, a linear mapping is

employed such that “blacker-than-black” pixel in-

formation is kept, with black (code 64 in the 13 bit

data) being defined as code 8 in the 10 bit domain.

The second option clips all pixel values less than

black (code 64 in the 13 bit data) to a programma-

ble offset value, offset1. This may be set to 0 if de-

sired. This option will lose the “blacker-than-

black” pixel information, but allow for slightly

more dynamic range. Note: If using the linear mode

(option 1), offset1 must be set to 8.

Registers x1 through x3 should be programmed

with the x coordinates of each one of the three

knees.

Registers slope1 through slope4 should be pro-

grammed with 256 multiplied by the calculated

slopes.

Finally, the offsets can be programmed following

the formulas below:

y1 = slope1/256 × (x1-64) + offset1

y2 = slope2/256 × (x2-x1) + y1

y3 = slope3/256 × (x3-x2) + y2

offset2 = y1 - (x1 × slope2 / 256)

offset3 = y2 - (x2 × slope3 / 256)

offset4 = y3 - (x3 × slope4 / 256)

(use integer division and discard the remainder)

When using the 10 bit companded output, be aware

of the non-linearity of the output data. If linear out-

put is needed to perform AWB or AGC, a linear

curve can be implemented to gather statistics. This

can be achieved by writing 8191 to x1 (set register

38h to 1fh and set register 39h to ffh) and setting

slope1 to 32 (set register 2eh to 00010xxxb and set

register 2fh to 20h). Once the statistics have been

gathered, all four registers should be returned to

their previous values before taking the actual pic-

ture.

14

DS301PP2

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