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STV0676

Overview

1.3

Auto exposure and gain control

The STV0676 automatically controls the sensor exposure, which is evaluated (and, where

necessary modified) once per frame, where a frame consists of 2 video fields. The video fields are

identical in length, that is, they do not contain any of the half line detail of the analogue video

standards like CCIR or NTSC. Two fields per frame are required by the internal sensor video timing

model. Integration time, sensor analogue gain and STV0676 digital gain are all used to control the

overall exposure. The STV0676 exposure algorithm uses an asymptotic approach in calculating the

change required in the present exposure value to approach the requested exposure target.

1.4

Defect correction

STV0676 automatically detects and corrects pixel defects without the need for any additional

components or sensor calibration procedures. This greatly simplifies camera assembly and test

when compared with previous EEPROM-based defect correction schemes. The pixel defect

correction scheme ensures that the STV0676 + ST CMOS sensor appears as a ‘defect free’

chipset.

1.5

Video compression (VC) engine

The video compression engine performs 3 main functions:

q

up scaling of input YCbCr 4:2:2 video stream from the VP (typically to scale from QVGA to CIF

image formats),

q

compression and encoding of YCbCr stream into Motion-JPEG (M-JPEG) format,

q

USB bandwidth monitoring.

The data stream from the VP can be up to VGA size. The scaler in VC can downsize this image.

Once scaled the video stream is then converted into M-JPEG format. M-JPEG simply treats video

as a series of JPEG still images. The conversion is realised via a sequential DCT (discrete cosine

transform) with Huffman encoding. After transfer over USB, the M-JPEG stream is decoded in the

device driver running on the host.

The VC module is capable of compression ratios of up to 100:1 although this is scene-dependent.

Image framerate produced by the STV0676 chipset is fixed and furthermore the available USB

bandwidth is also fixed (within the software driver). The VC module varies the compression ratio to

match the fluctuating input video data rates to the available USB bandwidth and required framerate.

The final stage of the VC block manages the data transfer from the local VC FIFO store to the USB

core. STV0676 performs this management automatically by employing long-term (frame-level) and

short-term (block-level) compression management.

1.6

Control processor

The embedded 8052 microprocessor core controls the data flow through the major sub blocks

from the device driver.

1.7

Power management

The chipset conforms to all power requirements specified by USB Version 1.1.