VS6724 datasheet(18/118 Pages) STMICROELECTRONICS

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Functional description

VS6724

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Quantizer

Quantizer scales the DCT coefficients with programmable luminance and chrominance Q-

tables. These Q-tables are programmed according to image characteristics, and modified by

the squeeze setting. The larger the squeeze setting, the more severe the quantization and

the greater the compression achieved.

Squeeze values can be manually set via the 8-bit register user_squeeze, or automatically

set, and dynamically adjusted, by the autosqueeze module.

Entcoder16

The entcoder16 (entropy coder with 16-bit output) converts the quantized data stream to a

symbol stream, including differential encoding of DC samples and run-length encoding of

zero-valued AC samples according to the JPEG standard, and finally Huffman-encodes the

symbol stream using hardwired baseline JPEG tables to compress the image. It also

includes the JPEG header and embeds restart interval markers (if requested by nonzero

restartinterval setting) as per the JPEG standard.

Autosqueeze

Autosqueeze is the VC’s in-built dynamic compression control system. It adjusts the

squeeze parameter based on measurements of two key criteria derived from the previous

frame: output file size, and FIFO fullness. Squeeze is subsequently driven iteratively

towards the optimal value, satisfying the two conditions:

FIFO stability, i.e. fullness has not exceeded a user-defined dead-zone, AND

output file size <= a user-defined target.

2.6.2

JPEG compression

At the simplest level the video compressor has one control which is the Squeeze setting:

this tells the system how hard to compress. A change in compression is achieved by scaling

the quantization tables.

Figure 7.

QCLK options

Video

compression

engine

UYV

422 to 420

JPEG

output

FIFO

Control

Squeeze

Firmware control

bOIFCLK

ratio

JPEG

YUV 422

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