STV-USB-VGA-R01 查看數據表（PDF） - STMicroelectronics

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STV-USB-VGA-R01

USB CIF/VGA Digital Camera Chipset

STMicroelectronics 

Customer Datasheet, Rev 3.1, 29 November 2000

STV0672-chipsetf-3-1.fm

2.2.6 Matrixing

This module performs a 3 x 3 matrix multiplication on the smooth RGB channels, to map pixel taking chromaticities onto nominal

display chromaticities. Depending on the characteristics, a general 3 x 3 matrix can effect the transformation; however we choose

to decompose this matrix into the form M = UT, where U is a neutral-preserving matrix, each of whose rows add to unity, and T is

a diagonal matrix of tilts, which form one component of the channel gains applied in the input processor. The combination of U

and T are coded in such a way as to require the user provision of 9 integers; three for the tilts T and six for the off-diagonal

components of U (on-diagonal coefficients are implicit in a re-ordered row computation). The strain parameter from the

housekeeper and a specially-cored version of the unsharp mask signal control the damper; a variable which desaturates the

effect of the matrix by attenuating off-diagonal components in the presence of noise (strain component, occurring in low ambient

light) or Bayer aliasing (unsharp component, occurring in image regions with high edge-content).

2.2.7 Peaking

The unsharp mask signal is added to each matrixed colour channel to compensate for edge information lost in Bayer-pattern

under-sampling and interpolation.

2.2.8 Gamma correction

Gamma correction provides a non-linear distortion of data amplitudes required for various video communication standards, as

well as cosmetic enhancement of image detail.

2.2.9 Coder

The coder module comprises a color-space convertor which takes RGB from the gamma module and produces luminance Y and

weighted colour-differences Cb & Cr. The data is now passed in YCbCr form to the Video Compression block.

2.3 Video Compression (VC) Engine

The video compression engine performs 3 main functions:

• Up scaling of input YCbCr 4:2:2 video stream from the VP (typically to scale from QVGA to CIF image formats)

• Compression and Encoding of YCbCr stream into Motion-JPEG (M-JPEG) format

• USB Bandwidth monitoring

Figure 3 below gives a block overview of the VC module. The data stream from the VP can be upto 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. Please note that the JPEG specific header information need not be transmitted in a MJPEG stream.

The conversion is realised via a sequential DCT (Discrete Cosine Transform) with Huffman encoding. After transfer over USB the

M-JPEG stream will be decoded in the Video-for-Windows (VfW) device driver running on the host.

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

produced by the STV0672 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, that vary according to scene

dynamics, to the available USB bandwidth and required framerate.

The final stage of the VC block manages the data transfer rate from the local VC FIFO store to the USB core. STV0672 can

perform this management automatically, by employing long-term (frame-level) and short-term (block-level) compression

management. The former is achieved by varying a scalar quality-factor from frame to frame, to drive expected data rates upwards

or downwards. The latter is achieved by truncating the zig-zag sequence of AC coefficients more or less severely according to

how many preset thresholds of FIFO usage have been crossed. As FIFO usage approaches maximum, this truncation process

reduces instantaneous data rates until stability is regained, at the cost of local loss of detail in the image. The latter process is

transparent to the decoder. Statistics of threshold-crossing activity are subsequently used in the long-term quality setting

decision.

29 November 2000

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Commercial in confidence

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