VV0670P001 View Datasheet(PDF) - Vision
Part Name
Description
MFG CO.
'VV0670P001' PDF : 36 Pages View PDF
Colour Processor Interface ASIC
2.2.2 Video Compression
CPiA
modules
CP MODULE
USB/PP
INTERFACE
HOST
PC
VIDEO COMPRESSION
Difference Engine &
Run Length Encoder
DATA STREAMER
Run Length Encoder
Framestore
decode
encode
Run-Length
Buffer
EXTERNAL DRAM
Video compression components and VC Module interfacing to External DRAM
In the discussion to follow the CPiA device is assumed to be interfaced to the the host PC via the USB,
however the discussion equally well applies to the parallel port interface option that CPiA also provides.
The VP module provides the video data stream to VC. The pixel-based compression algorithm operates by
considering the differences between consecutive frames and encoding those differences in a Run-Length
Buffer (RLB) held in external DRAM. The result is a run-length encoded stream of the changes from frame-to-
frame. The stream consists of run-lengths of regions where no change has occurred, coded YCbYCr
(YUV422) values of pixel-pairs which have changed significantly, additional codes to mark the start/end/length
of frame rows, a frame header detailing parameters associated with the capture settings of the encoded data
and finally codes to mark the end of the encoded frame.
The VC module implements the compression algorithm conceptually by way of two processes. The first
process is the Differencing Engine/Run-Length Encoder (DE/RLE):
• Each pixel-pair of the current video frame is compared in turn with the corresponding pixel-pair stored in
the DRAM FrameStore (FS). If this comparison indicates that there exists a significant difference in value
for the given pixel-pair then that pixel-pair’s value is stored at the next locationin the RLB .The stored value
is further coded to indicate its status as a pixel-pair value rather than a run-length . Conversely, if the
comparison indicates that the difference is not significant, a run-length counter is incremented and stored
at the current position in the RLB. The level of significant difference is dynamically calculated by the CP
module from frame-to-frame and can be influenced by the host PC. When the DE/RLB finishes processing
a frame the CP module is alerted indicating subsequent processes can commence.
The second process is the Data Streamer/Run-Length Decoder (DS/RLD):
• In parallel with the DE/RLE but delayed with respect to the current position in the RLB, the Data Streamer
(DS) streams data from the RLB to USB interface module for transfer to the host. In addition, the DS also
decodes the RLB stream back into the FS thus updating it. The software driver also decodes the received
RLB stream on the host PC. When the DS/RLB reaches the end of the data stream the CP module is again
alerted and the process repeats.
It is important to note that the two processes can run concurrently, that is the DE/RLE can be encoding the
current frame into the RLB while at the same time the DE/RLE can be streaming out the previously encoded
frame to the USB.
V:\apps\cpia\docs\cpia datasheet\cpia_datasheet4.fm
02/07/98
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Commercial In Confidence
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