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README for the Dirac video codec hardware implementation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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by Thomas Davies and Peter Bleackley, BBC R&D (dirac@rd.bbc.co.uk)
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1. Executive Summary
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~~~~~~~~~~~~~~~~~~~~
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Dirac is an open source video codec. It uses a traditional hybrid video codec
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architecture, but with the wavelet transform instead of the usual block
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transforms. Motion compensation uses overlapped blocks to reduce block
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artefacts that would upset the transform coding stage.
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Dirac can code just about any size of video, from streaming up to HD and
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beyond, although certain presets are defined for different applications and
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standards. These cover the parameters that need to be set for the encoder to
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work, such as block sizes and temporal prediction structures, which must
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otherwise be set by hand.
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Dirac is intended to develop into real coding and decoding software and hardware, capable of plugging
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into video processing applications and media players that need compression. It is intended to develop
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into a simple set of reliable but effective coding tools that work over a wide variety of content and
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formats, using well-understood compression techniques, in a clear and accessible software or hardware
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structure. It is not intended as a demonstration or reference coder.
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This release of the hardware implementation comprises VHDL modules for a prototype arithmetic coder
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and decoder. These implement a fixed probability model.
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2. Documentation
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~~~~~~~~~~~~~~~~
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A user guide and a guide to the software is in progress. More details on
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running the codec can be found at http://dirac.sourceforge.net/
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Documentation specific to the hardware will be posted the directory /docs.
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3. Synthesis
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~~~~~~~~~~
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All modules are built in RTL style, and are synthesizable. Modules common to
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the encoder and the decoder are in /src/common. Modules specific to the encoder are in /src/encoder
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and those specific to the decoder are in /src/decoder.
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Alongside each vhdl module (.vhd) is a .prj file. This specifies the compilation sequence for the
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module.
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The initial development of the vhdl was done with Xilinx ISE Webpack.
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4. Simulation
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~~~~~~~~~~~
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The encoder may run at clock rates of up to 100MHz, and process an input symbol every 4 clock cycles,
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thus giving a possible troughput of 25 Msymbol/second. The decoder can run at the same clock rate as
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the encoder, and decode the encoded stream at the same rate that it is produced by the encoder.
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A testbench is provided in /src/testbench, together with a test input file /src/rawdata. This
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testbench provides an end-to end simulation of the encoding and decoding process, for a data stream
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with a fixed source entropy of 0.498 bits/symbol. The testbench also provides a process that counts
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the number of bits produced by the encoder.
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Simulations have been perfomed with Modelsym XE.
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