Add some top level text for entropy encoding.
Added some high level text on entropy coding in AV1 to the
overview document. Still needs function level references and
resolution of link to AV1 technical overview document.
Change-Id: Icece17ed844b711727d39158133f7156089ba56e
diff --git a/doc/dev_guide/av1_encoder.dox b/doc/dev_guide/av1_encoder.dox
index 4137454..7f7e90c 100644
--- a/doc/dev_guide/av1_encoder.dox
+++ b/doc/dev_guide/av1_encoder.dox
@@ -1191,7 +1191,39 @@
\section architecture_entropy Entropy Coding
-\subsection architecture_entropy_aritmetic Arithmetic Coding Model
+\subsection architecture_entropy_aritmetic Arithmetic Coder
+
+VP9, used a binary arithmetic coder to encode symbols, where the propability
+of a 1 or 0 at each descision node was based on a context model that took
+into account recently coded values (for example previously coded coefficients
+in the current block). A mechanism existed to update the context model each
+frame, either explicitly in the bitstream, or implicitly at both the encoder
+and decoder based on the observed frequency of different outcomes in the
+previous frame. VP9 also supported seperate context models for different types
+of frame (e.g. inter coded frames and key frames).
+
+In contrast, AV1 uses an M-ary symbol arithmetic coder to compress the syntax
+elements, where integer \f$M\in[2, 14]\f$. This approach is based upon the entropy
+coding strategy used in the Daala video codec and allows for some bit-level
+parallelism in its implementation. AV1 also has an extended context model and
+allows for updates to the probabilities on a per symbol basis as opposed to
+the per frame strategy in VP9.
+
+To improve the performance / throughput of the arithmetic encoder, especially
+in hardware implementations, the probability model is updated and maintained
+at 15-bit precision, but the arithmetic encoder only uses the most significant
+9 bits when encoding a symbol. A more detailed discussion of the algorithm
+and design constraints can be found in (TODO REF) <b>A Technical Overview of
+the AV1 Standard</b> (TODO add link to Jingning's AV1 overview paper).
+
+TODO add references to key functions / files.
+
+As with VP9, a mechanism exists in AV1 to encode some elements into the
+bitstream as uncrompresed bits or literal values, without using the arithmetic
+coder. For example, some frame and sequence header values, where it is
+beneficial to be able to read the values directly.
+
+TODO add references to key functions / files.
\subsection architecture_entropy_coef Coefficient Coding and Optimisaztion
