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components/spotify/cspot/bell/external/libhelix-aac/readme.txt

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+Fixed-point HE-AAC decoder
+Developed by RealNetworks, 2005
+===============================
+
+Overview
+--------
+This module contains a high-performance HE-AAC decoder for 32-bit fixed-point 
+processors. The following is a summary of what is and is not supported:
+
+Supported:
+ - MPEG2, MPEG4 low complexity decoding (intensity stereo, M-S, TNS, PNS)
+ - spectral band replication (SBR), high-quality mode
+ - mono, stereo, and multichannel modes
+ - ADTS, ADIF, and raw data block file formats
+
+Not currently supported:
+ - main or SSR profile, LTP
+ - coupling channel elements (CCE)
+ - 960/1920-sample frame size
+ - low-power mode SBR
+ - downsampled (single-rate) SBR
+ - parametric stereo
+
+Highlights
+----------
+ - highly optimized for ARM processors (details in docs/ subdirectory)
+ - reference x86 implementation
+ - C and assembly code only (C++ not required for codec library)
+ - reentrant, statically linkable
+ - low memory (details in docs/ subdirectory)
+ - option to use Intel Integrated Performance Primitives (details below)
+
+Supported platforms and toolchains
+----------------------------------
+This codec should run on any 32-bit fixed-point processor which can perform a full 32x32-bit 
+multiply (providing a 64-bit result). The following processors and toolchains are supported:
+ - x86, Microsoft Visual C++
+ - x86, GNU toolchain (gcc)
+ - ARM, ARM Developer Suite (ADS)
+ - ARM, Microsoft Embedded Visual C++
+ - ARM, GNU toolchain (gcc)
+
+ARM refers to any processor supporting ARM architecture v.4 or above. Thumb is not required.
+
+Generally ADS produces the fastest code. EVC 3 does not support inline assembly code for
+ARM targets, so calls to MULSHIFT32 (smull on ARM) are left as function calls. This incurs
+a significant performance penalty. For the fastest code on targets which do not normally use 
+ADS consider compiling with ADS, using the -S option to output assembly code, and 
+feeding this assembly code to the assembler of your choice. This might require some 
+syntax changes in the .S file.
+
+Adding support for a new processor is fairly simple. Simply add a new block to the file 
+real/assembly.h which implements the required inline assembly functions for your processor. 
+Something like
+
+...
+#elif defined NEW_PROCESSOR
+
+/* you implement MULSHIFT32() and so forth */
+
+#else
+#error Unsupported platform in assembly.h
+#endif
+
+Optionally you can rewrite or add assembly language files optimized for your platform. Note 
+that many of the algorithms are designed for an ARM-type processor, so performance of the
+unmodified C code might be noticeably worse on other architectures. 
+
+Adding support for a new toolchain is straightforward. Use the sample projects or the
+Helix makefiles as a template for which source files to include.
+
+Multichannel
+------------
+For multichannel, just set AAC_MAX_NCHANS in pub/aacdec.h to the desired max number 
+of channels (default = 2) and recompile. This increases RAM usage since more memory 
+is required to save state for multiple channels. See docs/memory.xls for details.
+  
+Directory structure
+-------------------
+fixpt/           platform-independent code and tables, public API
+fixpt/docs       memory and CPU usage figures, callgraphs
+fixpt/hxwrap     Helix wrapper code and makefiles
+fixpt/ipp        source code which uses IPP for decoding (see the "IPP" section below)
+fixpt/pub        public header files
+fixpt/real       source code for RealNetworks' AAC decoder
+fixpt/testwrap   sample code to build a command-line test application
+
+Code organization
+-----------------
+fixpt/
+  aacdec.c       main decode functions, exports C-only API
+  aactabs.c      common tables used by all implementations
+fixpt/pub/
+  aaccommon.h    low-level codec API which aacdec.c calls
+  aacdec.h       high-level codec API which applications call
+  statname.h     symbols which get name-mangled by C preprocessor to allow static linking
+fixpt/ipp        source code for wrapper files which link in IPP libraries
+fixpt/real       full source code for RealNetworks AAC decoder, including SBR
+fixpt/real/asm   optimized assembly code files for certain platforms
+
+To build an AAC decoder library, you'll need to compile the top-level files and EITHER 
+real/*.c OR ipp/*.c and the appropriate IPP library. 
+
+Decoder using Real code: aacdec.c + aactabs.c + real/*.c + real/asm/[platform]/*.s (if necessary)
+Decoder using IPP code:  aacdec.c + aactabs.c + ipp/*.c + ippac*.lib
+
+IPP 
+--- 
+For certain platforms Intel<65> has created highly-optimized object code libraries of DSP 
+routines. These are called the Intel<65> Integrated Performance Primitives (IPP). If IPP 
+libraries are available for a platform, this AAC decoder can link them in and use them 
+instead of the RealNetworks source code. To use IPP, you still need to build the top-level 
+files (aacdec.c, aactabs.c). You also build the files in ipp/*.c. These are just thin 
+wrappers which provide the glue logic between the top-level decode functions in 
+aacdec.c and the optimized DSP primitives in the IPP libraries. IPP libraries are not 
+included in this module. You must obtain them WITH A LICENSE directly from Intel. 
+Further info on the latest versions of IPP (as of the date of this readme) is available 
+from the URL below:
+
+http://www.intel.com/software/products/ipp/
+
+This site explains how to obtain IPP and the terms under which IPP libraries may be used.
+The code in this module is merely wrapper code which calls IPP functions. You are fully 
+responsible for adhering to the license agreement under which you obtain the IPP 
+libraries from Intel.
+
+readme.txt last updated 02/25/05