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authorMichał Cichoń <michcic@gmail.com>2011-11-15 22:28:48 +0100
committerMichał Cichoń <michcic@gmail.com>2011-11-15 22:28:48 +0100
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+
+ libmad - MPEG audio decoder library
+ Copyright (C) 2000-2004 Underbit Technologies, Inc.
+
+ $Id: README,v 1.4 2004/01/23 09:41:32 rob Exp $
+
+===============================================================================
+
+INTRODUCTION
+
+ MAD (libmad) is a high-quality MPEG audio decoder. It currently supports
+ MPEG-1 and the MPEG-2 extension to Lower Sampling Frequencies, as well as
+ the so-called MPEG 2.5 format. All three audio layers (Layer I, Layer II,
+ and Layer III a.k.a. MP3) are fully implemented.
+
+ MAD does not yet support MPEG-2 multichannel audio (although it should be
+ backward compatible with such streams) nor does it currently support AAC.
+
+ MAD has the following special features:
+
+ - 24-bit PCM output
+ - 100% fixed-point (integer) computation
+ - completely new implementation based on the ISO/IEC standards
+ - distributed under the terms of the GNU General Public License (GPL)
+
+ Because MAD provides full 24-bit PCM output, applications using MAD are
+ able to produce high quality audio. Even when the output device supports
+ only 16-bit PCM, applications can use the extra resolution to increase the
+ audible dynamic range through the use of dithering or noise shaping.
+
+ Because MAD uses integer computation rather than floating point, it is
+ well suited for architectures without a floating point unit. All
+ calculations are performed with a 32-bit fixed-point integer
+ representation.
+
+ Because MAD is a new implementation of the ISO/IEC standards, it is
+ unencumbered by the errors of other implementations. MAD is NOT a
+ derivation of the ISO reference source or any other code. Considerable
+ effort has been expended to ensure a correct implementation, even in cases
+ where the standards are ambiguous or misleading.
+
+ Because MAD is distributed under the terms of the GPL, its redistribution
+ is not generally restricted, so long as the terms of the GPL are followed.
+ This means MAD can be incorporated into other software as long as that
+ software is also distributed under the GPL. (Should this be undesirable,
+ alternate arrangements may be possible by contacting Underbit.)
+
+===============================================================================
+
+ABOUT THE CODE
+
+ The code is optimized and performs very well, although specific
+ improvements can still be made. The output from the decoder library
+ consists of 32-bit signed linear fixed-point values that can be easily
+ scaled for any size PCM output, up to 24 bits per sample.
+
+ The API for libmad can be found in the `mad.h' header file. Note that this
+ file is automatically generated, and will not exist until after you have
+ built the library.
+
+ There are two APIs available, one high-level, and the other low-level.
+ With the low-level API, each step of the decoding process must be handled
+ explicitly, offering the greatest amount of control. With the high-level
+ API, after callbacks are configured, a single routine will decode an
+ entire bitstream.
+
+ The high-level API may either be used synchronously or asynchronously. If
+ used asynchronously, decoding will occur in a separate process.
+ Communication is possible with the decoding process by passing control
+ messages.
+
+ The file `minimad.c' contains an example usage of the libmad API that
+ shows only the bare minimum required to implement a useful decoder. It
+ expects a regular file to be redirected to standard input, and it sends
+ decoded 16-bit signed little-endian PCM samples to standard output. If a
+ decoding error occurs, it is reported to standard error and decoding
+ continues. Note that the scale() routine in this code is only provided as
+ an example; it rounds MAD's high-resolution samples down to 16 bits, but
+ does not perform any dithering or noise shaping. It is therefore not
+ recommended to use this routine as-is in your own code if sound quality is
+ important.
+
+Integer Performance
+
+ To get the best possible performance, it is recommended that an assembly
+ version of the fixed-point multiply and related routines be selected.
+ Several such assembly routines have been written for various CPUs.
+
+ If an assembly version is not available, a fast approximation version will
+ be used. This will result in reduced accuracy of the decoder.
+
+ Alternatively, if 64-bit integers are supported as a datatype by the
+ compiler, another version can be used that is much more accurate.
+ However, using an assembly version is generally much faster and just as
+ accurate.
+
+ More information can be gathered from the `fixed.h' header file.
+
+ MAD's CPU-intensive subband synthesis routine can be further optimized at
+ the expense of a slight loss in output accuracy due to a modified method
+ for fixed-point multiplication with a small windowing constant. While this
+ is helpful for performance and the output accuracy loss is generally
+ undetectable, it is disabled by default and must be explicitly enabled.
+
+ Under some architectures, other special optimizations may also be
+ available.
+
+Audio Quality
+
+ The output from MAD has been found to satisfy the ISO/IEC 11172-4
+ computational accuracy requirements for compliance. In most
+ configurations, MAD is a Full Layer III ISO/IEC 11172-3 audio decoder as
+ defined by the standard.
+
+ When the approximation version of the fixed-point multiply is used, MAD is
+ a limited accuracy ISO/IEC 11172-3 audio decoder as defined by the
+ standard.
+
+ MAD can alternatively be configured to produce output with less or more
+ accuracy than the default, as a tradeoff with performance.
+
+ MAD produces output samples with a precision greater than 24 bits. Because
+ most output formats use fewer bits, typically 16, it is recommended that a
+ dithering algorithm be used (rather than rounding or truncating) to obtain
+ the highest quality audio. However, dithering may unfavorably affect an
+ analytic examination of the output (such as compliance testing); you may
+ therefore wish to use rounding in this case instead.
+
+Portability Issues
+
+ GCC is preferred to compile the code, but other compilers may also work.
+ The assembly code in `fixed.h' depends on the inline assembly features of
+ your compiler. If you're not using GCC or MSVC++, you can either write
+ your own assembly macros or use the default (low quality output) version.
+
+ The union initialization of `huffman.c' may not be portable to all
+ platforms when GCC is not used.
+
+ The code should not be sensitive to word sizes or byte ordering, however
+ it does assume A % B has the same sign as A.
+
+===============================================================================
+
+BUILDING AND INSTALLING
+
+Windows Platforms
+
+ MAD can be built under Windows using either MSVC++ or Cygwin. A MSVC++
+ project file can be found under the `msvc++' subdirectory.
+
+ To build libmad using Cygwin, you will first need to install the Cygwin
+ tools:
+
+ http://www.cygwin.com/
+
+ You may then proceed with the following POSIX instructions within the
+ Cygwin shell.
+
+ Note that by default Cygwin will build a library that depends on the
+ Cygwin DLL. You can use MinGW to build a library that does not depend on
+ the Cygwin DLL. To do so, give the option --host=mingw32 to `configure'.
+
+POSIX Platforms (including Cygwin)
+
+ The code is distributed with a `configure' script that will generate for
+ you a `Makefile' and a `config.h' for your platform. See the file
+ `INSTALL' for generic instructions.
+
+ The specific options you may want to give `configure' are:
+
+ --enable-speed optimize for speed over accuracy
+
+ --enable-accuracy optimize for accuracy over speed
+
+ --disable-debugging do not compile with debugging support, and
+ use more optimizations
+
+ --disable-shared do not build a shared library
+
+ Note that you need not specify one of --enable-speed or --enable-accuracy;
+ in its default configuration, MAD is optimized for both. You should only
+ use one of these options if you wish to compromise speed or accuracy for
+ the other.
+
+ By default the package will build a shared library if possible for your
+ platform. If you want only a static library, use --disable-shared.
+
+ It is not normally necessary to use the following options, but you may
+ fine-tune the configuration with them if desired:
+
+ --enable-fpm=ARCH use the ARCH-specific version of the
+ fixed-point math assembly routines
+ (current options are: intel, arm, mips,
+ sparc, ppc; also allowed are: 64bit, approx)
+
+ --enable-sso use the subband synthesis optimization,
+ with reduced accuracy
+
+ --disable-aso do not use certain architecture-specific
+ optimizations
+
+ By default an appropriate fixed-point assembly routine will be selected
+ for the configured host type, if it can be determined. Thus if you are
+ cross-compiling for another architecture, you should be sure either to
+ give `configure' a host type argument (--host) or to use an explicit
+ --enable-fpm option.
+
+ If an appropriate assembly routine cannot be determined, the default
+ approximation version will be used. In this case, use of an alternate
+ --enable-fpm is highly recommended.
+
+Experimenting and Developing
+
+ Further options for `configure' that may be useful to developers and
+ experimenters are:
+
+ --enable-debugging enable diagnostic debugging support and
+ debugging symbols
+
+ --enable-profiling generate `gprof' profiling code
+
+ --enable-experimental enable code using the EXPERIMENTAL
+ preprocessor define
+
+===============================================================================
+
+COPYRIGHT
+
+ Please read the `COPYRIGHT' file for copyright and warranty information.
+ Also, the file `COPYING' contains the full text of the GNU GPL.
+
+ Send inquiries, comments, bug reports, suggestions, patches, etc. to:
+
+ Underbit Technologies, Inc. <support@underbit.com>
+
+ See also the MAD home page on the Web:
+
+ http://www.underbit.com/products/mad/
+
+===============================================================================
+