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authorAvuton Olrich <avuton@gmail.com>2006-07-16 17:49:15 +0000
committerAvuton Olrich <avuton@gmail.com>2006-07-16 17:49:15 +0000
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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/
-
-===============================================================================
-