
 mad - MPEG audio decoder
 Copyright (C) 2000-2001 Robert Leslie

 $Id: README,v 1.46 2001/10/18 08:34:41 rob Exp $

===============================================================================

INTRODUCTION

  MAD 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 and copyrights 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 the author.)

===============================================================================

ABOUT THE CODE

  The code currently consists of an MPEG audio decoding library (libmad) and
  a simple front-end called `madplay' that supports several audio output
  modules:

    - an Open Sound System interface module (for Linux, et al.)
    - a Sun audio interface module (for Solaris, NetBSD, et al.)
    - a Win32 audio interface module (for Windows 95/98/NT, et al.)
    - a raw PCM output module
    - a WAVE file output module (*.wav)
    - a Sun/NeXT audio file output module (*.au, *.snd)
    - a hex output module (for debugging and compliance testing)
    - a null module (for timing the decoder)

  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 which can be easily
  scaled for any size PCM output, up to 24 bits per sample.

  The API for libmad is likely to change, so it is not yet documented.
  However, the header file `libmad/mad.h' may be informative, as may also be
  the sample code in `minimad.c'. Note that the `libmad/mad.h' file is
  automatically generated, and will not exist until 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 `madplay' front-end is written to use the high-level synchronous API.

  The file `minimad.c' contains an example usage of the libmad API which is
  less complex than `madplay.c' and 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. It will abort at the first discovery of a
  decoding error. 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 which is much more accurate.
  However, using an assembly version is generally much faster and just as
  accurate.

  More information can be gathered from the `libmad/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 tested and 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, `madplay' implements a
  dithering algorithm when truncating samples for output. This produces high
  quality audio that generally sounds superior to the output of a simple
  rounding algorithm. However, dithering may unfavorably affect an analytic
  examination of the output (such as compliance testing), and therefore it
  may optionally be disabled at runtime.

  It is recommended that applications using the MAD library also implement
  some form of dithering to obtain the best quality audio.

Portability Issues

  GCC is preferred to compile the code, but other compilers may also work.
  The assembly code in `libmad/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 `libmad/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

  The MAD library can be built under Windows using either MSVC++ or Cygwin.
  A MSVC++ project file can be found under `msvc++/libmad'. In addition, a
  demo project (based on madplay and complete with ID3 support) can be found
  under `msvc++/demo'.

  To build MAD or the MAD plug-in for Winamp 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.

POSIX Platforms (including Cygwin)

  The code is distributed with a `configure' script which will generate for
  you a `Makefile' and a `config.h' in each of the various subdirectories
  which require them. 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

      --enable-shared		build a shared library if possible

  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.

  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 or to use an explicit --enable-fpm
  option.

  If an appropriate assembly routine cannot be determined, the fast
  approximation version will be used. In this case, use of an alternate
  --enable-fpm is 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 AND AUTHOR

  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:

      Robert Leslie <rob@mars.org>

  See also the MAD home page on the Web:

      http://www.mars.org/home/rob/proj/mpeg/

===============================================================================

