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mad - MPEG audio decoder
Copyright (C) 2000-2001 Robert Leslie
$Id: README,v 1.1.1.1 2001-11-04 19:00:21 lampret 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 other copyrights. MAD is NOT a derivation of the ISO
reference source or any other code.
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 and will 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.
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 probably required to compile the code, but your mileage may vary.
Besides the assembly code notation, a GNU extension was used in
`libmad/huffman.c' to specify union initialization of a member other than
the first.
The code should not be sensitive to word sizes or byte ordering, however it
does assume A % B has the same sign as A.
===============================================================================
INSTALLATION
If you are trying to build MAD under Windows, you will almost certainly
want to use Cygwin:
http://www.cygwin.com/
The code is distributed with a `configure' script which will generate for
you a `Makefile' and a `config.h' in both the `libmad' and top-level
subdirectories. 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/
===============================================================================
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