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Therefore, for the most part, comparison of decoders is almost
exclusively based on how computationally efficient they are (i.e., how
much memory or CPU time they use in the decoding process).
ID3 and other tags
Main articles: ID3 and APEv2 tag
A "tag" is data stored in an MP3 (as well as other formats) that
contains metadata such as the title, artist, album, track number or
other information about the MP3 file to be added to the file itself.
The most widespread standard tag formats are currently the ID3 ID3v1
and ID3v2 tags, and the more recent APEv2 tag.
APEv2 was originally developed for the MPC file format (see the APEv2
specification). APEv2 can coexist with ID3 tags in the same file, but
it can also be used by itself.
To edit tags of MP3 files, you need a tag editor. See the list of tag
editors for an enumeration of common tag editors.
Volume normalization
As compact discs and other various sources are recorded and mastered
at different volumes, it is useful to store volume information about a
file in the tag so that at playback time, the volume can be
dynamically adjusted.
A few standards for encoding the gain of an MP3 file have been
proposed. The idea is to normalize the volume (not the volume peaks)
of audio files, so that the volume does not change between consecutive
tracks.
The most popular and widely used solution for storing replay gain is
known simply as "Replay Gain". Typically, the average volume and
clipping information about audio track is stored in the metadata tag.
Alternative technologies
Many other lossy audio codecs exist, including:
MPEG-1/2 Audio Layer 2 (MP2), MP3's predecessor;
MPEG-4 AAC, MP3's successor, used by Apple's iTunes Music Store and
iPod
Ogg Vorbis from the Xiph.org Foundation, a free software and patent
free codec.
MPC, also known as Musepack (formerly MP+), a derivative of MP2;
mp3PRO from Thomson Multimedia combining MP3 with SBR;
AC-3, used in Dolby Digital and DVD;
ATRAC, used in Sony's Minidisc;
Windows Media Audio (WMA) from Microsoft.
QDesign, used in QuickTime at low bitrates;
AMR-WB+ Enhanced Adaptive Multi Rate WideBand codec, optimized for
cellular and other limited bandwidth use;
RealAudio from RealNetworks, frequently in use for streaming on
websites;
Speex, free software and patent free codec based on CELP specifically
designed for speech and VoIP.
mp3PRO, MP3, AAC, and MP2 are all members of the same technological
family and depend on roughly similar psychoacoustic models. The
Fraunhofer Gesellschaft owns many of the basic patents underlying
these codecs, with Dolby Labs, Sony, Thomson Consumer Electronics, and
AT&T holding other key patents.
There are also some lossless audio compression methods used on the
Internet. While they are not similar to MP3, they are good examples of
other compression schemes available. These include:
FLAC stands for 'Free Lossless Audio Codec'
Monkey's Audio
SHN, also known as Shorten
TTA
Wavpack
Apple Lossless
Listening tests have attempted to find the best-quality lossy audio
codecs at certain bitrates. At 128 kbit/s, Ogg Vorbis, AAC, MPC and
WMA Pro tied for first place with LAME MP3 a little behind. At 64 kbit/s,
AAC-HE and mp3pro performed marginally better than other codecs. At
high bitrates (128 kbit/s+), most people do not hear significant
differences. What is considered 'CD quality' is quite subjective; for
some 128 kbit/s MP3 is sufficient, while for others 200 kbit/s or
higher MP3 is necessary.
