MP3

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Peter 4May14: This page is a potential candidate for deprecation, the material should be fully covered in the Manual. If it stays it needs a big shake-up for non-legacy Wiki.
  • Gale 07May14: ToDo-S I updated it for current Audacity, though some of what I updated is duplicative of the Manual and could be trimmed. I think we need an MP3 page because of the importance of the format, even if we lose most of the Lame Installation content. I think some of the "troubleshooting content" on that page e.g. the list of valid sample rate/ bit rate combinations is ideal material for this page. MP3 encoding and decoding is such a complex matter that I doubt you could ever fit everything into the Manual.
  • Gale 17Jun14: ToDo-2 As per my comment above I reverted your deletion of Lame Installation and set a P2 to incorporate useful material from there to this page (or possibly elsewhere). Lame Installation is also linked to quite a lot so when it is finally deleted, it needs a redirect. I suggest it redirects here because if it goes to the Manual that may not address the question the link was addressing.
MP3  is a size-compressed, lossy audio format. This means that it uses less disk space to store audio than uncompressed formats like WAV, but loses some of the original sound quality in doing so. To reduce the quality loss, the audio removed is mainly outside the adult range of hearing, and the artefacts  it introduces are chosen so they are largely masked by louder or more prominent sounds. The actual quality achieved depends on the bit rate you encode at, higher bit rates giving better quality but resulting in larger files.
Other compressed lossy formats exist that work in a broadly similar way, such as MP2 and the more modern and arguably superior OGG and M4A formats.
 
Related article(s):


Contents

MP3 support in Audacity

Audacity can both import audio in MP3 format, and export to MP3 format. Audacity can use the LAME  MP3 encoding library in order to export MP3s, but we do not distribute LAME because of patent restrictions. Instead, you need to download LAME separately. Instructions for doing this are here in the Audacity Manual.

Settings

ToDo-2Add information about what the bit rate actually means in practice and what the other settings do. Explain stereo MP3 does not tend to use 2x as much space as mono, unlike wav and other uncompressed formats.

ID3 Tags

MP3 files contain "metadata" at the start of the file as ID3 tags . These tags typically contain information such as Track Title, Artist Name, Year and Genre. Some of this information may be visible in the playing window of software and hardware players. The tags can be edited with Audacity's tag editor.

By default, the Metadata Editor will always appear at export time. Enter any metadata you require in the editor, then click OK (not "Save"). The Export window lets you choose the file name, path and any custom encoding settings (by clicking the Options button).

To prevent Metadata Editor appearing at export time, click Edit > Preferences, then the Import / Export tab, and in the "When exporting tracks..." section, uncheck "Show Metadata Editor prior to export step". The tags can still be viewed or edited at any time prior to export at File > Edit Metadata..., and the tags in the editor at export time will still be exported.

When using Export Multiple, Metadata Editor pops up as many times as there are files to export. This gives flexibility to adjust the tags separately for tracks which form compilations with different artists or genres. If all the tracks to be exported have common information except for Track Title and Track Number, you may prefer to set Preferences as above so that Metadata Editor does not appear prior to export. Then make any necessary edits to the common tags at File > Edit Metadata... prior to export, and the multiple export will proceed silently with the Track Title and Track Number tags added automatically. The Track Title tag will be the same as the file name chosen in the label or track name, and the Track Number tag will be generated according to the order of the labels or tracks.

Each time an MP3 is imported in to a project window, the tags for that file replace the previous content of the Metadata Editor, so if the last imported MP3 has no tags, Metadata Editor will then have no content. If you want to export tags that a previously imported MP3 had, open File > Edit Metadata... to change the tags before export. Alternatively you could use File > Open... to import each MP3 into separate project windows which would preserve the tags for each file.

ID3v2.3 tags are exported. Note that some players don't yet fully support these tags, and so may not see all the tags. To prevent any tags being exported, uncheck "Show Metadata Editor..." in Preferences, open the Editor and delete any text showing in the "Tag Value" fields. Note that when using Export Multiple, the automatically generated Track Title and Track Number tags will still be produced, unless you show Metadata Editor at export and empty all the fields.

ID3v1 is only supported if you use Audacity's command-line encoder and add the --id3v1-only option. ID3v1 should only be needed for very old software or hardware players.

Limitations

In addition to the degrading of sound quality, MP3 files also have a short silence at the beginning. This is not a bug in Audacity or in Lame but is a limitation of the MP3 format. If this is an important issue (for example when making audio loops) you will need to use a format that does not have this limitation, such as WAV, Flac or Ogg.

Re-encoding to MP3

Every time you export from Audacity as an MP3 (or other lossy audio format), this encoding necessarily degrades some of the original quality of the audio. If you import an MP3 into Audacity, edit it then export it as an MP3, you are thus losing quality twice - once in the original MP3 encoding of the imported audio, then again when you export it from Audacity as MP3. Therefore when you are exporting as MP3, work with the highest quality copy of the audio that you can - preferably a copy in a lossless format such as WAV, AIFF or FLAC. You can always obtain a lossless copy of an audio CD by extracting its audio to a WAV or AIFF file. Never extract the audio from a CD to MP3 if you want to export it from Audacity as an MP3.

If you can't avoid importing an MP3 into Audacity and then re-encoding to MP3, don't believe what you sometimes hear that using the same or higher bit rate as the original file will prevent quality loss. This is incorrect. All you can say is that the higher the bit rate you re-encode to, the less will be the quality loss that results.

If you only want to perform simple edits on your MP3 (cut, copy, paste, join, fade or normalize), you may prefer an application that can edit MP3s directly without having to decompress then re-encode them as Audacity does. Examples:

  • MP3 DirectCut for Windows
  • Audion for Mac OS 8, OS 9 and OS X 10.0 to 10.5 (PPC only)
  • Macsome for OS X (Intel Mac only). It can perform lossless split, trim, cut and volume adjustments on M4A files as well as MP3.
  • Mp3Splt for Windows, Mac and GNU/Linux. It can split MP3, OGG and FLAC files by silence detection or by time point (it accepts an Audacity Label Track.txt file as input). It does not perform volume adjustments. It is a command-line tool but you can download the GTK version if you want an interface.

For more advanced edits such as effects, the audio needs to be decompressed in an editor like Audacity, so you must then accept any perceptible quality loss from re-encoding the MP3.

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