MP3Surround

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Revision as of 17:08, 28 September 2006 by Ojay (talk | contribs) (→‎The mp3SX converter)
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MP3 Surround

MP3 Surround is a new type of MP3 that supports 5.1 channels of audio. MP3 Surround files have the extension *.mp3 like normal stereo MP3 files. That is possible as MP3 Surround encoded MP3-files are completely backwards compatible with standard MP3. Because of that, any hardware and software MP3 player should be able to play MP3 Surround encoded files without any problem - although in stereo only. Normal Hardware and Software MP3-players treat and report MP3 Surround encoded MP3 files as standard stereo MP3 as they do not recognize the surround extension.

So everyone might already have some MP3 Surround encoded MP3 files without even knowing it...

For the playback of the 5.1 content of MP3 Surround files, no hardware-based and only three software-based possibilities exist at the moment: there is a Winamp plugin and a standalone player, both provided by all4mp3.com. The newest DivX player in the version 6.3.x and above can decode DivX-encoded movies with an MP3 Surround encoded audio layer. Internet streaming of MP3 Surround is not supported by any player up to now.

The MP3 backwards compatibility of MP3 Surround has been achieved by downmixing six discrete audio channels to two stereo channels (!) while a third channel (the "surround" channel) outside the stereo MP3 frames contains the dispatching information (the surround info) necessary to get back the full 5.1 content. This third channel is invisible for normal MP3 players (as it is outside the defined MP3 frame structures), they treat the surround channel as they would treat id3 tags: they don't recognize them for playback. The data overhead needed for the third surround channel is 16 kbit/s, which allows for file sizes similar to standard stereo MP3 files.

Encoding MP3 Surround

There are two ways of encoding MP3 Surround files, for TMB interesting is only the mp3SX converter.

The mp3sEnc encoder

The mp3sEnc encoder allows to encode mono MP3, stereo MP3, and to encode 6-channel MP3 Surround with 6 discrete channels by encoding one interleaved 6-channel WAV file or by encoding five (5.0) and six (5.1) mono WAV files. It supports only cbr bitrats: 128 / 160 / 192 / 224 / 256 / 320kbps. For TMB uploads this encoder is rather useless as there are no available non-commercial live DJ mix sets or broadcasts in 5.1 surround sound. However, one can also use it as a high quality free to use Fraunhofer stereo MP3 encoder.

The mp3SX converter

The mp3SX converter (mp3SX = MP3 Stereo eXtended) is much more important for TMB members as it works with already existing 44.1kHz stereo MP3-files as input (at least 95% of all files on TMB can be upgraded with this converter).

It losslessly upgrades them to MP3 Surround files with 5.1 channels. For doing so, mp3SX analyzes the existing natural ambience of the stereo material and plays it back through the rear channels. The sound sources remain in the front channels, but are played back through the Left, Center and Right channel, providing a stable front image even for off-sweet-spot listening. Mono content (e.g. vocals added by microphones) will be moved to the Center. Apart from that, mp3SX preserves the original stereo sound stage. The whole surround upgrade process requires only 15 kB/s additional information added as a third surround channel to the stereo MP3, finally resulting in a typical MP3 Surround file.

Using the mp3SX converter is quite easy for error-free stereo MP3-files as the converter features a graphical interface that allows to convert stereo MP3-files to MP3 Surround files with simple drag 'n' drop actions. Required are always error-free stereo MP3-files at 44.1kHz and with bitrates of at least 128kbps. The mp3SX converter accepts both cbr- and vbr- encoded MP3-files:

  • cbr-encoded: For these files the mp3SX converter works at stereo bitrates between 128-192kbps. The surround channel of the re-written MP3-bitstream enlarges the MP3 Surround file by 32kbps although the surround info requires only 15kbps. That is due to a general limitation of cbr-encoded MP3-files with the following allowed bitrates at 128kbps and above: 128 / 160 / 192 / 224 / 256 /320. As an example, a stereo 128kbps file will become a 5.1 channel 160kbps file after the conversion process.
  • vbr-encoded: These need to be 44.1kHz files at "medium to high" bitrates, that means bitrates from 128-256kbps. Only the really needed 15kbps of the surround channel are added to these vbr-encoded files. That results in only 8% file size increase for typical stereo vbr-MP3-files at around 192kbps - almost invisible for a typical downloader. Both the possibility to convert high quality vbr-encoded MP3-files to MP3 Surround as well as the small surround overhead indicate essential advantages over the solution with cbr-encoded files.

It has already been mentioned that the mp3SX converter requires error-free MP3-files otherwise it terminates the conversion process with an error. However, broken MP3-files are the rule rather than the exception and need to be repaired before the conversion process. Recommended for doing this is MP3packer.

A separate TMB wiki page provides a step-by-step guide for repairing broken MP3files with some tricks to get almost any stereo MP3-file converted to MP3 Surround - and back from MP3 Surround back to normal stereo MP3-files.

External links

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