View source

==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 four software-based possibilities exist at the moment: the [[Winamp]] audio player (version 5.5+ required) now natively supports mp3Surround files. There is also a standalone player for Windows, Mac OS X and Linux, provided by [http://www.all4mp3.com all4mp3.com]. The newest [http://labs.divx.com 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 supported now by ''Winamp 5.5+'' in combination with the shoutcast mp3Surround streamer that brings 6-channel surround streaming capabilities to any shoutcast server.

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 17 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.

===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 vbr encoding and also the following cbr bitrats: 128 / 160 / 192 / 224 / 256 / 320kbps. For normal users this encoder is rather useless as almost nobody has ever seen 5- or 6-channel WAV files. However, one can also use it as a high quality '''free to use ''Fraunhofer stereo MP3 encoder''''' where it might find a lot of friends. 

===The mp3SX converter===

The '''mp3SX''' converter (mp3SX = '''MP3 S'''tereo e'''X'''tended) is much more important as it works with already existing 44.1kHz stereo [[MP3]]-files as input having a mean bitrate of at least 128kbps. The mp3SX converter works with 44.1 kHz stereo MP3-files only as the additional surround information needs to be stored outside the frame structures in the ''padding data''. Padding is not properly defined for 32/48 kHz MP3 files. 

The mp3SX converter losslessly upgrades stereo MP3 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. The whole process is called ''Spatial Audio Coding''.

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 cbr-encoded input 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 17kbps. 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''': An undocumented feature of the mp3SX converter. The input files need to be 44.1kHz files at "medium to high" bitrates, that means bitrates from 128-300kbps. Only the really needed 17kbps 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 user. 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. The converter works well with vbr-encoded MP3-files - especially fresh-encoded ones - but it has to be taken in mind that this feature is undocumented (yet) and can in several cases lead to "MPEG stream synchronization errors". These ''Sync errors'' become audible during playback as "skips" in the MP3 bitstream. 

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]]. 

[http://wiki.themixingbowl.org/MP3optimized A '''separate TMB wiki page'''] [[MP3optimized | 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==

* http://all4mp3.com Player, Winamp plugins, encoders & decoders, shoutcast streamer, more information, and various mp3 resources
* http://www.mp3surround.com - Twin page to all4mp3.com, features additional information and samples
* http://winamp.com

[[category:Audio Formats]][[category:Audio Tools]]