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 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 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 Stereo eXtended) 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.
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.