MP3Surround

MP3 Surround

Introduction

MP3 Surround is a new type of MP3 that supports 5.1 channels of audio. MP3 Surround is a new format, it has been developed by the fathers of MP3, the Fraunhofer IIS and Thomson in 2004. 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. 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 (but does not know it) ...

Players for MP3 Surround

Due to the backwards compatibility to MP3, any hardware and software MP3 player should be able to play MP3 Surround encoded files without any problem - although in stereo only. For the playback of the 5.1 content of MP3 Surround files, 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

Streaming solutions for broadcasting MP3 Surround across the internet do not exist at the moment - but that might change sooner or later. Inofficial streaming solutions might be possible after tweaking the Icecast streaming server or by using the built in streaming capabilities of the VLC media player.

Encoding MP3 Surround

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.

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. 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 well-behaving 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. Two possibilities for input files exist in contrast to the mp3sEnc encoder, with the vbr-encoding definitely being the preferred variant:

• The converter accepts cbr-encoded 44.1kHz stereo MP3-files at 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.

• In contrast to the mp3sEnc encoder, the mp3SX converter accepts also well-behaving vbr-encoded MP3-files. These need to be 44.1kHz files at "medium to high" bitrates. There is no specification what is meant with "medium to high", however, several successful conversion runs indicate working bitrates from 128-256kbps. That is much more than for cbr-encoded MP3 files. In addition, only the really needed 15kbps of the surround channel are added to vbr-encoded files in contrast to the 32kbps for cbr-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.

It has already been mentioned that the mp3SX converter requires well-behaving MP3-files otherwise it terminates the conversion process with an error. 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 with some tricks to get almost any stereo MP3-file converted to MP3 Surround.

External links

• http://all4mp3.com Players, Winamp plugin, encoders & decoders, information, and various mp3 resources
• http://mp3surround.com - Twin page to all4mp3.com, features additional information and samples