Audio codec
MPEG-1 Audio Layer III HD (more commonly known by its abbreviation mp3HD) was an audio compression codec developed by Technicolor, formerly known as Thomson. 
It featured lossless compression of audio data, and was usually backwards compatible with the MP3 format by storing two data streams in one file.
Development.
mp3HD was released in March 2009 as a lossless competitor to the already popular FLAC, Apple Lossless, and WavPack. In theory, the format provided a convenient container in the form of a single file, which included the standard lossy stream playable on any mp3-capable device and the lossless data which was stored in the ID3v2 tag. 
To play the lossless data, the user needed a compatible mp3HD player with decoder. Otherwise, only the lossy data would be played. Files produced by the algorithm were substantially smaller than the uncompressed source files, though they are roughly comparable to other lossless formats.
Since 2009, Technicolor updated the format and encoding tools to make it more efficient, while adding a plugin for Winamp (Windows only), a DirectShow filter for Windows Media Player, and an mp3HD converter.
As of April 2013, the MP3HD website, specification and encoder software are no longer available, and promotion of the format has been abandoned.
Encoding.
The only available mp3HD encoder was the Technicolor toolkit which contained a command-line encoder and decoder. This could be used with the Exact Audio Copy to rip CDs into WAVE files and then automatically convert them to mp3HD files. Another method was the mp3HD Converter, which could convert WAVE files to mp3HD and decompress them back to WAVE files. mp3HD supported CD audio (PCM) at 44.1 and 48 kHz sampling rates at 16 bit/sample. Nero Burning Rom also supports converting to mp3HD with Nero Recode.
Audio quality.
The lossless mp3HD stream allowed for exact replication of CD-quality audio tracks. Average bitrates varied between 500 kbit/s to 900 kbit/s depending on genre, similar to other lossless codecs. The lossy stream used the same bit rates as a normal mp3 file and the lossy portion could use VBR or CBR depending on the user's preference for compression and quality.
Pros and cons.
Pros
Cons
Products that support mp3HD.
Hardware
Software
Alternative technologies.
Lossless
References.
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