Sustainability of Digital Formats: Planning for Library of Congress Collections

Known by its file extension M2TS, this format stores multiplexed audio, video, and other streams in BDAV (Blu-ray Disc Audio/Visual) and AVCHD (Advanced Video Coding High Definition) using a modification of MPEG-2 Transport Stream.

Summary of BDAV (Blu-ray Disc Audio/Visual)

The Blu-ray Disc (link via Internet Archive) format, first released in 2006, is detailed in a series of specifications from the Blu-ray Disc Association (BDA). FYI, BDA's White paper Blu-ray Disc Format - General (Aug 2004) notes that "the spelling of 'Blu-ray' is not a mistake. The character 'e' is intentionally omitted because a daily-used term cannot be registered as a trademark" (p. 5). Extensions to the format specifications and capacities have continued including recent updates for Ultra HD Blu-ray Disc. In brief, according to Blu-ray.com, Blu-ray is an "optical disc format that ... was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data. The format offers more than five times the storage capacity of traditional DVDs and can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc." Blu-ray uses the Universal Disk Format (UDF) specification developed and maintained by the Optical Storage Technology Association (OSTA), standardized as ISO/IEC 13346 (1995) and ECMA-167 (1997). UDF is used for computer data storage for a broad range of media, including DVDs and Blu-ray disks, supplanting ISO 9660. Blu-ray specifies the use of UDF 2.5 released in 2003 because it offers better support for larger capacities and is compatible with more devices.

The file structure for BDAV is defined in BDA's White paper: Blu-ray Disc Format, 2.B Audio Visual Application Format Specifications for BD-ROM (March 2005) (link via Internet Archive) and declares the supported video streams shall be MPEG-2 video format (ISO/IEC 13818-2) compliant, MPEG-4 AVC video format (ISO/IEC 14496-10) compliant or SMPTE VC-1 video format compliant. The BDAV MPEG 2 Transport Stream is "constructed from one or more “Aligned units”, where: 1) The size of an Aligned unit is 6144 bytes (2048*3 bytes); 2) An Aligned unit starts from the first byte of the source packets; 3) The length of a source packet is 192 bytes. One source packet consists of one TP_extra_header structure and one MPEG2 transport packet structure. The length of the TP_extra_header structure is 4 bytes and the length of the transport packet structure is 188 bytes; 4) One Aligned unit consists of 32 source packets.... In addition, aligned units are recorded in three consecutive logical sectors on the BD-ROM disc. The size of one logical sector is 2048 bytes. The maximum multiplex rate of the BDAV MPEG-2 Transport Stream is 48Mbps."

Overview of AVCHD (Advanced Video Coding High Definition)

AVCHD is a simpler form of the Blu-ray disc standard with just one video encoding algorithm and two audio encodings. As described in a press release by Panasonic and Sony in 2006 in 2006 announcing the format, AVCHD "is an HD digital video camera format for recording 1080i*1 and 720p*2 signals onto 8cm DVD media by using highly efficient codec technologies. The format employs MPEG-4 AVC/H.264 codec for video compression, and Dolby Digital (AC-3) or Linear PCM for audio codec. This makes it possible to develop HD video camera recorders which achieve compact size as well as high-quality video and audio.AVCHD Compared to Blu-ray disc format, AVCHD can use various storage media, such as DVD media, memory cards or hard disk drives." The BDAV container contains videos recorded using AVCHD camcorders, such as Sony's HDR-SR(xx) series models. Panasonic, Canon and other brands of AVCHD camcorders also store recorded video in the BDAV container format.

Overview of M2TS Implementation

For both BDAV and AVCHD, all application files are stored within the the directory as defined in White paper: Blu-ray Disc Format, 2.B Audio Visual Application Format Specifications for BD-ROM (link via Internet Archive). Section 2.3 describes the structure including the M2TS file (where the zzzzz is a generic placemarker for the file name): "The “zzzzz.m2ts” files contains a BDAV MPEG-2 transport stream. The names of these files are in the form “zzzzz.m2ts”, where “zzzzz” is a 5-digit number corresponding to the Clip. The same 5-digit number “zzzzz” is used for an AV stream file and its associated Clip information file."

In addition, whereas MPEG-2 Transport Stream allows for both CBR (Constant Bit Rate) and (VBR) Variable Bit Rate, M2TS files uses VBR in order to optimize the storage size.

M2TS is widely adopted in Blu-ray and AVCHD implementations and many software tools and applications exist for encoding and decoding. Wikipedia mentions that "M2TS files can also be played on Sony PlayStation 3s, Sony Bravia TVs, Western Digital WDTVs, Xtreamer media player, Amkette FlashTV HD Media Player and Panasonic Viera TVs supporting playback of AVCHD."

Depending on the need, a license may be required to produce a Blu-ray disc. See https://blu-raydisc.info/index.php for details. A license is required for AVCHD from https://www.avchd-info.org/license/index.html.

Licenses for MPEG-2 is managed by Via LA (Licensing Alliance) (https://www.via-la.com/licensing-2/mpeg-2/). Licenses pertain to tools and not to streams or files per se.

See MPEG-2 Transport Stream.

Accessibility FeaturesMPEG-2 TS and more specifically M2TS do not have specific features that enhance or support accessibility but the MPEG-2 family overall provides moderate accessibility support. According to the System Implementation of CEA-708 and CEA-608 Closed Captioning and Program-Related Data, "files that use MPEG-2 video compression encoding may carry closed caption data in the video user data, as per ATSC A/53 Part 4". Typically, accessibility features such as captions and subtitles are carried in containers and wrappers, not in encoded video data.See W3C's Making Audio and Video Media Accessible for more general information about accessible sound and moving image media.

The lack of metadata of the type called bibliographic by librarians motivated the MPEG group to develop MPEG-7, a separately standardized structure for metadata to support discovery and other purposes.

Moderate to very good, given that the options for audio employ lossy compression.

For all types of compressed audio, the quality of the outcome will depend on the type and extent of compression, and the encoder used.

Multi-channel sound is supported in both the backwardly compatible (BC) audio structures shared with MPEG-1, which includes layer 3 audio (MP3_ENC), and in the non-backwardly compatible AAC audio structure. MPEG-2 BC can deliver five main channels and an optional LFE (Low Frequency Encoding or Effects) channel, i.e., 5.1 surround sound. AAC provides a capability of up to 48 main audio channels, 16 LFE channels, 16 overdub/multilingual channels, and 16 data streams.

The use of AC-3 compressed audio (aka Dolby Digital), described above in connection with AES3_SMPTE data, provides another means for presenting surround sound.