BDM File Documentation

Summary

A BDM (AVCHD Information File) is a small binary index file that an AVCHD camcorder writes inside its BDMV folder, either as INDEX.BDM or MOVIEOBJ.BDM. It holds no video: it catalogs and orders the actual MTS/M2TS clips in the STREAM folder. Its MIME type is application/octet-stream. Never delete a .bdm before importing your footage, and to use the video, import the whole AVCHD folder into an editor rather than opening the .bdm itself.

Technical details

FeatureValue
Full nameAVCHD Information File (BDMV index)
File extension.bdm (as INDEX.BDM / MOVIEOBJ.BDM)
MIME typeapplication/octet-stream
Format typeBinary index / management sidecar
Part ofAVCHD (BDMV) folder structure
DeveloperPanasonic & Sony (AVCHD specification)
Introduced2006 (AVCHD 1.0)
Typical size~0.5–2 KB
Contains video?No — pointers and metadata only
Magic (INDEX.BDM)49 4E 44 58 30 31 30 30 (“INDX0100”)
Magic (MOVIEOBJ.BDM)4D 4F 42 4A 30 31 30 30 (“MOBJ0100”)
Actual video clips.mts / .m2ts in the STREAM folder
Related index files.cpi (clip info), .mpl (playlist)
Open standardNo — proprietary AVCHD specification
Related extensions.mts, .m2ts, .cpi, .mpl, .bdmv, .tod, .mod
Safe to delete?Not until the footage has been imported
File signature (magic bytes)
49 4E 44 58 30 31 30 30

Offset 0, 8 bytes. This is the header of INDEX.BDM and reads INDX0100 in ASCII (the tag INDX plus a version 0100). The sibling MOVIEOBJ.BDM instead starts 4D 4F 42 4A 30 31 30 30 = MOBJ0100. The two version digits after the four-letter tag can differ between camcorder models. Both files are only about half a kilobyte to two kilobytes and contain no picture data.

What is a BDM file?

A BDM file is an AVCHD Information File, a small binary index that a high-definition camcorder writes as part of the AVCHD recording structure. AVCHD (Advanced Video Coding High Definition) was created jointly by Panasonic and Sony in 2006, and cameras from Canon, JVC and others adopted it. When an AVCHD camera records, it does not drop a loose movie file onto the memory card. It builds a folder tree modelled on Blu-ray Disc, and the .bdm files are the small management files that catalog and order the actual video clips.

The key thing to understand is that a .bdm holds no video at all. The footage lives elsewhere, as MTS or M2TS clips inside a STREAM subfolder. The .bdm files are pointers and metadata that video software reads to reassemble those clips in the right order. That is why you should never delete them before importing your footage, and why copying a single .mts out of the folder can break an import.

Where BDM sits in the AVCHD folder tree

An AVCHD recording is a directory structure with a top-level AVCHD or BDMV folder. The index and management files sit near the top, and the heavy video data sits in STREAM. A typical layout looks like this:

AVCHD/  (or BDMV/)
 └─ BDMV/
     ├─ INDEX.BDM        <- index of the recordings in this directory
     ├─ MOVIEOBJ.BDM     <- movie/playback objects for those clips
     ├─ PLAYLIST/
     │    └─ *.MPL        playlists (ordering / edit lists)
     ├─ CLIPINF/
     │    └─ *.CPI        clip information (timing, stream maps)
     └─ STREAM/
          └─ 00000.MTS    the ACTUAL video (H.264 in MPEG-2 TS)

On Blu-ray the equivalent files use the .bdmv, .mpls and .clpi extensions; AVCHD uses the DOS-friendly 8.3 forms .bdm, .mpl and .cpi so they fit the older FAT naming that camcorder cards used. The whole tree is meant to move as a unit: an editor imports the folder, reads the index, and reconstructs each recording from the clips it points to.

INDEX.BDM versus MOVIEOBJ.BDM

Two distinct .bdm files appear in every AVCHD folder, and they are told apart by their eight-byte header at offset 0. INDEX.BDM begins with the ASCII tag INDX0100 (49 4E 44 58 30 31 30 30). It is the title index: a catalog of the recordings held in the directory, which the camera and playback software use to present the list of clips you shot. MOVIEOBJ.BDM begins with MOBJ0100 (4D 4F 42 4A 30 31 30 30). It holds the movie objects, the playback-program entries that describe the format, location and playback order of the clips.

Both files are binary, typically only half a kilobyte to two kilobytes, and both are direct AVCHD analogues of the Blu-ray index.bdmv and MovieObject.bdmv files. The four version digits after the tag (0100) can vary slightly between camcorder models, which is normal and not a sign of corruption.

Why you import the folder, not the BDM

Because the .bdm, .mpl and .cpi files only describe the video, importing works correctly only when the entire AVCHD tree is present and intact. Software such as Sony Catalyst Browse, DaVinci Resolve, PowerDVD and Roxio read INDEX.BDM to enumerate the recordings, then follow the playlists and clip-info files to stitch the .mts segments (a long recording is often split across several .mts files) back into a single continuous clip. If you copied only the STREAM folder, or renamed clips, the index no longer matches and the import fails.

The practical rule when offloading a card is to copy the top-level AVCHD/BDMV folder whole, keeping its subfolders and file names exactly, and only archive or delete it after your footage has been imported and exported to an editable format. To play a single clip immediately without importing, open the .mts file in a player that reads MPEG-2 transport streams such as VLC; the .bdm itself will not open in a media player because it is an index, not a stream.

A note on other BDM uses

The .bdm extension has a few unrelated meanings. PTC Pro/ENGINEER (now Creo) has used .bdm for a bill-of-materials drawing file, and a small number of legacy applications use it for their own binary settings. These are rare compared with the AVCHD camcorder use, and they have completely different internal structures. You can tell which one you have by reading the first eight bytes: an AVCHD index starts with INDX0100 or MOBJ0100, while a CAD or app-specific .bdm will not.

References