What does “1080MBAFF” mean?
For fans of The Walking Dead checking out their favourite scene pre database, you may have seen an oddly tagged release of the season 4 premiere from a group named BWB.
While this is the first scene release tagged with MBAFF, there have already been several P2P releases tagged with it. Why not tag it as 1080i or 1080p, but rather 1080MBAFF? As you may already know, the i in 480i/576i/1080i stands for interlaced. As defined by Wikipedia, interlaced video is a technique of doubling the perceived frame rate introduced with the signal without consuming extra bandwidth. Interlaced channels are typically 29.97fps (NTSC) or 25fps. (PAL) All television channels with the exception of 720p channels are broadcasted as interlaced. The p in 720p stands for progressive, which means the video does not have the interlacing lines throughout. The frame rate is doubled, however in most cases the frames are doubled, meaning every other frame is a duplicate rather than an interlaced frame. For NTSC that’d mean they’re 59.96 fps and 50fps for PAL. Click the image below to clearly see what an interlaced frame looks like. Any progressive material will not have such frames. All scene releases are deinterlaced, meaning the interlaced frames are ironed out to make the full video progressive.
In the case of 1080MBAFF, however, it’s a combination of both interlaced and progressive video. This means BWB did not reencode their video, as they mentioned in their NFO of the release.
Pointless to re-encode such grainy source.
Macroblock-adaptive frame-field (MBAFF) coding, using
a macroblock pair structure for pictures coded as frames,
allowing 16×16 macroblocks in field mode (compared with
MPEG-2, where field mode processing in a picture that
is coded as a frame results in the processing of 16×8
MBAFF allows a single frame to be encoded partly progressive and partly interlaced. AfterDawn has a little more information on this:
MBAFF, or Macroblock-Adaptive Frame/Field Coding, is a video encoding feature of MPEG-4 AVC that allows a single frame to be encoded partly progressive and partly interlaced. Maintaining the quality of interlaced video can be a challenge in video encoding because of the larger spaces between horizontal lines in the same field. MBAFF allows an AVC encoder to examine each block in a frame to look for similarities between interlaced fields. When there is no motion the fields will tend to be very similar, resulting in better quality if you encode the block as progressive video. For blocks where there is motion from one field to another the quality is more likely to suffer if encoded progressive, so these blocks can remain interlaced.
In addition to quality improvements, progressive frames require fewer bits making them more compressible for the same quality as interlaced frames. This also applies to individual macroblocs, meaning that by using MBAFF you may greatly improve quality for a given bitrate. Since all the decisions required for MBAFF are made by the encoder it can slow encoding speeds greatly. Being a relatively new technology it’s also not supported well among AVC playback software. Unlike many of AVC’s advance features, though, MBAFF doesn’t affect decoder speed.
Currently there are no rules disallowing 1080p or 1080MBAFF releases in the scene so such releases cannot be nuked.