What's wrong with ABC's 'Lost' broadcasts?
Lot of people asked what's up with the new 'Lost' episodes lately, why don't they show up on the net in a timely fashion? Not so much a problem for the SD/XviD crowd as they and their encoders don't seem to care, but for the people interested in HD/x264 content.
Well, the problem is that ABC used some kind of alternating frame rate in their broadcast for the whole season to squeeze in more time for advertisements. This is not so much a problem in itself, as the viewer subscribing to the broadcast won't really notice if he doesn't keep track of advertisement time. But it is a problem for distribution on the net. Here's why:
Source material for most movies and series nowadays is 24 FPS. That gets pulled down 3:2 for TV to broadcast the video in 60 Hz, that means 2 frames get broadcasted as 3 frames by interlacing them (Wikipedia has the complete explanation). ABC now cuts away some of those "dupe" frames during some scenes during transmission to save time. This won't get noticed in 60 Hz/30 FPS, as some of these frames exist as dupes anyways. Just speeds up the video a bit.
Now for encoding this for distribution on the internet the pulldown has to be reversed to get 24 FPS again to not waste precious space/bitrate on dupe content (actually it's 24/1.001 FPS, since audio is still slowed down and encoders prefer not to touch it). Unfortunately the programs that do this are quite "dumb" and can't recognize where ABC has meddled with its content.
After application of this "reverse pulldown" the result should be a steady stream of unique progressive pictures (again, Wikipedia knows more about it). Since there are fewer frames aired then there should be the reverse pull down algorithm skips over more frames then he should be in turn, resulting in choppy video. This wouldn't be a problem if ABC somehow manipulated the whole broadcast the same way, stupid reverse pulldown algorithms could be adapted to that. Unfortunately they only use that in some scenes, not all, so it's hard avoid.
SkyHD broadcasts that get released a little later don't suffer from that problem, that's why everyone is seeing the really "good" encodes so late. Just this time one of the big groups seems to have decided not to bother with the bad ones anymore, too. As always smaller groups have jumped in to fill the gap, but this took some time.
Well, the problem is that ABC used some kind of alternating frame rate in their broadcast for the whole season to squeeze in more time for advertisements. This is not so much a problem in itself, as the viewer subscribing to the broadcast won't really notice if he doesn't keep track of advertisement time. But it is a problem for distribution on the net. Here's why:
Source material for most movies and series nowadays is 24 FPS. That gets pulled down 3:2 for TV to broadcast the video in 60 Hz, that means 2 frames get broadcasted as 3 frames by interlacing them (Wikipedia has the complete explanation). ABC now cuts away some of those "dupe" frames during some scenes during transmission to save time. This won't get noticed in 60 Hz/30 FPS, as some of these frames exist as dupes anyways. Just speeds up the video a bit.
Now for encoding this for distribution on the internet the pulldown has to be reversed to get 24 FPS again to not waste precious space/bitrate on dupe content (actually it's 24/1.001 FPS, since audio is still slowed down and encoders prefer not to touch it). Unfortunately the programs that do this are quite "dumb" and can't recognize where ABC has meddled with its content.
After application of this "reverse pulldown" the result should be a steady stream of unique progressive pictures (again, Wikipedia knows more about it). Since there are fewer frames aired then there should be the reverse pull down algorithm skips over more frames then he should be in turn, resulting in choppy video. This wouldn't be a problem if ABC somehow manipulated the whole broadcast the same way, stupid reverse pulldown algorithms could be adapted to that. Unfortunately they only use that in some scenes, not all, so it's hard avoid.
SkyHD broadcasts that get released a little later don't suffer from that problem, that's why everyone is seeing the really "good" encodes so late. Just this time one of the big groups seems to have decided not to bother with the bad ones anymore, too. As always smaller groups have jumped in to fill the gap, but this took some time.