I get that the thread started off being about .ts files (as per the title), and has generalized to FF/RW in any container.
I have a Gen1 WDTV HD. I don’t have a Live or a Live Plus so I have no experience in them, other than what comments I see others make.
But I still believe my .mkv comment about chaptering is valid, as it indicates the issues of GOP length. Yes, when I try to jump to a chapter in an .mkv, it’s never more than 10 seconds off, but it’s still up to ten seconds off because the stream has a huge GOP. The same with VLC. If I use the slider to fast-forward or reverse, depending on the file, it can take up to 10 seconds of playback before I get a “true” picture without garbled pixels. These issues are because of the GOP. And the large GOP that causes chaptering inaccuracies, also causes FF/REV inaccuracies.
Yes, GOP lengths of 12-15 frames are somewhat standard on a purchased DVD or BluRay (which gives you about a half-second of accuracy). But it’s possible to have an _ entire video stream _ be a single GOP. There’s nothing in the standards that says you can’t do it that way. The stream would play fine when you start it from the beginning, but you would _ never _ be able to get a mid-stream picture generated. You would have a tiny file, that would play fine from the start, but couldn’t be searched or indexed.
I’m going to go out on a limb here and say that the my .mkv files that only end up with 10-seconds of “accuracy” or so, do not have 2 or 3 I-frames per second (GOP length of 12) but only have one I-frame per 10 seconds (GOP length of around 300).
There are many encoders that have used GOP lengths of above 300 (and, as I said, even entire streams as a single GOP). Your average of 15 or so, is for DVD compliance… it’s not a standard of GOP lengths used in other video streams, or even of MPEG streams themselves. Would a GOP of 18000 not be ten minutes?
Streaming over the internet, like YouTube, you want a very small GOP size, so that frames can be dropped if the bandwidth is too low, without losing the picture – often it’s only 2 or 3. Playing from media like a DVD or a BluRay, you want a compromise between file size and frame-accuracy – you don’t want 10-minute jumps in FF/REV – so they’ve selected their standards.
But, for someone designing an encoder that they are intending for people to use to make small rips, played on a PC from a hard drive, there is no real reason for them to stick to a small GOP, or a DVD-compliant GOP. I mean, think of how much room the actual chaptering information takes up… it’s almost nothing. Now do a quick Google for pirated rips of BluRay discs… how many have retained the chapter structure? Almost none. Just about every byte counts, or so it seems. How do you think you can get a 1080p movie down to 2 GB? Sure, the 15GB files you see may still have a small GOP, but the GOP on the 2GB ones can’t possibly be 12 or 15 or 18.
So, since no one has come out and shown a GOP size of 12 on a file with an apparent “accuracy” of 5 minutes, it’s only natural to assume they’ve been encoded with huge GOPs for tiny file sizes. The same scene groups who think people don’t want chapters, apparently also think people don’t want frame-accuracy for FF/REV, but would rather have small file sizes.
As I said, my Gen1 has no problem with FF or Reverse or chaptering when I play a DVD .iso I’ve created, but when I started changing them to .mkv files because drive space was becoming a concern, my files lost accuracy – both when played on my WDTV and on my PC. This tells me that handbrake does _ not _ use a GOP of 12-18. If it was just the WDTV, then I could question Western Digital, but since the same .mkv file has the same “error” (although exhibited somewhat differently) when played through VLC, it’s obviously in the encoding of the file.
Again, it’s not a format issue. One .mkv can FF/REW well with the slider in VLC, and another one can need a while to “fix” the picture. It depends on what program did the encoding, and what GOP length was used – they _ don’t _ all use 12-18; not even close.
And, as far as FF/REV previewing goes, just because the decoder keeps a rough track of the B- and P- frames and generates a rough approximation of what the evolving video probably looks like, doesn’t mean it has the full frame information for the playback frame. Again, I refer back to VLC. While I’m actually scrolling with the slider bar to FF or REV, the picture is intact. It only garbles when playback re-starts.
(And for the record, the only programs that I’ve noticed a selectable GOP length or GOP pattern, are DVD authoring programs, which need to ensure DVD compliance.)