Bluray subtitles timing precision

[mp3dom]

I'm asking this to all who author blurays. From the specs, the subtitles should have a frame-precise playback, but this doesn't seems to be the case in practical terms. I saw this a lot of times, and especially with video at 23.98/24fps (29.97 seems to be more precise). If I put a subtitle to start at a scene change (for example) I see that during playback the subtitle can appear randomly earlier or afterwards that scene change (about 2-3 frames). The fancy thing is that on the same player the same subtitle can appear in a different way (sometimes earlier and sometimes afterwards) without a real cause.
This happens not only with software players (like TotalMedia or PowerDVD) but also on standalone (tried on Samsung, LG, Panasonic and other brands... and all do the same, they're not frame-precise)
Generally it's not a problem for dialogues subtitles (it's not a 1-2 frames that made the difference) but this is a real pain and problem when subtitles are used for soft-postproduction (for example signs, or to mask and re-design a title). In this case it is very ugly to sometimes see the original title being covered by the subtitle or see a 'patch' over the movie for a very short period of time.
Am I missing something?

[Ghitulescu]

AFAIK the subtitles are timed, not frame-linked.

[mp3dom]

Ok, but If a scenechange is at (example) 00:02:30:00 (hh:mm:ss:ff,@23.98) and I put at that timecode a subtitle, I expect that at that timecode the subtitle constantly appear on screen. In practical terms this is not true. Sometimes it appears at 00:02:29:22 and sometimes at 00:02:30:03 without a real cause, even on the same player! And almost all players seems to works in the same way. For this needs (precise subtitle display) it is more precise the dvd format which is absurd.

[Ghitulescu]

I am not the right person in this matter but I would guesstimate that the HW of the player takes the display time code and then fits the subtitle in the most close (or probably the next) frame to be displayed. The subtitles cannot be displayed other than in the context of a frame.
Therefore I speculate that the fps conversion would play a role - for instance when the TV accepts only 60Hz (most older LCDs) but the movie is 24/1.001.

[moffy]

I was reading the DVB subtitle specification and there was a clause that dealt with late delivery of subtitles because the hardware designers hadn't allowed enough bandwidth in the subtitle decoding path. Sounds like it is a known issue.

[mp3dom]

Interesting, moffy. Thank you for pointing this out. So it seems no workaround exists to avoid or fix this problem. Too bad.

[Ghitulescu]

And what has this to do with the blu-ray?

DVB may have timing problems, as its packets arrive via unreliable channels (ether) and reception problems may cause that packets are lost.

BD does not have this problem. It doesn't loose packets (only when the medium is defective), and the bitrate is high enough.

Also, the HW problem of insufficient resources is not an issue for BD. DVB-receivers belong to the category of cheap appliances (some are given "for free" with an abo), therefore cheap DSPs are built in. BD players were high end machines by the times the BD specs began to be laid down. When you impose a new technique, you ensure this doesn't have "stupid" problems or you'll face bankruptcy. High end DSPs (like RealTek or SigmaDesigns are also found in premium SAT-receivers).

[moffy]

Not all bluray players are of similar quality. The specs of newer machines e.g. processor/speed and RAM(1GB) for the newer Java Virtual machine are much higher than older ones. It is also possible for engineers to make what seems to them acceptable compromises. The delay in subtitles is not related to terrestrial propagation but internal decoder bandwidth limitations. The advice was that if a subtitle is decoded late that it is better to display it late than skip it, as you might for a video frame.
Also how early a subtitle appears in the stream depends on the multiplexer. This could impact the decoder especially if they are appearing at the decoder just before display.

[Ghitulescu]

The point was that the BD specs allowed for some twice the bandwidth any DVB-S2 stream may have - therefore the BD format should not have this issue.

Indeed it's the job of the multiplexer to arrange the additional streams according to their PTC/DTC. I think that any BD compliant multiplexer knows how to do this (and equally does it).

I still think it's a frame conversion issue done internally in the player.

[moffy]

You make a good point.