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Old 29th July 2014, 17:43   #1  |  Link
Deftoner
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Bitrate of Subtitle Streams

Hi guys!

I am tryig to author a DVD with 24 Subtitle Streams in Scenarist 3.2.
Shouldnt be a problem i thought because as far as i know there are 32 Subtitle Streams possible and each should have a bitrate of about 0.04kbps. Is that correct?

The problem is when i add the subtitle streams to my audio/video streams with an overall bitrate of about 7mbps, scenarist creates an error "The total Bitrate is too high". Even deleting audio and reducing the bitrate to 6mbps doesnt help.
When i delete all but four subpicture-streams it works again. So i guess the bitrate of the subtitle streams is way higher than expected (maybe around 1mbps!).

Any ideas, what i did wrong or how i can create subtitle streams with a lower bitrate?
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Old 29th July 2014, 19:12   #2  |  Link
TheSkiller
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Quote:
Originally Posted by Deftoner View Post
and each should have a bitrate of about 0.04kbps. Is that correct?
No, subtitle streams do not have a fixed/constant bitrate; for sections where there's no subtitle in any of the streams the bitrate of all subtitle streams together is 0, and whenever a new subtitle appears in any of the streams, the size of it's RLE compressed bitmap adds to the total bitrate of video + audio (+ overall mux overhead). If the total bitrate (inculding subtitles and mux overhead) exceeds 10.08 MBit/s even once, the authoring fails.
With as much as 24 subtitle streams (and probably all sharing very similar subtitle timing) there might be quite some sharp bitrate spikes in some places... However, I think the problem lies in the video stream's maximum bitrate, see below.

Quote:
Originally Posted by Deftoner View Post
The problem is when i add the subtitle streams to my audio/video streams with an overall bitrate of about 7mbps
Overall (average) bitrate does not count; it's the maximum bitrate that counts here.

Try the following: use Muxman and add your video and all audio streams (no subtitles), and let it mux a dvd. Then post Muxman's log here. It will tell us the maximum bitrate of video + audio + mux overhead so we can see how much spare bitrate there is left for the subtitles.

Last edited by TheSkiller; 29th July 2014 at 20:18.
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Old 30th July 2014, 09:56   #3  |  Link
Deftoner
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Thanks for the reply but with "overall bitrate" i didn't mean "average bitrate" but "combined bitrate of video and audio streams". I used a constant bitrate of 6,4mbps for the encoding.
This bitrate is also shown in the scenarist data editor, so it seems to be correct.
Without any audio streams there is a maximum of two subpicture streams i can add before i get the error.
So one subpicture stream seams to have a maximum bitrate of more than 1mbps. This can't be correct!?

Am i doin something wrong in the sst-header?
st_format 2
SubTitle Joja_ALB_subs
Display_Start NON_FORCED
TV_Type PAL
Tape_Type NON_DROP
Base_Time 00:00:00:00
Pixel_Area (2 574)
Display_Area (0 2 719 574)
Color (1 3 3 4)
Contrast (0 0 15 15)
BG (255 255 255 = = = )
PA (0 0 0 = = = )
E1 ( 255 0 0 = = = )
E2 ( 0 0 255 = = = )

Do i have any influence on the encoding/muxing of the subpictures in scenarist?
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Old 30th July 2014, 10:53   #4  |  Link
Deftoner
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I did some more tests:
When i create the subtitles within the track editor of scenarist, i can mux more than 24 Subtitle streams on the same video track i tried it before.
That doesnt help much, because it would take years to create all the subtitles manually, but i think it shows that the problem must have something to do with the subtitle script that i import or the tif-files that my customer delivered to me.
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Old 30th July 2014, 13:56   #5  |  Link
Deftoner
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Ok i thinkt i found the issue:
The Subpicture Stream startet directly at the beginning of the Video. The Scenarist hardware simulation seems to have a problem to start directly with multiple subpictures.
I did another encoding with one second of black before the program and now it works.
When i try to start the simulation at a later part of the track that contains subtitles, the simulation doesn't work because of the bitrate problem again.
Thanks for listening
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Old 30th July 2014, 19:50   #6  |  Link
TheSkiller
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You're welcome. Good to know that having the first subpicture start at the video time code of zero is not recommendable.
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