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Old 20th August 2011, 03:25   #1  |  Link
lovelove
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difference between subtitles and closed captions

Quote:
Originally Posted by Dimmer View Post
[...] closed captioning! ... often it's a priceless resource of information, much more then just subtitles with dialog. I've been using your tools for several months to make DVDs out of TV captures, and the possibility to keep the CC still amazes me. Nowadays anybody could make a DVD with subtitles, but it's a whole new level of authoring to have your DVD closed-captioned.
can someone please explain this to me? I have been reading wikipedia and forum threads for quite a while now, but I still don't really understand the issue. I was told elsewhere that CC are often used as subtitles for the hearing impaired. But why can't normal subtitles be used for them? I have not yet seen CC on any of my DVDs, only normal subtitles it seems. But this might be because I don't know how to spot/detect them. I was told that MediaInfo identifies subtitles as RLE encoded text streams, whereas CC are also text streams in EIA-608 format. DVD subtitles are bitmaps and need OCR ... what about CC? Are they bitmaps too? Or do they come as text streams right away?

Why the need for two different formats (subtitles/CC) anyway? You see, I'm quite lost. And the comment quoted above only contributes to that. Why is CC "much more then just subtitles with dialog" ? And while "anybody could make a DVD with subtitles", why is it "a whole new level of authoring to have your DVD closed-captioned" ?

Many thanks if you can clarify this a little bit for me and guide me into the right direction. Thank you.


Last edited by lovelove; 20th August 2011 at 03:27.
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Old 21st August 2011, 12:25   #2  |  Link
Blue_MiSfit
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To rant for a moment, closed captions are dreadful. They are a huge pain in the ass to deal with, and this is all because of the hard requirement for backwards compatibility.

To be more constructive - closed captions are very different from subtitles for a few reasons.

1) From a high level perspective, captions are usually supposed to be for the hearing impaired, meaning they also contain descriptions of sound effects.

2) Originally, (in NTSC anyway), closed captions were encoded into the vertical blanking interval (VBI) portion of the analog video signal, typically on line 21. This means that the caption data is part of the video signal, and not carried as a separate stream (as is often the case with subtitle formats that we PC folks are familiar with). This caption data is also known as EIA-608, and is basically hex encoded text.

3) Nowadays with digital video compression, the closed caption data (using the same encoding format for backwards compatibility, plus some newer fancy stuff for HDTV a-la CEA-708) is usually encoded into MPEG-2 user data, or as H.264 SEI messages. In this sense, the closed captions are carried as part of the video elementary stream, but are not directly encoded into the actual picture.

4) There are many subtle differences in the ways that caption data (carried in an SCC file for example) can be embedded into a video stream. These tiny differences produce wildly different results depending on what hardware and software you use to decode captions. VLC for example can usually decode closed captions from MPEG-2 or H.264. Not always, though. Sometimes you get nothing, sometimes you get duplicated characters, sometimes you get a garbled mess. There are some professional products that can reliably decode captions and simulate a set top box, but they are usually quite expensive and are often MUCH more than a simple media player.

5) Sometimes a larger than usual image (720x486 or 720x512) can be encoded containing all of the VBI including caption data lines. This isn't seen very often outside of professional applications where backwards compatibility is necessary

6) To make things even more complex, consider this. 608 captions can be upconverted into 708 captions via two methods: simple upconversion and true upconversion. The former really just stuffs 608 data into the 708 format, and the original data can be extracted without issues. The latter actually translates the data into real 708 data, so it cannot be translated back.

7) When you encode video for broadcast or traditional cable / satellite / telco VOD platforms. captions are usually mandatory. There are many standards which specify how the captions are to be encoded (CableLabs etc). You usually end up with all 3 possible types of caption data in the video file (real 608, fake 708, and real 708).

8) Again, captions suck.

I won't even go into the issues with caption transport over HDMI versus HD-SDI VANC data, or the SMTPE 436M standard that defines carrying this VANC data in an MXF file, or the INSANE logistical problems associated with inserting captions into live video streams in cases where you receive the captions long after you've encoded the video.

Stay away. Don't deal with captions unless you have to

Derek
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Last edited by Guest; 21st August 2011 at 12:59. Reason: rule 4
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Old 21st August 2011, 12:30   #3  |  Link
Blue_MiSfit
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Also, read this whole frustratingly designed but wonderfully informative website:

http://www.cpcweb.com/faq/what_is_cc.htm

In my book, these guys are basically the definitive word on closed captions and their associated workflows in the pro/broadcast space.
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Old 23rd August 2011, 16:12   #4  |  Link
lovelove
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many for this knowledgeable insight


note: I also found the answer to whether CCs are text or images:

Quote:
Originally Posted by http://ccextractor.sourceforge.net/

- DVD subtitles are image based (they could be anything, not just text) while Closed Captions are text based (they can only be text).

Last edited by lovelove; 23rd August 2011 at 16:25.
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Old 29th August 2011, 05:20   #5  |  Link
Blue_MiSfit
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No problem, and this is correct. Captions are hex encoded text values. This is defined by the 608 specification.

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