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20th August 2011, 03:25 | #1 | Link | |
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difference between subtitles and closed captions
Quote:
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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21st August 2011, 12:25 | #2 | Link |
Derek Prestegard IRL
Join Date: Nov 2003
Location: Los Angeles
Posts: 5,988
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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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These are all my personal statements, not those of my employer :) Last edited by Guest; 21st August 2011 at 12:59. Reason: rule 4 |
21st August 2011, 12:30 | #3 | Link |
Derek Prestegard IRL
Join Date: Nov 2003
Location: Los Angeles
Posts: 5,988
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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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23rd August 2011, 16:12 | #4 | Link | |
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many for this knowledgeable insight
note: I also found the answer to whether CCs are text or images: Quote:
Last edited by lovelove; 23rd August 2011 at 16:25. |
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29th August 2011, 05:20 | #5 | Link |
Derek Prestegard IRL
Join Date: Nov 2003
Location: Los Angeles
Posts: 5,988
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No problem, and this is correct. Captions are hex encoded text values. This is defined by the 608 specification.
Derek
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