Need information on TV subtitle/captioning

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cornishandy
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Need information on TV subtitle/captioning

Post by cornishandy » Fri Jan 20, 2017 2:47 am

Hello all, as stated I am looking for information about subtitles and captions on TV. I'm a volunteer with a group of deaf people who rely 100% on subtitles or captions for everything they watch. It's a really great invention and a big help to all deaf people world wide.

There are some problems. Sometimes the captions are late. That is to say they arrive after the person has spoken. This is really difficult because you are now trying to follow separate sound and vision! Sometimes the captions are garbled, you see nonsense words words like fwrpft. Sometimes the captions stick onscreen and don't change for the whole programme. This can be fixed by turning them off and back on again. Other times there is a long pause in the subtitles and then they suddenly race by too fast to read. There are sometimes whole gaps in the subtitle stream. It does look to me as if the processor in the TV is responsible for at least some of this. The rest might be a poor signal or interference, I don't know.

What I am trying to do here is to find out what causes these things. I have been in touch with several TV broadcasters and they all assure me that their signal leaves the building in perfect condition. It's clear from talking to the deaf groups that it isn't arriving in the same condition on their screens and that is what I am trying to investigate. The problem here is that it is very hard to find out anything about how subtitling is actually processed in the TV set. Nobody seems to know! There must be specialised circuitry and perhaps a purpose designed chip. Perhaps all TV's use the same chip, or do the manufrs all invent their own? Are there "good" chips and ones to avoid?

Basically I am trying to find the cause of the subtitle glitches and from there work out how to fix them but I am very short of information. I am a computer engineer not a TV engineer although I have some electronics background. So I'm appealing to anyone with the required knowledge, can you help? you would be helping an awful lot of people to move forward because subtitle faults are a complete PITA for deaf people.

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PID_Stop
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Re: Need information on TV subtitle/captioning

Post by PID_Stop » Fri Jan 20, 2017 12:28 pm

There are basically four ways captioning happens, and each has its practical effect on caption quality:

First, scripted pre-produced programming (e.g., what you watch on the networks and syndicated programs) have captions applied during post-production. This is the ideal case, since the exact text can be entered and formatted to fit the picture -- including positioning the text block according to who is speaking, or to avoid blocking relevant picture content. It also allows the captions to be synchronized with the audio.

Second, scripted live programming like local news broadcasts often take data from the prompter computer to drive the caption encoder. This can have a high degree of accuracy since the captions derive from the script, and the timing can be pretty close to the audio if the prompter operator is smooth. Naturally, this cannot account for ad-libbed content, and in cases where a piece of the script is skipped for whatever reason, you can see a sudden flurry of text flying by.

Third, many live programs have a live captioner who listens to program audio and operates a stenotype-like machine, similar to a court reporter. The accuracy of the captions is lower because these sorts of systems typically work on the basis of how the words sound, and can be fooled by similar words. Names are especially problematic. On the upside, manned live captioning can handle spontaneous situations. The biggest typical limitation behind this is that even with a really skilled operator, the captions will be significantly behind the audio simply because the operator has to hear what is being said, interpret it to the stenotype machine, and the computer it's driving has to construct actual text. I think there might be an short intentional delay to give the operator a chance to veto the computer if it misinterprets the text in a particularly inappropriate way.

Fourth, there are voice recognition systems (ENCO makes one I've seen) that drive caption encoders directly. In very simple cases they can be driven directly from program audio, but more often it's necessary to have someone repeating the air audio into a mic strictly to feed the voice recognition. These systems do very poorly when music or other extraneous sounds are in the mix, and they also don't handle cases where people talk over each other. The accuracy of these systems is variable, and they can only follow what they hear... so some delay is inherent here.

On the technical end, there are three standards by which captions have been encoded in the United States: analog standard definition systems put it on line 21 of the video, and you could actually see it flashing just above the top of the active picture. Digital video has a special space for ancillary data like captions; standard definition uses EIA-608 protocol, which is comparable in capability to the old line 21 encoding; HD video typically uses EIA-708, which provides quite a bit more flexibility in how the captions are displayed. From the end user's point of view, the technical standard makes little or no practical difference: each is theoretically capable of being completely accurate, and providing captions that are simultaneous with the audio. Differences between decoder/display chipsets within the television have little or no effect on system performance. Except for corruption that might result from poor signal conditions, the limiting factors to accuracy and simultaneity lie entirely in how the text gets entered.

Regards,
Jeff

cornishandy
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Re: Need information on TV subtitle/captioning

Post by cornishandy » Wed Jan 25, 2017 1:03 am

Thank you for the information. The quest continues.....

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PID_Stop
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Re: Need information on TV subtitle/captioning

Post by PID_Stop » Wed Jan 25, 2017 7:50 am

cornishandy wrote:
Wed Jan 25, 2017 1:03 am
The quest continues.....
As it does for those of us at the station end. Not sure what can possibly fix the inherent issues with live captioning, short of hiring psychics with impeccable typing skills who know what's going to be said ten seconds into the future. :wink:

Jeff

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Re: Need information on TV subtitle/captioning

Post by KPJL FM » Wed Jan 25, 2017 8:07 am

PID_Stop wrote:
Wed Jan 25, 2017 7:50 am
As it does for those of us at the station end. Not sure what can possibly fix the inherent issues with live captioning, short of hiring psychics with impeccable typing skills who know what's going to be said ten seconds into the future. :wink:

Jeff
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