For Engineers: Troubleshooting Digital Captioning Problems Beyond the TV

[dmulvany]

The purpose of this thread is to facilitate the resolution of technical problems with CEA-708 and CEA-608 captions on digital television programs caused by problems with equipment under the control of television networks, stations or other video programming distributors. As such, this thread is intended to be a resource to assist engineers in troubleshooting these problems and in otherwise improving the provision of 608 and 708 captions on digital programming.

An example of such a known problem is missing and/or garbled captioning problems caused by an untouched Evertz 608 to 708 captioning bridge (which needs to be resolved by installing the latest firmware update).

This thread was created because there appears to be no other centralized resource to help engineers identify the cause of and solutions for digital captioning problems. Furthermore, if more information could be shared here about captioning-related equipment that *is* working well at delivering captions intact to all digital television equipment, as well as equipment that is not, the hope is that this information would help engineers find solutions to their captioning problems much more quickly.

This first posting will be updated over time to organize information that becomes available in the thread. Feel free to post links to online resources that would apply to this thread.

Here are some informative links explaining some of the technical requirements for closed captions for digital programming:


Implementing Closed Captioning for DTV, by Graham Jones, National Association of Broadcasters
http://cn.evertz.com/resources/cc-imp-paper.pdf

Delivering Captions in DTV (both 608 and 708 captions must be broadcast for digital programs)
http://ncam.wgbh.org/dtv/toolkit/gen...tvcapbrief.pdf

FCC DTV Order (technical standards for display of closed captioning on digital television receivers)
http://www.fcc.gov/Bureaus/Engineeri...0/fcc00259.doc



Note to consumers:

Consumers experiencing problems with captions for a specific station generally should notify that particular station and/or their own pay TV service provider about those problems and are encouraged to request from the station and then share technical information here about the cause of those problems. To keep this thread highly useful for engineers, however, problems with captions from end-user equipment, such as televisions, converter boxes, or other equipment not supplied by a pay TV service provider should not be discussed here but on more relevant threads on the AVS Forum such as:


Status of Captions for Digital Channels

Evaluating Digital to Analog Converter Boxes for Users of Captioning

CC Problems

New and more specific threads could also be developed to address consumer concerns.


To report problems about captioning from a specific station, guidance is available at:

http://www.nvrc.org/content.aspx?pag...on=5#Complaint

[dmulvany]

Here's a question for local television station engineers to ponder: at what point, if any, might weak signal strength cause problems with captioning and not the picture quality of the program?

I've noticed that changing the antenna position made a significant difference in the appearance of captions from the local PBS affiliate even though the picture quality was very good throughout. Many deaf and hard of hearing people thus may not realize that signal strength could still be an issue even if the picture quality is good. (If there are audio problems that generally signal hearing people to improve the signal, hard of hearing people may not pick up on them.)

However, I haven't seen such sensitivity problems with captioning on other stations. I'm wondering if there's an interaction with the captioning equipment the affiliate is using combined with the signal strength.

Something to check out, perhaps, is what's the minimum level of signal strength necessary in order to get intact captions and to see if this varies among different stations in the area. In other words, is the captioning transmission somehow more robust from one station than from another at the same weak signal level? Or is there a common range of signal strength at which captioning will start becoming intermittent and changing from 708 to 608 styles and then disappear altogether while the picture is still being shown?

Is this already known? If so, please let us know where this is documented.

I imagine also that some DTV receivers could be more sensitive to signal strength issues with captioning than other ones. My Sharp HDTV, which is less than 2 years old, does not perform as well as digital-to-analog converter boxes in picking up captions from some local stations, although it picks up captions reliably from the NBC and CBS affiliates. The question remains, however, why the captioning is so consistently transmitted and received from these affiliates and not from others.

Dana

[dmulvany]

Over the phone today with an engineer at a local PBS affiliate, I learned that a probable cause of occasional glitches with digital captions had been identified. I had noticed on two separate PBS affiliates, WETA and MPT, that 708 captions occasionally were disappearing for some seconds and then flitting across the screen extremely quickly (too quickly to be read). This appears to be caused by a mismatch with the frame rate selected by the station for the captions and the actual frame rate used by the station. There are at least two different frame rates available---30 fps or 24 fps. Curiously, it takes quite a while for the problem to show up after the beginning of the program.

I very infrequently see digital captions for syndicated episodes of "Stargate Atlantis" from Fox affiliates, but they don't persist. (Another person in the area, afiggat, has confirmed that he can't get digital captions for these programs from two local Fox affiliates.) There'll be line fragments of digital captions once in a great while but no complete sentences. I wonder now if there's a timing problem with the encoder. Perhaps the digital captions are being sent out but they can't match up properly.

[Konrad2]

> at what point, if any, might weak signal strength cause
> problems with captioning and not the picture quality of
> the program?

With ATSC/8VSB, the captioning information is digital,
and is the same as all other info in the RF signal. No
more or less likely to get corrupted than any other
info.

> Something to check out, perhaps, is what's the minimum
> level of signal strength necessary in order to get intact
> captions

My notes say that ATSC/8VSB needs 15.5 dB signal to noise
in theory. In practice you need more.

Note that signal strength (signal to noise ratio) is only
part of the reception equation. Multipath and interference
are common problems.

In analog (NTSC), a signal strength (signal to noise ratio)
problem shows up as "snow". Multipath causes "ghosts".
Most other problems are some form of interference.

> I've noticed that changing the antenna position made a
> significant difference in the appearance of captions
> from the local PBS affiliate even though the picture
> quality was very good throughout.

Any error in captioning data is likely to be very noticeable.
A small error in the picture might not be noticed.

You may have found a useful technique for aiming antennas
for the least errors.

> I imagine also that some DTV receivers could be more
> sensitive to signal strength issues with captioning than
> other ones.

Differences in signal strength sensitivity are probably minimal.

Newer generations of demodulators are better at handling
multipath.

[dmulvany]

Here's some guidance about the Evertz HD9084 caption encoder from:

http://www.cpcweb.com/forums/index.php?showtopic=185

"If you have difficulty getting captions out with Evertz HD9084 encoder, choose EEG 470 as caption device."

[dmulvany]

Check out one kind of CEA-708 captioning problem associated with how some local stations are transmitting CEA-708 caption data over digital channels:

http://picasaweb.google.com/dana.mulvany/WTTGAndWDCA#

I've uploaded to this album a video of gradually accumulating CEA-708 captions, which was taken last month (January 2009), and also pictures of accumulated captions filling up much of the screen---taken today, February 14th.

Another station, WJLA, is also showing the same kind of accumulating caption problem for "Lost" and some other prime-time programs---just a few days ago. So WJLA is having problems processing caption data from nationally transmitted programming, while WDCA and WTTG are having problems processing caption data for locally transmitted programming, but the end result is the same.

The same kind of problem has been noted by other owners of Samsung DTV equipment, one of which lives in Rome, Georgia, for captions from some but not all stations.

A cable subscriber, though, has said that her Scientific Atlantic set-top box from Time Warner will also show accumulating captions like this as well (though I haven't seen pictures of them).

I'm wondering if Samsung and Scientific Atlanta might use some of the same components, like the same caption decoder chip. My Sharp HDTV and other people's Sony HDTVs simply won't show any captions from programs for which the Samsung has the above trouble decoding captions, so I believe there's a general problem of the caption data being corrupted somehow.

At least two stations in the DC area are causing this kind of problem within the past week.

For what it's worth, I did find that the CEA-608 caption data was coming through fine. It seems it's only the CEA-708 caption data that is getting converted into accumulating captions this way. (Thus people who aren't decoding CEA-708 data may not see this problem.)

My Samsung HDTV equipment is able to decode CEA-708 caption data from other stations just fine, like WRC (NBC) and WETA (PBS). It's only having a problem with the CEA-708 caption data that's broadcast by some stations, and my Sharp HDTV tends to have captioning problems as well from those same stations. (In addition, my Insignia converter box tends to crash only when tuned to those same stations, so I think the stations with problematic captioning is sending out corrupted data that is causing problems for a variety of equipment.) The stations don't know what they're doing wrong, though, and since I'm not an engineer, I don't know exactly what's wrong either.


Dana

[dmulvany]

FYI, the FCC held a meeting of a working group on May 18th, 2009, which discussed digital captioning problems. I myself spoke at that meeting about problems with captioning from local broadcast stations.

Some stations had been unable to resolve digital captioning problems after more than six months. After extensive troubleshooting, it appears that iIn some cases, the MPEG encoder may be the cause of the captioning problem, not the caption encoder, though I don't yet have enough details about the exact nature of the problem.

There will hopefully be a centralized database of problems and solutions about digital captioning problems so that it *doesn't* take more than six months to fix the problems, but in the meantime, it would be really helpful if engineers could share details about problems and solutions on this thread.