[mythtv-users] ATSC caption question

Jim Stichnoth stichnot at gmail.com
Tue Dec 22 21:30:08 UTC 2009

On Tue, Dec 22, 2009 at 10:10 AM, Yeechang Lee <ylee at pobox.com> wrote:
> I've been testing Taylor Ralph's patches to EIA-708 support (#7739,
> #7775, and #7776). As he says, they do indeed finally make ATSC
> captions usable. A few questions/observations:
> * In what situations would using ATSC captions over VBI (which I'd
>  used until these patches) be preferable, if both are available? I
>  know ATSC supports color foregrounds and backgrounds, and multiple
>  typefaces, but are such feature ever actually seen in the real
>  world?
>  On the other hand, during the past couple of days using Taylor's
>  work I've found a few situations in which VBI captions are
>  preferable to ATSC:
>  * A PBS program with almost-completely corrupt ATSC but intact VBI
>    captions.
>  * A movie in which, during dialogue, VBI captions' positions vary to
>    indicate who is saying what but ATSC captions don't. VBI captions
>    sync with dialogue more accurately, while ATSC captions tend to
>    appear/disappear much more quickly at the end of the lines they
>    depict.
> * I enable black backgrounds for captions for visibility. VBI
>  captions' backgrounds cover the letters and no more. ATSC captions
>  apparently reserve a fixed amount of screen space and so their
>  backgrounds cover more than just the words on the screen at a given
>  moment.
> * VBI captions depict musical notes properly while ATSC captions show
>  square boxes (an issue fixed with VBI captions some time ago). I use
>  the same fonts for both so it can't be a font issue.

I also recently tried out the patch in #7775 and had many of the same

* Out of the box, ATSC captions are less readable than VBI captions,
but that's just a (fixable) combination of the font size and the

* I also see the issue where ATSC captions tend to show up late and
stay on for only a short time before they disappear, in comparison to
their VBI counterparts.

* In the one PBS program I tried, it seemed that only about half the
ATSC captions would even show up.

* With ATSC captions, a couple of programs crashed the frontend, with
no suspicious messages in the log.  These programs had both been
losslessly transcoded to remove commercials, which may have something
to do with it.  (I have seen problems where mythtranscode sometimes
corrupts the VBI caption timestamps, which then manifests as "stuck"
captions since the bogus timestamp is far in the future.)


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