stichnot at gmail
Jan 22, 2012, 8:32 AM
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Re: [mythtv-commits] Ticket #10273: CEA-708 captions are delayed because of incorrect byte-pair handling
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On Sat, Jan 21, 2012 at 6:51 PM, <noreply [at] mythtv> wrote:
> #10273: CEA-708 captions are delayed because of incorrect byte-pair handling
> ----------------------------------+-------------------------
> Reporter: argilo@… | Owner: danielk
> Type: Patch - Bug Fix | Status: new
> Priority: minor | Milestone: unknown
> Component: MythTV - ATSC | Version: Master Head
> Severity: medium | Keywords: cea-708
> Ticket locked: 0 |
> ----------------------------------+-------------------------
> ATSC closed captions (CEA-708) are sometimes delayed due do a bug in the
> way the DTVCC transport layer is packetized. According to the CEA-708
> specification:
>
> "The end of the packet is indicated by either: (1) receipt of the header
> (cc_valid = 1, cc_type = 11) of the next DTVCC packet, or (2) receipt of a
> byte-pair where cc_valid = 0 and cc_type = 10 or cc_type = 11."
>
> But only case (1) is handled currently. As a result, a received packet
> will not be processed until the next one arrives, which in some cases only
> occurs after a long time has passed. This usually manifests in roll-up
> captions that are missing the last letter or two until more text appears,
> on in pop-on captions that appear too late, not leaving enough time for
> the viewer to read them before they disappear.
>
> The attached patch adds handling for case (2). I have tested it with the
> PBS test material (http://ncamftp.wgbh.org/DTV/CEA%20test%20material/) and
> recorded programs from various local ATSC stations. The delays are gone,
> and I have not observed any new problems.
Thanks for this fix! Now, on my favorite test recording, according to
mythccextractor, EIA-708 caption durations and gaps between adjacent
captions differ by at most 3 frames from their EIA-608 equivalents.
As a result, I would say EIA-708 captions are now truly usable in
mythtv.
Jim
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Re: [mythtv-commits] Ticket #10273: CEA-708 captions are delayed because of incorrect byte-pair handling