[mythtv-commits] Ticket #10273: CEA-708 captions are delayed because of incorrect byte-pair handling
MythTV
noreply at mythtv.org
Sun Jan 22 02:51:58 UTC 2012
#10273: CEA-708 captions are delayed because of incorrect byte-pair handling
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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 |
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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.
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Ticket URL: <http://code.mythtv.org/trac/ticket/10273>
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