Ticket #10273 (closed Patch - Bug Fix: fixed)
Opened 16 months ago
Last modified 16 months ago
CEA-708 captions are delayed because of incorrect byte-pair handling
|Reported by:||argilo@…||Owned by:||danielk|
|Component:||MythTV - ATSC||Version:||Master Head|
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.