[FFmpeg-devel] [PATCH] Add A53 Closed Captions to MPEG header if they are available.

Brian Matherly brian.matherly at yahoo.com
Wed Jun 7 01:42:55 EEST 2017

>> Indeed, multiple entries of the same type are really a bad idea and we
>> basically made them impossible with stream sidedata, although maybe
>> not with frame side data yet. We should not add API for them or
>> encourage their use.
>> If there is a real need for multiple of the same type, maybe the type
>> should be expanded to hold more information.
> The cc_count is only 5 bits, which mean that only 31 3-byte "closed caption
> constructs" will fit in a "block".    Testing this with 1080i60 material, I
> found that 2 or 3 blocks was often necessary to hold all of the CC data.
> I tried ignoring that limit of 31 "constructs" per block, and ended up with
> corrupt captions.   By preserving the 2 or 3 separate blocks I observed
> from the original source, the captions are perfect.
According to CEA-708, in the case of 1080i60, the correct number for 
cc_count is 10. The highest number that cc_count is allowed to be is 30 
in the case of repeating a frame three times for film mode. Exceeding 
the correct cc_count for the frame rate would cause the CC channel data 
rate to exceed 9,600bps.


More information about the ffmpeg-devel mailing list