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

John P Poet jppoet at gmail.com
Thu Jun 8 00:46:32 EEST 2017

On Tue, Jun 6, 2017 at 4:48 PM John P Poet <jppoet at gmail.com> wrote:

> On Tue, Jun 6, 2017 at 4:40 PM Kieran Kunhya <kierank at obe.tv> wrote:
>> >
>> > 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.
>> >
>> Odd, ATSC specifies specific bitrate requirements in this area. Are you
>> sure your insertion process isn't  bursting?
>> Kieran
> The source is SDI with embedded 708 captions.  I supposed there may be an
> issue there.  I have not tried this with any other source.
> John

>From my SDI source, most frames have cc_count=20.  Then there can be two or
three frames without any CC packets, which are often followed by a single
frame with with two or three VBI lines of CC packets:  e.g. two or three
groups of cc_count=20 in a single frame.

When you asked if it is "bursting", I assume this is what you mean?  If so,
what would you suggest I do about it?



More information about the ffmpeg-devel mailing list