Removing EIA-608 Closed Captions from H.264 without reencode

Question

I'm looking to remove the closed captions (EIA-608) from a H.264 video (contained as mkv) without reencoding.

The closest I've got is using ffmpeg:

    ffmpeg -f lavfi -i movie=input.mkv[out+subcc] -map 0:0 output.mkv

In order to separate the video into rawvideo and subrip components and export out the rawvideo. However this results in a file close to 200GB, which isn't really a sustainable solution.

An ffmpeg based solution would be preferable, but I'm fine using whatever software is necessary.

Tantra

Not possible at present with ffmpeg - trac.ffmpeg.org/ticket/5283 – Illuse

I assume your captions are CEA-608 (and not EIA-618). Please provide a sample file. Your captions may be embedded inside the H.264 stream (via SEI) or your captions may be in a separate stream inside the MKV container. – Lanza

You're right, they are 608, edited the question to correct my error. I've uploaded a brief clip to Mega – Tantra

Answer 1

This is actually possible using bitstream filters. As far as I know I discovered this myself, since everywhere I have looked this is supposed to be unsupported.

The first thing to understand is that for EIA-608 and similar closed captioning standards, the captions are embedded directly in the video bitstream as user data. H.264 bitstreams are stored as a sequence of NAL (network abstraction layer) units. Each unit has a type; user data is stored in a NAL unit of the supplemental enhancement information (SEI) type.

It turns out that ffmpeg has a bitstream filter called filter_units, which allows you to pass or reject NAL units by type. So we can use this to remove all the SEI NAL units, which strips out the captions.

The filter documentation for filter_units says that we have to specify the types by number. According to the latest H.264 spec (Table 7-1), SEI units have type 6.

So the following command will remove embedded closed captions:

ffmpeg -i input.mkv -codec copy -bsf:v "filter_units=remove_types=6" output.mkv

This has worked for me on several files without any problems or side effects.

Redmon

Works like a charm. Thank you so much <3 – Tantra

How can I make ffmpeg to see the EIA subtitles? I'm trying ffmpeg -i and ffprobe but it doesn't show up the subtitle at all! – Adjourn

Unknown bitstream filter filter_units with ffmpeg version 3.4.4-1 you need 4+ – Adjourn

I attempted to remove the CC1/EIA-608 by outputting video & audio into separate files. Running each through MediaInfo, neither showed the closed caption track. So I figured it was successful. Then I muxed the video & audio back together into a new MP4 & found the CC track to still be shown in MediaInfo. The filter_units solution did work for me. Thank you. – Duodecillion

Amazing! - And it's really really fast. – Torrey

Answer 2

I came across this post while attempting to remove closed captions from an MPEG-2 video (from an NTSC DVD). filter_units=remove_types=6 didn't work and actually caused a thick black horizontal bar to appear, presumably because crucial video information was removed since the codecs differ. After some experimentation, I discovered that remove_types=178 worked.

Thought I'd post here for anyone else trying to achieve the same in the future. I would've just commented on the answer but I don't have the reputation necessary to do so.

Zenobia

Answer 3

For anyone looking to do this with h265 remove_types=39

It's important to note that for AVC/HEVC, SEI is also used for other information like HDR10+, so using this will result in that metadata being removed, there is an existing open ticket regarding this issue. So if you don't want to lose HDR then it would be best not to remove closed caption.

https://trac.ffmpeg.org/wiki/HowToExtractAndRemoveClosedCaptions

Vasomotor