ac-3 | Interactive implementation of Arc Consistency Algorithm | SDK library

My plan was to put a transparent red box behind a video. This box should only be present from second 1-45. But if the videos are 3 hours long, the process takes a long time although it only has to process 45 seconds.

My first attempt takes too long:

ffmpeg -i %1 -vf drawbox=0:9*ih/10:iw:ih/10:t=fill:color=red@0.5:enable='between(t,1,45)' "%~dp0transpred\%~n1%~x1

Then i tried splitting the video into two parts. put the box on the first video, and then put the two back together again.

ffmpeg -ss 00:00:00.0000 -i %1 -to 00:00:45.0000 -vf drawbox=0:9*ih/10:iw:ih/10:t=fill:color=red@0.5:enable='between(t,1,45)' "%~dp0transpred\%~n1A%~x1"

FFMpeg -ss 00:00:45.0000 -i %1 -c:v copy -c:a copy -avoid_negative_ts make_zero "%~dp0transpred\%~n1B%~x1"

But i don't even have to try to put these two together, because they are not separated exactly at the second. I have read that this is due to "timestamps" and the different video and audio streams.

Now I'm trying an approach to create a stream with the bar, and then overlay it with the finished video. I haven't quite managed that yet, and I don't know if it's faster. Shortening the video is very fast.

EDIT (Added as a replacement for the comment later)

Thanks for your help I have almost done it with a slightly different approach. Unfortunately, the second part now always has no sound. No matter if I put A and B (B no sound) or B and A (A no sound) together.

1. First split with mkvmerge so i have no worrys about the keyframes and get the exact time mkvmerge --split timestamps:00:00:45.100 A.MKV -o splitmkm.mkv
2. Then add the Bar (Black because of easier testing): ffmpeg -i splitmkm-001.mkv -vf drawbox=0:9*ih/10:iw:ih/10:t=fill BAR1.MKV
3. Merge (mkvmerge ends with error): ffmpeg -safe 0 -f concat -i list.txt -c copy output1.mkv

EDIT (Answer to kesh)

This was the error Again, audio codec config's must match across all your concat files. The drawbox changed the audio Codec from AC-3 to Vorbis.

the procedure is now:

1. mkvtoolnix\mkvmerge --split timestamps:00:00:05.100 %1 -o A_splitmkm.mkv with mkvmerge i have an exact split at the time, and i don't have to learn about keyframes.
2. ffmpeg -i A_splitmkm-001.mkv -vf drawbox=0:9*ih/10:iw:ih/10:t=fill:color=red A_BARmkm.MKV create the Bar
3. ffmpeg -i A_BARmkm.MKV -i A_splitmkm-001.mkv -map 0:v -map 1 -map -1:v -c copy A_BARwithAudio.mkv redo the step with the changed audio from drawbox
4. ffmpeg -safe 0 -f concat -i list.txt -map 0 -c copy A_output1.mkv merge

Now everything works. Thanks alot!