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I was looking into cutting a file and I came across two flags. Could anybody explain them in layman's terms. -c copy - It doesn't re-encode the file. What does

I was looking into cutting a file and I came across two flags. Could anybody explain them in layman's terms. -c copy - It doesn't re-encode the file. What does re-encoding mean? -async 1 -which is deprecated in favour of aresample. If the correct syntax is aresample=async=1 then would it just cut off the audio timestamp and match it with video timestamp.

dualogy 6y ago
> What does re-encoding mean It's something you may want done or not whenever a decoding step is happening by necessity anyway. The "possible benefit" would be if the original encoding was partially broken somewhere (which players/decoders can handle usually but they complain to stderr) or otherwise sub-par. Not sure about video but audio files floating about out there from the last 25 years are a wild mess, with "mp3"s really being layer 2 or 1 or not even mpeg at all inside etc.
- zerocrates 6y ago
  ffmpeg's default mode of operation is to decode the input and re-encode it. Unlike the default of re-encoding, "copy" is extremely fast and doesn't involve loss of quality from the input because it's just, well, copying the streams from the input to the output. The downside is that you can't do lots of things when you're just copying: making any changes to the actual video or audio, or even cutting video accurately at non-keyframes requires the standard decode/re-encode steps. Typical video-file-related reasons for using the "copy" codec would include just switching between container formats: MP4 vs. Matroska vs. AVI vs. whatever else. As long as the new container supports the types of audio and video you have, just copying them will save both time and quality. Or maybe you want to alter the video but keep the audio untouched, or vice-versa. Or you just want to add a track to something and not disturb the existing ones. Even doing "copy" for all streams and using the same container can be useful, even though nothing would seem to change in that situation: "remuxing" like this is often all that's necessary to fix a wonky file.
dylan604 6y ago
re-encoding = transcoding. which means decoding one video codec and then encoding it into a different codec. OR, reading the source, but then recompressing it into the same codec. Either way, the new video is not the same as the original. `-c :v copy` literally takes the video from the source and places it into the output without doing anything than making it fit in the output.

dualogy 6y ago

> What does re-encoding mean It's something you may want done or not whenever a decoding step is happening by necessity anyway. The "possible benefit" would be if the original encoding was partially broken somewhere (which players/decoders can handle usually but they complain to stderr) or otherwise sub-par. Not sure about video but audio files floating about out there from the last 25 years are a wild mess, with "mp3"s really being layer 2 or 1 or not even mpeg at all inside etc.

zerocrates 6y ago
ffmpeg's default mode of operation is to decode the input and re-encode it. Unlike the default of re-encoding, "copy" is extremely fast and doesn't involve loss of quality from the input because it's just, well, copying the streams from the input to the output. The downside is that you can't do lots of things when you're just copying: making any changes to the actual video or audio, or even cutting video accurately at non-keyframes requires the standard decode/re-encode steps. Typical video-file-related reasons for using the "copy" codec would include just switching between container formats: MP4 vs. Matroska vs. AVI vs. whatever else. As long as the new container supports the types of audio and video you have, just copying them will save both time and quality. Or maybe you want to alter the video but keep the audio untouched, or vice-versa. Or you just want to add a track to something and not disturb the existing ones. Even doing "copy" for all streams and using the same container can be useful, even though nothing would seem to change in that situation: "remuxing" like this is often all that's necessary to fix a wonky file.

dylan604 6y ago

re-encoding = transcoding. which means decoding one video codec and then encoding it into a different codec. OR, reading the source, but then recompressing it into the same codec. Either way, the new video is not the same as the original. `-c :v copy` literally takes the video from the source and places it into the output without doing anything than making it fit in the output.