Editing Openai/6957d2e8-b738-8008-987a-09a5d9f29bc8 (section)

=== Assistant: They’re not required to cut — they’re just the “safe defaults” that make the output widely playable and keep the cut ac… ===
They’re not required to cut — they’re just the “safe defaults” that make the output widely playable and keep the cut accurate.

Here’s what each one does, and what happens if you remove it:

===== Chooses the video encoder (H.264). =====
* If you don’t specify -c:v, ffmpeg will pick a default encoder for the output container (often still H.264, but not guaranteed).
* You could also use -c:v copy to avoid re-encoding, but then your cut won’t be millisecond-accurate (it’ll snap to keyframes / packet boundaries).

===== Quality target for x264 (“constant quality” mode). Lower = higher quality/bigger file; higher = lower quality/smaller file. =====
* Not required; if omitted, x264 uses its default CRF (commonly 23), which is a bit lower quality.
* Typical range: 18–23.

===== Encoding speed vs compression efficiency. =====
* Not required; x264 defaults to medium anyway.
* Slower presets = smaller file for same quality (but take longer).

===== Re-encodes audio to AAC at 160 kbps. =====
* Not required; ffmpeg might pick AAC anyway, but again not guaranteed.
* If your source audio is already AAC and you don’t need frame-accurate audio boundaries, you might do -c:a copy. But for “exact cuts”, re-encoding audio avoids weird edge cases (missing samples, bad timestamps, concat issues later).

===== Moves the MP4 “moov atom” to the beginning so the file can start playing before fully downloaded (web-friendly). =====
* Not required for local playback.
* It doesn’t change quality, only file layout.

==== You said: separate clips + millisecond precision if needed. ====

To cut with ms precision reliably, you generally need to decode and re-encode (at least video), because -c copy can only cut cleanly on keyframes and packet boundaries.

So the shortest “ms-accurate” version is basically:

<syntaxhighlight lang="bash">ffmpeg -i input.mp4 -ss 00:01:02.000 -to 00:02:13.000 -c:v libx264 -c:a aac clip_01.mp4

</syntaxhighlight>

Everything else (-crf, -preset, -b:a, +faststart) is tuning / compatibility / convenience.

==== This is still ms-accurate for video (because it re-encodes video), and keeps audio untouched: ====

<syntaxhighlight lang="bash">ffmpeg -i input.mp4 -ss 00:01:02.000 -to 00:02:13.000 -c:v libx264 -crf 20 -preset medium -c:a copy clip_01.mp4

</syntaxhighlight>

Caveat: if you later concatenate clips, audio-copy can sometimes make concat pickier depending on timestamps/codec parameters.

==== <syntaxhighlight lang="bash">ffmpeg -ss 00:01:02.000 -to 00:02:13.000 -i input.mp4 -c copy clip_01.mp4 ====

</syntaxhighlight>

Fast, no quality loss — but cuts may be off by up to a few seconds if keyframes are sparse.

If you tell me whether your priority is (1) perfect accuracy or (2) no re-encode / perfect quality, I’ll give you the best exact command for that.