[mythtv-users] slightly OT: Can dd not be used on mp4?

[Jean-Yves Avenard]

On 21 September 2013 10:43, Ian Evans <dheianevans at gmail.com> wrote:
> Trying to create a clip of an mp4 file for debug purposes for another
> thread.Was asked for a 50mb sample since that's easier to send/share than
> the 900mb original.
>
> Created the file with:
>
> dd if=1019_20130918060000.mp4 of=1019_20130918060000_shorttake3.mp4 bs=1024
> count=50000
>
> ffmpeg -i doesn't recognize the new file:
>
> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x339d080] moov atom not found
> 1019_20130918060000_
> shorttake2.mp4: Invalid data found when processing input
>
> is this a command line error or is it not possible to create a debug sample
> from an mp4?

for many mp4, the moov is located at the end of the file...
There was a bug in 0.26 that prevented to play mp4 files made by an
iphone for that reason (myth couldn't read the last kB of a file under
some circumstances)

so with those files unfortunately, you can't "cut" them.