> Hi
>
>
> In the example below the resulting 5 second mp4 shows noise
> (horizontal coloured lines) for the first few seconds then a very
> pixilated few seconds of the actual footage. I thought if I used
> –ovc copy it does just that copies each frame
Yes, and this is exactly why you get a distorted image!
All MPEG-ish video streams have I-Frames and, between them, P- (and
possibly B-) Frames. I-Frames are much like JPEG still-images, P-Frames
are just _differences_ from the preceding I-Frame (this is also where
motion compensation takes place), dramatically saving filesize.
Now guess what happens, if you take a random frame from the video
stream? You'll probably a P-Frame (there is only one I-Frame in 10
seconds or something), meaning you cannot display the image (because you
dont have the preceding I-Frame).
This is what happens with your "-ss 1 -ovc copy", and therefore you get
garbage - up to the next I-Frame in the stream.
If you want the correct image, then you need to either re-encode this
first few seconds, or you need to start the copied section at an
I-Frame. Both is not easy with mencoder - better use some video editing
software like avidemux.
Or you just re-encode the whole cut-out section, which may result in
some quality loss.
> I also note when encoding the message 'Skipping Frame' many many
> times is this normal?
That depends. A few skips can be neccessary to keep audio and video in
sync, always depending on output and input framerate. The same is true
for "duplicate frames".
Regards,
Alex
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