14 Mar
2007
14 Mar
'07
12:48 p.m.
On 3/14/07, Adrian Graham <witchy at binarydinosaurs.co.uk> wrote:
Ah, my bad. I naturally assumed that cat would strip
those off and just have
a single pair, otherewise how does the new header report the newly expanded
filesize? Or doesn't it do that and apps just deal with it as a filestream?
Cat just, um, cats. It has no knowledge of the contents of a file,
but will happily squeeze together any amount of data you can fit from
any source you can read from. Unlike the AVI format, MPEG-2 isn't
utterly reliant on every detail in the headers being precisely
correct. A well-behaved player will use what's in the header, but
players I've used will keep attempting to play until they hit EOF or
until the data looks so screwy that it causes serious problems to the
decoder routines.
What you may see from glommed MPEGs is a blip on the screen or a chirp
from the audio as the decoder moves across the headers
embedded in the
file, but even that's not guaranteed to happen - it all depends
on the
particulars of the CODEC in use. MPEG, though, is reasonably tolerant
of cruft passing though - like when there's a scratch or a speck of
dust on a DVD, so it's unlikely to throw up its hands because it finds
a few dozen bytes that don't decode to sensible video.
With all of that said, I _think_ there's tools out there that can
identify and remove those embedded headers. You wouldn't need the
tool if everyone used "proper" tools to merge video, but since lots of
people stitch MPEGs together with cat or COPY or whatever else,
there's demand for the tool.
-ethan