On Thu, 13 May 2004, Derek Fountain wrote:
> I've used mpeg2enc for ages now, and I use -f 1 to set VCD format, as per the
> help:
>
> --format-f fmt
>
> This has always made sense and I've never really thought about it. Somewhere
> in my mind I associated this with an operation which scales the image to VCD
> size. Only today, it finally dawned on me that that /isn't/ what it does. I
> plugged in a 512 x 384 video, and that's what came out!
>
> So, for curiousity's sake, what does do? What is a "pre-defined mux format"?
It tells the encoder if the stream is to be MPEG-1 or if it's MPEG-2
and what constraints the stream must meet. Constraints might
include "CBR" (ConstantBitRate), the maximum bitrate, maximum GOP
size, and so on.
The '-f' to mpeg2enc does not know or care abou the frame size - which
is why it let you create a 512x384 VCD. The encoder did however
make sure the bitrate didn't exceed 1152Kb/s ;)
Obviously a couple of the -f values stand out - the values 6 and 7
for VCD and SVCD still images are do not have a bitrate as such.
The data needs to be appropriately scaled (and in 4:2:0) before
hitting the encoder. Eventually (hopefully) 4:2:2 might be supported
but that's just for "studio" work - it's nothing that can be put on
a DVD.
Cheers,
Steven Schultz
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