[MPlayer-users] MENCODER: AuDIO OF SYNC when recording from TV

[D Richard Felker III]

On Thu, Aug 07, 2003 at 03:01:24PM -0300, Andre Bruce wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> i forgot to ask an other thing.
> What are the ideal fps ?!? why 25? How many fps does a normal TV have
> ?:P

PAL (European) TV is 25 fps. NTSC (US, Japan, etc.) is 29.97. You MUST
record at the actual framerate or you'll get horrible bogus choppy
video. There's no way to resample video in time (because there are too
few samples to begin with) so if you try to record at a different
framerate, you'll throw away or duplicate images.

This problem is greatly amplified by the fact that TV is not frames,
but *fields*. A field is half a frame, either the even lines, or the
odd lines, shown alternately so you have an illusion of smoother
motion without using too much bandwidth on the air. So you really have
50 fields per second with PAL, or 59.94 fields per second with NTSC.

When you capture, you get 2 fields at a time, combined as one frame,
but they're actually images from 1/50 or 1/59.94 second apart, so
you'll see an effect known as "combing". This looks rather ugly when
played on a non-interlaced display device such as a computer monitor
or projection screen.

How do you deal with it?

First point: when dealing with interlaced video, whatever you do, DO
NOT SCALE IT VERTICALLY. If you do, it will be permenantly ruined and
there's no way you will get a clean picture out of it ever again! :(

Unfortunately there is no "correct" ideal way, but you can just record
the video as-is (interlaced) and play it back on an interlaced display
device, or with deinterlacing.

MPlayer can deinterlace in several ways (see -pphelp), but all of
these essentially throw away half the framerate and lose lots of
information. With MPlayer G2, you can use the "tfields" filter to
deinterlace to 50 (or 59.94 with NTSC) fps, giving quite nice output,
if your system is fast enough.

On the other hand, if you're watching a movie on PAL tv, it's probably
not interlaced at all. That's because film is recorded at 24 fps, then
sped up slightly to 25 fps for PAL tv. So, pairs of fields come from
the same original frames in the movie, rather than distinct points in
time.

For NTSC, dealing with movies gets more complicated. I won't bother
explaining here since it sounds like you're in a PAL region, but if
you care to learn about it, browse the archives for info about
"telecine" or "pulldown".

> What about the bitrate?

Analog video has no such thing as bitrate. IMO you should not use
bitrate when recording live from TV capture, but instead use vqscale=2
for constant (near-max) quality, then figure out an appropriate
bitrate later and do 2pass encoding with mencoder to make a copy at
the desired quality for archival purposes.

Rich