[MPlayer-users] Converting DivX4 -> MPEG ?

[Novak Levente]

Tuukka Toivonen tuukkat at ee.oulu.fi wrote:

>No, the same number of lines. Nyquist frequency corresponds to half of
>the number of video scan lines. Why? Imagine that every other line
>is black and every other white. This is the maximum frequency that can
>presented. What is this frequency? 0.5, i.e. every full wave with
>one black and one white line takes in total two lines.
>

I told about horizontal, not vertical lines, because that's how it's
called in the VCR/camcorder world. It corresponds to the _horizontal_
resolution of an analog output. For most PAL Hi8 camcorders, the
resolution is 480 analog horizontal lines per 576 of vertical scanlines
(imagine why SVCD spec has the same -- but digital -- resolution). For
digital sources, there is no need for an A/D conversion, you just get the
same resolution for the target file.

And why should you use _twice_ the analog "resolution" of your source? If
eg. you have a Hi8-camcorder output, you have a nominal horizontal analog
resolution of 480 lines (not vertical scanlines!). If you shoot a regular
black-white-black-white-... pattern (one "analog pixel" per black and one
per white component) onto tape, then during capture (A/D conversion), if
the digitalisation is not exactly in sync and you use a horizontal
resolution of 480 digital pixels, you won't get a black-white-black-...
dot pattern, but a grey-grey-grey-... one: the information is lost. If you
digitalise with a resolution of 2x480=960 pixels, maybe you won't get a
very contrasted, sharp black-and-white pattern if you're not completely in
sync, but at least it  will be definitely dark-clear-dark-clear-...,
approaching enough your original pattern filmed to tape (the information
is preserved). You can capture with even further increased resolution, but
it helps only to restore better the contrast with respect to the original.
A theoretical "infinite" resolution produces the exact same contrast
pattern than your source.

Levente