[MEncoder-users] settings to convert ATSC HDTV 1080i30 to 720p24

[Scott Larson]

On Wed, 2007-05-30 at 19:24 +0100, Dieter wrote:
> The obvious next question is: how does one detirmine whether the video
> is interlaced or telecine?

All American dramas and comedies broadcast in HD have been telecined
from film or HD video. The rest are 60i. The only weird exception I can
think of was the Victoria Secret's modeling special which had some
sequences in 30p with a mix of 24p and 60i sequences.

> I read somewhere that most or all 1080i content started life as 24 fps
> progressive.  I have to wonder why they decided to mangle that into
> 30 fps interleaved instead of transmitting 24 fps progressive?

Because that would prevent them from transmitting 60i for sports and
other programming. Broadcast networks can't switch formats like that.
There was nothing in the ATSC standard on how to handle that so ATSC
receivers don't handle the change very well.

The telecine flags allow you to effectively broadcast 24 fps. Some 1080i
cable channels like HBO use the telecine flags correctly for filmed
content. Unfortunately broadcast network distribution from the 1080i
networks can't use these flags because the stations receive a high
bitrate feed (24-45 Mbps) and reencode it into ATSC. Some stations have
smart encoders that try very hard to match fields and mark them with the
telecine flags when the reencode. They're not perfect so you'll see them
switch back and forth.