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

[Scott W. Larson]

> This would explain why my recording of "Numb3rs" was switching back
> and forth between progressive and interlaced.

Yes, the station's encoder was trying hard to use the telecine flags but
it's better for it to pass some interlaced frames than mismatch incorrect
fields when it's not sure what to do.

One place this will always happen is when an animated network bug appears
over filmed content. These are always 60i.

> I assume that these points are where the telecline flags got
> screwed up, possibly where bit errors occurred, or where there was a
> commercial.  Hmm interresting, this may be a good way to detect where
> commercials start and end in the broadcast. It appears that the
> majority of the commercials are a full interlace video while the
> actual show is teleclined.

The station's encoder should also try to put the telecine flags on
commercials with filmed content which are almost all national commercials
these days. The station ID's before the commercials will be 60i.

> Anyway, it's far easier to recover the
> 24fps film from 720p content then it is with 1080i content.

That also depends on the encoder. You'll almost certainly find that every
frame in 720p is slightly different, even on frames that should be exact
duplicates. For ABC stations, this is a result on the encoder trying to
reencode the 45 Mbps network feed into ATSC. I've found that a new frame
doesn't have as much detail as the "duplicate" frame that follows it. Some
people have found the opposite.

Apparently this is because the station's encoder is trying to "keep up"
with the faster network feed. At 60 fps, your eyes won't notice the
less-detailed frame after it's been enhanced by the "duplicate" frame.
I've found that decimate will sometimes find more difference between
"duplicate" frames than in frames that aren't duplicates. This makes
finding the best 24 out of 60 frames every second somewhat difficult.

Fox shouldn't have this problem because the stations are receiving an ATSC
feed straight from the network which supports variable length GOPs. Yet
for some reason Fox's ATSC feed has similar characteristics. Maybe this
idea of using the duplicate frames to add detail in "duplicate" frames is
a common practice when encoding 24 fps material in 720p.