[MEncoder-users] interlaced/non-interlaced

[Andrew Berg]

 On 10/6/2010 11:25 PM, Rolf Ernst wrote:
> Not a single premium network channel is broadcasting interlaced. Not 
> sure about standard over the air HD but it's my guess that none or only 
> very few broadcast interlaced. (1080i would be the only option in the 
> U.S. and an idiotic one at best).
Most stations in the US (this includes network, cable and premium
channels) broadcast 1080i60, the rest do 720p60. Many programs
(especially sports) are recorded 1080i60. 1080p30 and 1080p60 are not
options. On 1080i channels, either the original 60i material is
broadcast or 24p material is telecined to 60i. SDTV is 480i60 except in
a few rare cases.



> Show me any TV station that's sending out it's interlaced
> material deinterlaced. Or DVDs for that matter.
I can't be 100% certain, but I would say that deinterlacing would cost
money (we're dealing with extremely expensive commercial encoders, not
the free ones that are available for personal use) and CPU cycles that
broadcasters are not willing to spend and therefore deinterlacing is not
part of the standards.
As for DVDs (and BDs for that matter), the bitrate available is usually
enough to accommodate interlaced material, and of course there's money
issue, so I can see why deinterlacing is not part of those standards either.
And in both cases, they want to provide the original format. There are
situations where deinterlacing would be good, but everyone has to follow
the standards.
> Well, i thought that the encoder (ffmpeg/mencoder) would ultimately
> need to have real 25 frames/sec when i want it to do progressive encoding,
> and if the input is 50i fields (originating from an actual 25p material),
> then the final 25 frames/sec are created by combining two fields back into
> a frame - without any interpolations.
The only way to losslessly deinterlace would be to use a filter that
turns each full-height field into half-height frames (e.g. tfields=0).
This halves the resolution and doubles the framerate. Of course, the
picture will flicker because of the way interlacing works and it's not
as efficiently encoded as a stream that was deinterlaced using a
standard filter like yadif. Regardless of how the material was
originally recorded, you either have fields or frames. If the material
was recorded progressive, but broadcast interlaced, you can't magically
revert it to its original state. You will have to deinterlace with a
filter like yadif to get 25p again, and it won't be 100% lossless
(though it will be extremely close since it was never 50Hz to begin with).

> Hmm. remind me, what would be the right parameter to get encoding purely
> based on quality, not target bitrate ? I guess that would be possible then
> to do with single pass ?
With x264, it's CRF. Don't know about other encoders.
Yes, constant quality is done in one pass.

> How about part 10 ? I have been doing 900kbps for 1280x720 at 30frames.
900kbps is extremely low, even for x264, when dealing with 1280x720.
You'd get much better quality at that bitrate if you reduce the
resolution. With sane settings, you'd need at least 1500kbps to get
decent quality with x264 with 720p material, if your source is clean. It
all depends on how much noise and artifacting your source has, as well
as what kind of material it is. A noisy, artifacted program with a lot
of motion can have 5-8x as many bits as a clean and flat cartoon and
still have worse quality. If you're aiming to have everything the same
size, pick a size you're comfortable with and use 2-pass. If you want
everything to have the same quality (source quality matters here, but
overall you can have varied types of programs and bitrate will be
adjusted to keep a specified quality), use a constant quality setting.
This is a bit complicated and VERY subjective, so I can't tell you what
is best for you. Broadcast quality is pretty good in Europe (at least
what I've seen from Sky Sports), so a CRF value between 19 and 22 is
good. For a clean DVD, 18-20, 21 or 22 if it has heavy artifacting. For
BDs, 17-19 is good. Lower CRF=higher bitrate and higher quality. Type of
content (cartoons/sitcoms/sports) and resolution don't matter much, but
source quality does.