[MEncoder-users] interlaced/non-interlaced

[Toerless Eckert]

On Thu, Oct 07, 2010 at 11:13:20PM -0500, Andrew Berg wrote:
> Most stations in the US (this includes network, cable and premium
> channels) broadcast 1080i60, the rest do 720p60. 

Well. My comcast cable has way too many channels but only < 100 of them
are HD, eg: 1080i or 720p. The rest is all 480i. Sure, there is a bunch
of those that are just downsampled HD material, but likewise i commonly
see upsampled SD material on HD channels. I think There are just
480i cameras and HD cameras supporting 720p/1080i ;-)

So i guess the question is what is "most stations".
Maybe you mean "most stations except lots of strange programming on cable"? ;-))

> 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.

right.

> 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. 

Right. And i would also like to avoid doing any steps that introduce
new errors...

> There are situations where deinterlacing would be good,
> but everyone has to follow the standards.

Which standard ? DVD does support 24p and 30p, right ? And converts
it to 50/60i...

> 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.

No, i totally do not understand that. Assume i have 25p material and
want to broadcat it as 50i. What i do then is that i just split
up every frame into even and odd lines and broadcast first the odd
lines as one field and then the even lines as the second field. Of course
i shuold be able to recombine these two fields to a full frame without any
loss.

> 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).

Well, let's just assume original 25p -> 50i -> 25p and 30p -> 60i -> 30p.
Those cases should be lossless. Even 24p -> 60i -> 24p should be lossless.
Not sure which case you are thinking of. 24p -> 50i should be possible
to convert lossless for vide back to 24p, just the audio needs to
be speed up/down by 4%.

> > 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.

Thanks, but why would you use a high CRF == low bitrate for material
with heavy artifacting ? SHhouldn't that lower bitrate just aggravate
artifacts ?

Cheers
    Toerless