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Want the real truth about interlacing on LCD TVs
Alright, I’ve been editing commercial broadcast video since long before LCDs, HDTV or digital was the broadcast standard. I understand all the ins-and-outs of progressive and interlaced encoding and broadcast. However, there are so many misconceptions and conflicting reports about the current state of interlaced broadcast (1080i60), so if anyone can clarify the actual technical end of things:
Are LCD/Plasma TVs *ACTUALLY* delivering independent fields at 1/60th of a second? Everything I’ve read states that HDTVs “can handle” interlaced video, but that’s not exactly the same thing. I’m starting to suspect that LCDs are simply displaying each pair of fields simultaneously that have been run through an internal deinterlace process.
I’m fairly certain that LCDs do not display the “half-on/half-off” bursts that CRTs do, so that at any given moment all lines are lit. But up until now, what I thought that meant was that all fields would start a 60th apart, but remain lit until the next field of it’s type (upper or lower) would take it’s place (a 30th later). But maybe that’s not really true?
Can someone give a definitive answer as to what LCD TVs are really displaying to the viewer?
A) old-fashioned upper/lower, on/off combing (like CRTs)
B) 60th-of-a-second refreshing, but with each field remaining up for a 30th? (like I described above)
C) 30th-of-a-second refreshing, but with both fields going through a hardware deinterlace process (so what we’re really seeing is technically 30p).The reason I’m asking is that I’m encoding a very high end commercial shot and edited in 24p. When I output for broadcast at 60i, it brakes the frames up into a 3:2 field pattern (pulldown) to preserve the 24p cadence. My concern is that this only preserves the true 24p cadence if the display is actually refreshing each field individually at 1/60th of a second. If there’s deinterlacing going on with a 30th refresh rate… then I might as well do the deinterlacing myself (creating a 60i file with duplicated fields), and can choose my own deinterlacing method.
Television Producer
KTVF-11 Fairbanks, Alaska
video.ericbarker.com -
5 Replies
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A very valid question and, in my book, unfortunately one that will have as many answers as there are different makes of LCD / Plasma TVs.
I’d suggest you contact a few panel manufacturers to try and get the truth out of them. But in my experience you’re probably looking at a processed de-interlaced process that will not break a PSF image. That way they run the same process blanket for both. But that’s just my guess in what I’ve seen. If you want to be safe I’d say go PSF to 60i signal so you have control. You’re starting P anyhow. Unless you’re using Optical Flow to interpolate frames with your pulldown that actually breaks into combed fields from 24 to 60 you’re PSF already aren’t you?
Francois
FireTrigger Inc.
https://firetrigger.com -
In my experience it’s not at all complicated.
What I do know is that if you have a TV spot with the incorrect
field order interpreted, you will definitely see out of field
order jitter on CRT & Plasma displays. I’ve never seen
this happen on LCD but I supposed it’s possible.In the case of clients delivering TV spots in 24p which
does happen sometimes, I just place it in a 1080i upper field
first sequence and export it from there. It looks fine
and plays fine without any visual artifacts or ghosting. -
I’m really not familiar with PSF (just reading up on it). I don’t know why I would be. From what I’m reading, PSF is specific to 30p and 25p, as a way of producing not 24p.
The footage was shot on a Sony NEX-EA50 at true 24fps progressive (not 60i files with 24p pulldown integrated), and edited in a 1080p24 sequence. All nested after effects pre-comps and files are 24p as well.
I guess I’m not having any problems per say, but I’m interested in the technical end of what’s going on.
Television Producer
KTVF-11 Fairbanks, Alaska
video.ericbarker.com -
I ran into PSF with my AJA monitoring system back a few years before it could handle bandwidth of full progressive 1080 higher framerates. PSF is used there to send a progressive signal over an interlaced level bandwidth channel. The Progressive image is broken into 2 halves and then is played back as interlaced – exactly the process you’re following going from 23.98p to 29.97i, excluding the frame duplication of your pulldown, I believe. The catch is that if you play it back you’re actually looking at an re-assembled progressive image – it just comes to the screen in halves. Since the original was not combed to begin with the motion edges are not combed and appears exactly the same as when viewing it in progressive.
(The same is true for my BMD DeckLInk SDI – it has the option to send out 23.98p or 23.98psf – for monitors that couldn’t handle true P over SDI).
I might be wrong, but the bottom line is that if you don’t use Optical Flow or similar process to create new interpolated motion frames between your actual progressive frames, you don’t end up with combed motion edges anyhow. As long as the field order is correct in playback you’re golden.
So back to your question – It’s really something that the panel builders for the TVs can only answer, varied by make and model. They build TVs to sell against others in TV walls of consumer stores. Whatever they can do to make images “pop” on there will ultimately lead to a sale. And there is a lot of dark arts going on in consumer TVs to achieve that…
Francois
FireTrigger Inc.
https://firetrigger.com -
[Eric Barker] “Can someone give a definitive answer as to what LCD TVs are really displaying to the viewer?
A) old-fashioned upper/lower, on/off combing (like CRTs)
B) 60th-of-a-second refreshing, but with each field remaining up for a 30th? (like I described above)
C) 30th-of-a-second refreshing, but with both fields going through a hardware deinterlace process (so what we’re really seeing is technically 30p).
“It’s actually none of the above.
LCD and plasma displays are inherently progressive display technologies, which refresh the entire screen all at once rather than in alternating fields, like CRTs.
When given 1080i60 source material, circuitry in modern plasma and LCD displays perform realtime motion-adaptive deinterlacing on the fly, essentially converting 1080i60 into 1080p60. Of course it’s not as crisp as true 1080p60 would be, but the fast refresh rate increases the perceived resolution. It’s also why 720p60 is perceived as being as sharp as 1080i60. I’m not sure if it’s still the case, but ESPN actually used to broadcast in 720p60.
You can see a similar effect in practice when watching a DVD on a computer. Most DVD playback software (and definitely Apple’s DVD Player software) perform realtime motion-adaptive deinterlacing on interlaced SD footage, so what you end up seeing on your computer screen has the same temporal resolution as watching interlaced SD on a CRT. The software is actually converting 29.97 interlaced into 60p on the fly. That’s why DVD footage that originated as either NTSC 29.97fps interlaced (60 fields per second) or PAL 25fps interlaced (50 fields per second) will retain the “live video” look on playback on a computer screen rather than looking like 30p or 25p. An exception would be the web. It’s extremely rare to see any online video with a frame rate higher than 30p, although it’s technically possible to stream 60p.
As far as 24p in the broadcast TV world goes, 1080i60 will contain 3:2 pulldown with interlacing (exactly like NTSC SD), and 720p60 will contain 3:2 pulldown but using duplicate whole progressive frames instead of split interlaced frames. As far as what ends up being displayed on an LCD or plasma from a broadcast signal, it will either be 1080p60 (synthesized from 1080i60) or 720p60, each containing a pulldown cadence of duplicated frames. It can be illustrated using letters as sequential frames thusly:
AA BB CC DDD EEE (repeat)
Where things get more complicated is if you’re playing back true 24p source material from a DVD or Blu-ray player connected to an LCD or plasma. In that case, the footage can be displayed at its native progressive frame rate without pulldown being added. The refresh rate of the screen will vary depending on the model of TV. It might be refreshed at 24Hz, 48Hz, 72Hz or higher, but the frame rate doesn’t change – you’re just seeing the same frame refreshed at a multiple of the base frequency.
An exception to this is what is in my opinion the completely awful “smooth motion” 120Hz processing in many modern LCD and plasma displays that synthesizes new interpolated frames between existing frames, thereby making everything have the motion quality of live video. It’s the first thing I turn off in the menu settings of any TV I come into contact with. You’d probably not be surprised to learn that I absolutely hate the look of the 48fps HFR versions of part 1 and 2 of The Hobbit. The difference in temporal resolution results in a very different psychological experience for me, which I liken to watching an episode of a soap opera versus experiencing a cinematic masterpiece. For me, higher frames rates have the look of something that is happening, versus 24p looking like something that happened. I find the 24p look to be much more complimentary to dramatic material, as it helps to provide a sense of detachment from reality.
Hope this helps.
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