Introduction
I have hopes that this may become a "sticky" thread, or at least one that's referenced in a FAQ, as it covers issues that come up rather a lot. In particular I am getting very, very tired of people constantly claiming "You don't need a 1080p screen, as there won't be any 1080p around for years" without having
any understanding of what they're talking about.
Hopefully, while this post is tediously long
it will at least finally lay that particular question to rest.
To be fair, people use the terms "
1080i" and "1080p" to mean a
lot of different things, and it is understandable that some might be confused. I think it might help to go back to first principles and discuss all the possible meanings of those terms before we try and come to any conclusions about what is or is not important.
The Basics
First, let's make sure we actually understand the difference between an "
interlaced" signal and a "
progressive" one. A video signal is a sequence of pictures. Each picture is made up of dots or "pixels", laid out in a rectangular grid pattern. In a
progressive signal, you are given the first horizontal line of pixels, then lines 2, 3, 4, 5, 6, 7, and so on: each set of lines makes up a complete picture.
In an
interlaced signal, you are given lines 1, 3, 5, 7, 9, etc. (all the odd-numbered lines); then there's a break; then you get all the even-numbered lines - 2, 4, 6, 8, etc.; then another break; then another set of odd lines; another set of even lines; and so on.
So, "
progressive" means a sequence of complete pictures (known as "frames") while "
interlaced" means a sequence of half-pictures (or "fields"), with alternate fields containing odd and even lines.
The "1080" part refers to the number of horizontal rows of pixels in each full picture. Either
1080i or 1080p is intended to be viewed on a display with a resolution of 1920x1080.
Signal Formats Field/Frame Rate
Now, if we want to fully describe a video signal, we need more information than either "
1080i" or "1080p" tells us. First, we need to know something about the signal
frequency.
1080i, for example, might have 50 or 60 fields per second (which might or might not be equivalent to 25 or 30 frames per second - more on that in a moment). So, for example, "1080i/50" means we get 50 fields per second, or 50 half-frames per second.
"1080p/60" would mean we get 60 full 1920x1080 frames per second.
Film Or Video
Another piece of information we need to fully identify the signal is: was the original source of the images inherently
progressive or inherently
interlaced?
Suppose we're watching a movie on Sky HD. This will consist of a 1080i/50 signal. The way this was produced was:
- Start with a cinema film, which has 24 frames per second, and
scan it in.
- Scale each each frame to a resolution of 1920x1080. (So you now have a sort-of 1080p/24 signal).
- Split each frame into two fields, the first containing the odd lines, and the second containing the even lines. (You've now effectively got a 1080i/48 signal).
- This is converted to 1080i/50 simply by playing it back faster - the same fields are in the signal, but they're coming at you more quickly.
If you need to convert a film to a 1080i/
60 signal, then the process then is:
- Convert to 1080i/48 as above.
- Broadcast each four pairs of fields in this sequence:
Odd field 1
Even field 1
Odd field 2
Even field 2
Odd field 2 again
Even field 3
Odd field 3
Even field 4
Odd field 4
Even field 4 again.
An important point in either case is that, although the eventual signal is
interlaced, the
source material it was produced from was
progressive. Each pair of odd and even fields come from what was originally the
same single
progressive frame - the odd lines and the even lines were photographed at the same moment in time.
But this isn't always true. If you're watching a football match on Sky Sports HD, then it may well have been filmed using a native
1080i TV/video camera. In other words, the actual pictures the camera takes are
not 25 full-frame pictures per second - it is taking 50 pictures per second, but only recording half the lines in a full frame each time it does it.
Now, the odd and even fields in each pair do
not come from the same original
progressive frame, and were not taken at the same time.
Filtering
The final thing we need to know is whether there has been additional filtering (effectively downscaling) of the original source image.
1080i signals (indeed, all
interlaced signals) were originally designed to be displayed on CRT devices, which are capable of displaying an
interlaced signal directly: they display one line of the picture at a time, and they can actually display the lines in the order 1, 3, 5, 7, etc. If you're displaying
1080i on a system like this, and there is some feature in the picture that is exactly 1 vertical pixel wide (say, a bright dot on a black background), you will only see that dot thirty (or, in this country twenty five) times per second. This creates very visible and annoying flickering called "line twitter".
To prevent this,
interlaced signals were (and often are) vertically filtered i.e. made more blurry, so the effective vertical resolution of
1080i drops to about 800 lines. This guaranteea that nothing is ever only one vertical pixel wide, but it also causea a loss of detail. Many actual
1080i transfers were also filtered horizontally, lowering the effective horizontal resolution from 1920 to about 1440.
Summary
So, to fully describe a video signal, we need to know:
1) Is it
progressive or
interlaced?
2) How many fields or frames per second does it contain?
3) Was the original source material
progressive (film) or
interlaced (video)?
4) If the signal (but not necessarily the source) is
interlaced, has there been additional filtering of the source image to prevent line twitter?
Displays
We now have to worry about what we might or might not mean by describing a
display as "1080p".
Output
First, we might be referring to the actual output of the screen - the image that we actually look at.
There are a few plasma panels from Hitachi and Toshiba which use a technology called ALIS and which are inherently
interlaced - they display the odd lines and even lines of a display alternately, but never together. However these are a) fairly rare and b) crap, so we don't really have to worry about them.
CRT-based devices are also able to directly display
interlaced images. Some of these are most emphatically not crap - high-end CRT projectors still produce images at least as good as the best digital projectors, and have done so for many years. But these too are fairly rare, and direct-view CRT televisions capable of displaying native
1080i are almost unknown in this country.
So, for practical purposes,
any high definition display you are likely to be thinking about buying at the moment is
inherently progressive, that is they display all the pixels in the picture at once. This applies regardless of whether it's plasma, LCD, LCOS (which includes DILA and SXRD), DLP, or even future technologies like SED and OLED.
Because all of these displays are inherently
progressive it has become quite common to use the term "1080p screen" as shorthand for "screen with a resolution of 1920x1080".
Inputs
However, the output is not the whole story. We also have to worry about inputs. A screen that has a resolution lower than 1920x1080 might still be described as being "compatible with
1080i" or even "compatible with 1080p" if it is capable of accepting an
input signal in
1080i or 1080p format and displaying an intelligible image derived from it.
Many 50" plasma screens, for example, have a resolution of 1366x768. Most so-called high-definition 42" plasmas have a resolution of 1024x768. But they are still able to accept a
1080i signal at the input. That signal is deinterlaced and then downscaled to the resolution of the screen.
A second possibility is that a screen may actually have a resolution of 1920x1080 (i.e. it is able to display 1080p images) but still
not be able to accept a 1080p signal at its inputs. A startlingly
large number of so-called 1080p displays work like this: the picture may be 1080p/60, but you can't feed it
any kind of 1080p signal, only a
1080i one. (There are also screens that will accept a 1080p input over analogue - VGA or component - but not via a digital input).
Finally, even if a screen can accept a 1080p signal, you then have to ask what signal
frequencies it can deal with. Can it accept 1080p/100? 1080p/75? 1080p/72? 1080p/60? 1080p/50? 1080p/48? 1080p/24? The fact it can accept some of these frequencies
doesn't mean it can accept all of them, which becomes important when we think about deinterlacing - more on that in a moment.
There are some very odd devices out there. The Sharp Aquos LC45GD1E, for example, cannot accept a 1080p input at the media box. But if you bypass the media box you can pass 1080p/60 direct to the screen - but not 1080p/50. (Among other things this means the media box is converting 50Hz signals to 60Hz ones, which is very nasty
).
Deinterlacing
A full description of deinterlacing - that is, converting an
interlaced signal into a
progressive one - is outside the scope of this post. But there are a few important points to grasp.
- Because virtually all hi-def displays are inherently
progressive, they are incapable of displaying
1080i actually as
1080i -
somewhere along the line it
must be converted to 1080p.
- On a fixed-pixel display (i.e. more or less anything except CRT) if you're displaying a picture full-screen then the picture that is eventually displayed has to be the same resolution as the device it is being displayed on. So, if you (for example) feed a
1080i image to a 1024x768 plasma, somewhere along the line the picture will have be to resized or "scaled" from 1920x1080 all the way down to 1024x768. And,
before an
interlaced image can be scaled it must first be deinterlaced.
- All non-CRT display devices therefore contain a built-in deinterlacer and scaler.
- Deinterlacing is actually a very difficult computational problem. If you know for certain that your original source image was
progressive (e.g. it was shot on film) then weaving together each pair of fields into a single frame is simple. But how does the device
know whether or not the source material was
progressive?
And, if the original source material was inherently
interlaced, how do you go about combining two half-pictures taken at
different times into a single picture that is all shown at once?
The process of guessing whether the material was originally film or video and the process of doing video-type deinterlacing is really quite complex.
- So, the most important thing to understand about deinterlacing is that
virtually all of the deinterlacers built into displays are not very good.
This can cause a wide variety of objectionable picture problems
including loss of vertical resolution (so you end up watching something with only 540 lines of vertical resolution instead of 1080), "combing" (where the even rows of pixels seem to have been displaced sideways relative to the odd rows), "jaggies" (diagonal lines looking blocky and jagged rather than smooth), and jerky or irregular motion.
Sources
Now let's consider what types of HD source material are out there.
PCs
These will typically generate 1080p/60.
"Next gen" games consoles
The output from these remains somewhat uncertain, but there will certainly be 1080p/60 or 1080p/50 sometimes.
Video processors (sometimes inaccurately known as "scalers")
Because most built-in deinterlacers (and, to a lesser extent, most built-in scalers) are a bit crap, one important thing to think about when buying a hi-def display is whether or not you want to use some kind of external device to do the deinterlacing and scaling. If you do, then clearly it is very important for the display to accept a 1080p signal, otherwise there's no point in having an external deinterlacer.
Stand-alone scaler/deinterlacers are often able to convert a 1080i-encoded movie back to the original 1080p/24 movie frames (or 1080p/25). You will therefore not get the best out of an external video processor unless the display can accept 1080p/24 and 1080p/25 as well as higher frequencies.
BluRay/HD-DVD
Movies in these disc formats are often advertised as "1080p". What that means is:
a) They're 1080p/24.
b) There
hasn't been any twitter-supressing filtering applied. (By contrast, an awful lot of
1080i broadcast or D-Theatre material has been filtered).
So the film, as stored on the disc, is full-resolution 1080p/24. The actual output from current-generation players isn't 1080p/24, though - it's 1080i/60, with all the inherent deinterlacing problems that implies. (One would hope that the players released next year will actually output 1080p/24, or at least 1080i/48).
So, does 1080p matter?!
The answer, as you will have realised by now, is: "that depends what you mean by 1080p".
If we make the question more specific, then maybe we can finally answer it!
Q: Is it worth having a screen that has a resolution of 1920x1080?
A: Yes, very much so. Any
1080i video signal requires a resolution of 1920x1080 to be displayed without loss of detail. This does
not, of course, mean that
all 1920x1080 displays will automatically look better than all lower-resolution screens - there is a lot more to picture quality than just resolution.
However, if there are no other differences between two displays except that one is 1080p-native (1920x1080), and the other is (say) 720p native (1280x720), and
if you are sitting close enough to the screen, and if you're feeding them both a
1080i signal, which is being deinterlaced correctly, the 1080p display will look a great deal better than the lower-res one.
Let's just emphasise that one more time:
you cannot watch 1080i without loss of detail unless you have a 1080p screen - 1080p output resolution, anyway.
Q: Are there any sources of 1080p material?
A: Yes, loads. Nearly all PCs. (Some downloadable video material is - I believe - in 1080p/60 format). Next gen games consoles. The next batch of BluRay and HD-DVD players (hopefully). And, which is particularly important, stand-alone video processors (scaler/deinterlacers).
Q: Do I need a screen which can accept a 1080p input signal?
A: Yes, if you want to use any of the devices mentioned in the previous answer.
Q: Will there be any actual TV programmes shot and broadcast as 1080p/50 or 1080/60?
A: Not for some time.
Q: At what frequencies does a screen need to accept a 1080p signal?
A: For use with a PC, 60Hz. For use with a console, 60Hz and possibly also 50. For best use with an external video processor, definitely both 50 and 60Hz and also some multiple of 24Hz (e.g. 24, 48, 72, 96).
Q: What do I need to check to see if my screen is "true" 1080p?
A: The following:
- What is the actual resolution of the display? (Is it 1920x1080?)
- Can it accept 1080p
input or only
1080i?
- Can it accept 1080p over a
digital input that supports HDCP? (HDMI always supports HDCP. DVI sometimes does but usually doesn't.)
- What
frequencies of 1080p can it deal with?
Q: If my display is "HD Ready", am I covered for all eventualities?
A: No, definitely not. "HD Ready" means:
- The vertical resolution actually displayed must be at least 720 pixels (but the horizontal resolution can be absolutely anything!
)
- It must be able to accept both 720p and
1080i signals.
- It must accept both signals at 50 or 60Hz.
- It must accept both both resolutions and frequencies on a digital input that supports HDCP.
Note that there's no reference to 1080p anywhere in that definition. To see whether or not that matters, read the rest of the post!
Edit (9/3/2008):
As this thread is still linked to in a FAQ, and especially as it has just been bumped back up to the top of the forum anyway, I should probably make two small additions:
1) Since this post was first composed, we've seen a lot more HD-DVD and Blu-Ray players hit the market. (And we're now seeing HD-DVD on it way out!). HD-DVD and Blu-Ray players now quite often have the ability to output a 1080p signal with a 24Hz refresh rate. At the time I originally composed this, the only way you could easily get 24 frames-per-second playback of hi-def movie material was by using a video processor to correctly deinterlace a 60Hz
1080i signal. Now, most higher-end hi-def disc players will produce 1080p/24, and the result of doing this (on a display that correctly supports it) will look a lot nicer than it will if you have to convert 1080i/60 to 1080p/60. The ability to accept a 1080p signal, and specifically to accept one at 24Hz, is therefore significantly more important now than it used to be.
In fact most "1080p" or "full HD" screens these days can now take a 1080p input at 50 or 60Hz; a smaller number can accept 24Hz input.
If you have to watch a BluRay of HD-DVD movie as 1080p/60 (and this is what nearly all TVs will do if you feed them a 1080i/60 signal, even if they recognise it as film) then you will get a "judder" effect on motion. What actually happens is that some of the original film frames are shown twice, while others are shown three times; this makes motion much less smooth.
2) Another change to the market since I first composed this is that "upscaling
DVD players" are much more common than they used to be. Generally speaking, the way these players are marketed is a con; the salesman will try to convince you that it somehow converts a standard-definition image into a high-definition one. It doesn't. Actually it does the same thing as your hi-def TV already does: if you're watching something that is originally in standard definition (such as a
DVD) and the picture is filling the screen (as opposed to being displayed in a tiny little box in the middle) then it has
been upscaled: that's all upscaling means, blowing up the picture till it fills the whole screen. However, sometimes
DVD players will do a
better job of deinterlacing and scaling than the equivalent circuitry inside the television, and the picture quality will therefore be a bit better if you let the player do the processing and feed a
progressive signal to the TV at the native resolution of the screen.
If you plan to use an upscaling
DVD player, as with a stand-alone video processor. then it is important that the screen is able to accept a
progressive signal at its native resolution, otherwise you won't get the full benefit from the external scaling because the TV will be doing additional processing. If, therefore, you have a 1920x1080 display, and you want to use it with an upscaling player, it is important that it be able to accept a 1080p input signal.
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