Technical explanation for a "double image" effect in 30 fps games on current HDTVs

[ChrisT]

Hi!

Can anyone of you provide a technical explanation of a so called "double image" effect that can be observed on modern HDTVs?

The problem, which is also referred to as "frame doubling" in some threads, can be observed in 30 fps games like PS3's Killzone 3, Battlefield 2, Uncharted series, Little Big Planet, NFS Shift, Motorstorm. It cannot be observed in 60 fps games like PS3's GT5, Ridge Racer 7, Ratchet & Clank series, Wipeout HD or Call of Duty Modern Warfare.

The problem is easy to observe in the first and third person perspective games while looking around or left and right. All moving vertical contours / edges become doubled, which makes the whole background very difficult to observe. The doubled image has the same colour as the original, but seems more or less brighter. While moving, you can see two poles, trees, construction parts, characters, etc. Some people seem to be very insensitive to this problem. On the other hand, I read about others getting headaches and nausea. I personally experience it in some 2D like games with horizontal panning like Little Big Planet, Trine, Underground Garden.

At the end of this post, you can find a few links to videos, screenshots and forum threads related to this issue.

The problem is especially evident on plasma tvs, even the Panasonic top ones (G series, V series), as on plasmas, the doubled image is very sharp. I see it on my Samsung PS51D550. On LCD tvs the problem is also visible, but due to an inherent LCD technology blur, it is much less evident. Doubled image is hidden in the blur as if it was intended in the game. Enabling motion interpolation engines on LCDs like the Sony's motion flow technology at full settings can remove this effect. I have seen the doubled image on Samsung D550, LG LD551, Sony EX 500 and my Samsung 215TW monitor. The problem can be observed both with the use of HDMI and Component connection.

Since the problem is visible on both main technologies tvs (Plasma and LCD), I assume that it has something to do with the way modern consoles generate image and/or send the image to the tv. I understand that modern consoles may be not powerful enough to generate complex worlds with sophisticated physics and AI on an image frame basis, every 16ms (1/60th of a second) in 60Hz. Still, I would expect that even if 2 frames are required for that, the image being sent to the tv is somehow fixed to a 16ms frame boundary. To put it in other words, the same frame is displayed twice for example, while the console does the mentioned processing.

I do not think that anybody knows the way to overcome this issue, at least I have not found the solution on the internet. What I do in case of some of the mentioned games is to set my PS3 output to standard interlaced PAL 576i 50Hz signal and let my plasma do the image processing (deinterlacing, scaling, etc.). As you may expect, everything becomes very blurry, but it is easier on my eyes and possibly brain, than seeing crisp double images in 720p 30fps (majority of the currently available games).

Best regards!
Chris

References:

Panasonic G10 Motion handling Problem Video 1 - YouTube (the effect seems stronger than in real life

probably due to filming)
http://www.youtube.com/watch?v=qPBIETZLTNQ

Samsung plasma D550 owner's thread with X360 Gears of War screenshots (in Polish, but screenshots speak for

themselves and they are the closest to what I personally percieve)
http://www.hdtv.com.pl/forum/676046-post156.html

AVSForum thread: 30fps gaming at 60hz and the effects of frame doubling?
http://www.avsforum.com/avs-vb/showthread.php?t=1107325

AVForum thread: Panasonic 50 and 65inch VT20 pricing/reviews/owners thread (see camera panning)
http://www.avforums.com/forums/12206545-post996.html

 

[Shifty Geezer]

Since the problem is visible on both main technologies tvs (Plasma and LCD), I assume that it has something to do with the way modern consoles generate image and/or send the image to the tv. http://www.avforums.com/forums/12206545-post996.html

Absoutely, definitely not. Games render a single, 'perfect' image with no ghosting unless explicitly chosen to by the developer. There's no way to accidentally duplicate and overlap data within the rendering pipeline.

What you've described is image ghosting caused by the TV being unable to clear the image completely when the next frame is shown, but I don't know that it's as evident as you are saying though the range of TV's I've seens is limited. In a fast moving scene, this'll appear as double images, whereas when the change between images is slight it'll look more like motion blur.

 

[AlNom]

:s Odd, I haven't seen anything of the sort on my plasma (Sammy HP-5054T) in all the 30fps games I've played (aside from the ones that reuse the previous frame for performing pseudo-AA or other post-fx, like Reach or Rock Band). While I'm sure you're not confusing things with screen space motion blur, an intended post-fx, maybe the connection type has something to do with it, i.e. how the TV is handling the signal. I don't see the doubling at all (HDMI, PC connection for 360, 720p for PS3) *shrug*

If anything, I would expect plasma to not exhibit the issue at all because of how the tech works. The response time is instantaneous.

 

[ChrisT]

I agree that this effect does not seem to be related in any way to how the software and hardware renders the graphics. Still, I wonder how your answer relates to the fact that the problem is not observed with 60fps games or to put in other words why people are not bothered by it in case of 60fps games.

My understanding is that the tv receives full frame 60 times a second no matter whether the game runs in 30fps or 60fps. In case of 30fps games, the same frame is displayed twice. Would it be then the issue of contours/edges being further in their location in case of the 30fps games than it is in case of 60fps games making tv's struggle to remove the old image more evident? That would match my observation concerning some tv frame interpolation engines being capable of removing or minimizing the effect.

One thing that puzzles me though is that those are plasma tv owners that complain about this problem and I have always thought that the fast and accurate motion handling is believed to be one of the biggest advantages of that technology.

As I mentioned in my previous post, it seems to be a very individual thing whether the effect is noticed by a given person or not. People, who notice the problem, see it on both plasma and lcd tvs, but on the lcd the effect seems to be easier on the eyes/brain.

 

[L. Scofield]

I don't think it's ghosting, looks too severe for current gen TVs, particularly plasmas. It probably has something to do with some of the post processing those TVs do on the input signal. You know TV manufacturers love to screw with that.

 

[Thowllly]

Way back in the day 3dfx (I think?) released a little program to show this effect.

30 fps on 60hz screens give this double effect. 60fps on 60hz screen and the effect disappears. 60fps with 120hz refresh and the effect is back, but the distance between the double objects is halved.

Very obvious on CRTs, I guess plasmas flicker like the CRTs used to do, and some LCDs also turn off the backlight between frames.

I happens when your eyes tries to track an object moving across the screen. Your eyes moves in a smooth motion while the object blinks into excistence twice in each position (for 30fps games). Relative to your moving eyes these manifest as two different objects, separated by the distance your eyes moves in 1/60 of a second. If the game runs at 20fps you get triple objects and so on.

On displays that does not dim between frames the object just get blurred instead (while it should be perfectly sharp as long as your eyes can keep up with it)

It's a bit hard to explain without visual aids, maybe somebody can explain better, but it's all actually very simple...

 

[AlNom]

I don't think it's ghosting, looks too severe for current gen TVs, particularly plasmas. It probably has something to do with some of the post processing those TVs do on the input signal. You know TV manufacturers love to screw with that.

Hm... good point.

Have they tried game mode? PC HDMI input? Are they letting the console upscale or the TV upscale? Maybe there's something slower about the TV upscaling...

 

[Arwin]

That's really weird. My first guess would also be a post-processing bug in the TV's software. Maybe something that Digital Foundry can look into?

Things I would do to check:

- does behaviour change depending on whether you let your 360 do the upscaling or the TV? The list of PS3 titles you mention perhaps match in whether or not they support 1080p output or not (though I'm not sure).
- what effect do tearing frames have on it?

 

[Thowllly]

It's not really weird (note that my post above is not a description of the problem, but an explanation of why it happens), it's a natural strobing effect. Only way to get rid of it is to match the framerate to the refresh rate, either through native rendering at the refreshrate, or frame interpolation.

 

[ChrisT]

Well, there is a chance that I might have missed something, but I have spent a lot of time playing with both the TV (game mode, movie mode, PC mode) and PS3's settings and connectivity (HDMI, Component). I have even connected my old PS2 and Gamecube only to see the same difference between 30fps and 60fps (Metroid, Rogue Leader 2, GT4, Jak and Daxter) games.

Apart from 30 fps games, the TV performs very well in other areas, especially considering its price. It has got "highly recommended" from http://www.hdtvtest.co.uk. According to that site, it is supposed to be great at games due to its 16ms input lag (HDMI1 labeled to PC). Such low input lag in my opinion eliminates possibility of any sophisticated input signal processing (extending over multiple frames of the input signal).

The TV employs the 600Hz subfield drive, which means that every subpixel is excited 10 times every 1/60 second (~16ms frame). Every subpixel stays alight for around 2ms. The fact that anything from the preceding frame may remain on the screen seems to be completly against this description. Still, it may suggest problems with the circuitry processing the input signal and driving the panel.

The fact that the same double image, but with much softer edges/contours, can be observed on the LCD, which works in exactly opposite way (passing or blocking the light from the backlight), makes the whole thing even more confusing.

 

[ChrisT]

As Arwin suggested that earlier in the thread, it seems that all the 30fps games that exhibited the problem were 720p titles. I will try to download some demos of games that run at 1080p. I assume that not all of them will be running at 60fps.

 

[Shifty Geezer]

Panasonic G10 Motion handling Problem Video 1 - YouTube (the effect seems stronger than in real life probably due to filming)
http://www.youtube.com/watch?v=qPBIETZLTNQ

Samsung plasma D550 owner's thread with X360 Gears of War screenshots (in Polish, but screenshots speak for

themselves and they are the closest to what I personally percieve)
http://www.hdtv.com.pl/forum/676046-post156.html

I've just seen these now and...that's terrible!! Something wrong with them TVs for sure! This isn't an issue with the games rendering, whatever's causing it. My first guess would be the TV's own upscaling or image processing.

 

[ChrisT]

Shifty Geezer, the movie looks much worse than it actually looks when you play. My guess is that the camera frame rate put over tv frame rate somehow intensifies the effect. The same concerns the screenshots. They were made when the image in the background was moving fast horizontally. Nevertheless, the issue is definitely there.

 

[Kasersky]

i thought that the digital signal sent from the console to the television is 60hz regardless of its framerate. i feel this is related to display response time or possibly some image processing or crappy motion interpolation.

try this in bolded, i noticed this issue on my samsung display years ago. pc label changes the way the way the display processes the image.

We weren’t expecting the PS51D550 to do wonderfully or woefully when it came to gaming, because most of Samsung’s plasmas have managed respectable, but not leading amounts of responsiveness lately. The 51D550 exceeded our expectations with an exceptionally speedy 16ms, but we had to use the “PC” input label (accessed by pressing Source > HDMI1/DVI > Tools > Edit Name > PC) to achieve this speed. We’re also not sure if it will enable with all sources, although it did with our Xbox 360 Elite feeding the panel. (This also results in full uncompressed colour being displayed in the 4:4:4 format, for those of you who are both hardcore gamers and hardcore videophiles). Remember that this trick only works on the HDMI1 input, so gamers should give their favourite system priority when connecting.
Without this trick, the Samsung PS51D550 still managed a respectable, but slightly game-altering 31ms of input lag.

 

[ChrisT]

Kasersky, thank you for a response. That was the first thing that I did.

hdtvtest's review was one of my most important sources of information before I made my choice. Still, I think that the "double image" problem in 30fps games is somehow ignored by their editors.

I will write again what I stated in my first post. Once you notice the issue, you see it on both plasma and lcd tvs. Only frame interpolation engines can remove it, but I believe that at the cost of the immense input lag. The problem is that those engines on plasmas cannot be enabled for any type of input signal as it is the case of LCDs. In case of my TV, it can only be done for an interlaced signal. It seems that in case of plasmas, it is somehow assumed by the design that such engines are not needed for high definition progressive sources.

 

[function]

I've got some crazy action going on with my 100hz CRT. It uses motion interpolation that you can't turn off.

On 60 fps Megadrive stuff you get a blurred effect of where things have been and, more bizarrely, a white "preview" outline of where high contrast edges will appear in the future (either as part of an intermediate frame or the next actual frame, can't really tell). It's really, really freaky in Sonic the hedgehog, but also mucks up 25/30/50/60 fps Xbox and Dreamcast games. You get a weird blur + double image effect in all games. Looks fantastic in DVDs though, and really good in tv (better than any damn LCD I've seen), but means I have to keep another 60hz CRT around for older game systems.

This is a different phenomenon to the strobing effect that Thowllly is talking about though. I've played 30 and 60 fps games (including Sonic) on my crt monitor running at 120 fps (completely flicker free when you're looking at it btw, fantastic experience) and the effect that my telly shows isn't even remotely there.
Got to say, 60 fps on my PC with the monitor at 120hz didn't give me any problems. Felt just like a flicker free version of 60fps on 60hz.

(I'm still looking for flat panel tv that seems to do justice to SD consoles. If anyone has any HD TV's they can really recommend in this regard (LCD or plasma) I've love to hear about them.)

 

[Sigfried1977]

Way back in the day 3dfx (I think?) released a little program to show this effect.

30 fps on 60hz screens give this double effect. 60fps on 60hz screen and the effect disappears. 60fps with 120hz refresh and the effect is back, but the distance between the double objects is halved.

Very obvious on CRTs, I guess plasmas flicker like the CRTs used to do, and some LCDs also turn off the backlight between frames.

I happens when your eyes tries to track an object moving across the screen. Your eyes moves in a smooth motion while the object blinks into excistence twice in each position (for 30fps games). Relative to your moving eyes these manifest as two different objects, separated by the distance your eyes moves in 1/60 of a second. If the game runs at 20fps you get triple objects and so on.

On displays that does not dim between frames the object just get blurred instead (while it should be perfectly sharp as long as your eyes can keep up with it)

It's a bit hard to explain without visual aids, maybe somebody can explain better, but it's all actually very simple...

That explanation sounds very reasonable. I've always wondered about the cause of that particular effect myself, because I happen to think it's actually really noticeable.

 

[fearsomepirate]

(I'm still looking for flat panel tv that seems to do justice to SD consoles. If anyone has any HD TV's they can really recommend in this regard (LCD or plasma) I've love to hear about them.)

I've been quite happy with my Samsung LCD. IMO their TVs have the best post for SD content, and it's only gotten better since I bought my TV a couple years ago. I just played Halo 2 and SC:CT on the OXbox with component cables, and they looked great. The DNR got rid of all the artifacts in the analog signal. TBH, I've never noticed anything out of order in 30fps games other than the simple fact that 30 isn't nearly as smooth as 60. I've never thought to compare side-by-side with a CRT to see a difference. I've played GC, N64, and SNES games over the S-video cable, and again, the image looks good, none of those weird compression artifacts, and the response is adequate in game mode for the old twitch platformers.

 

[function]

I've been quite happy with my Samsung LCD. IMO their TVs have the best post for SD content, and it's only gotten better since I bought my TV a couple years ago. I just played Halo 2 and SC:CT on the OXbox with component cables, and they looked great. The DNR got rid of all the artifacts in the analog signal. TBH, I've never noticed anything out of order in 30fps games other than the simple fact that 30 isn't nearly as smooth as 60. I've never thought to compare side-by-side with a CRT to see a difference. I've played GC, N64, and SNES games over the S-video cable, and again, the image looks good, none of those weird compression artifacts, and the response is adequate in game mode for the old twitch platformers.

Thanks for the info. I had been looking at Panasonic plasma's but I've been worried about some of the PAL and SD image processing reports I've read.

How would you say the SD upscale compares to the 360 (just as a point of reference)? Do you have any control over softness/sharpness? I'd rather have more visible pixels than a softer, bilinear kinda look.

 

[fearsomepirate]

How would you say the SD upscale compares to the 360 (just as a point of reference)? Do you have any control over softness/sharpness? I'd rather have more visible pixels than a softer, bilinear kinda look.

Well, I have a PS3...but I'm assuming that it looks the same* for comparison's sake. Anyway, 480p looks as sharp as a PC monitor in my experience, so I've never tried to soften it at all, just messed around with dynamic contrast, DNR, and color temperature to get the exact setting I like (at the time, Samsung had the best user control options by far). The TV also does a very nice job deinterlacing a 480i component signal to the point where it is barely distinguishable from 480p (I had to check the box to be sure DQ8 didn't support 480p!). S-video is okay, but there's a pretty large drop-off in quality to straight AV cables, and I've never tried to use a composite cable. But that's just a plain signal fidelity issue; I can't say it looks any worse than AV did on my old CRT, which is why I bought S-video cables for everything. If you have older Nintendo consoles, you can get a Gamecube S-video cable for very cheap that will work on the SNES and N64 as well.

*BUT THE PS3 DOESN'T HAVE THE SUBTLE BLOOM EFFECT IN MW2 THIS GAME HURTS MY EYES OLOLOLOL!