What is input lag on a LCD TV?

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delerious

Weaksauce
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I've read that, in general, TN panels have little input lag, IPS panels have some input lag, and VA panels have the most input lag. What is the cause of this "input lag"? I've seen some people suggest that input lag is caused by the time it takes the TV to scale/process the incoming signal. So if you watched 1080p material on a 1080p TV, there would be less input lag than if you watched 720p material on that same TV. That makes sense, but then that applies to all of the panel types - TN, IPS, and VA. It doesn't explain why TN panels have a lot less input lag than VA panels.

I read somewhere else that TN panels have low input lag because their color accuracy is the worst, so the TV's processor doesn't have to spend much time processing the incoming signal. But that doesn't seem to make sense - I would think that a TV's processor is independent of the panel. How would the processor know what type of panel the TV is using - or does it? This makes me think that "input lag" is not the same thing as "scaler/processor lag". So what is input lag exactly?
 
input lag is completely unrelated to certain panel types
its entirely caused by the electronics that drive the panel

the electronics in modern hdtvs tend to buffer multiple frames of video instead of displaying them right away
doing that is required for certain gimmicks like motion interpolation and the main cause of input lag
 
Not completely true regarding panel type, TN panels generally perform better in regards to gaming.
 
dowant120hz said:
input lag is completely unrelated to certain panel types
its entirely caused by the electronics that drive the panel
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Then why is there the idea that TN panels have less input lag than IPS panels, which have less input lag than VA panels?

the electronics in modern hdtvs tend to buffer multiple frames of video instead of displaying them right away
doing that is required for certain gimmicks like motion interpolation and the main cause of input lag
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Isn't motion interpolation something that only TVs with 120Hz and higher refresh rates do? But even TVs with just a 60Hz refresh rate suffer from input lag, so I'm not sure that's it.
 
delerious said:
Then why is there the idea that TN panels have less input lag than IPS panels, which have less input lag than VA panels?



Isn't motion interpolation something that only TVs with 120Hz and higher refresh rates do? But even TVs with just a 60Hz refresh rate suffer from input lag, so I'm not sure that's it.
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As for TN having less lag than IPS or VA, that's got to due with pixel response times mostly but *VA panels tend to have stuff like overdrive and other stuff like that with the possibility of adding input lag.

Well but the scalars do everything from scaling up the image to fit the screen if it's not the right resolution to allowing the OSD to show up on the TV. One of the reasons my 30 LCDs have such low input lag is because it has no scalar, so no OSD just brightness control and a input button.

All in all though, it's all down to electronics that drive the panel FAR more than any minute difference panel type may cause.
 
As others have said, it mostly comes down to frame buffering. When you look at PRAD's input lag measurements, they fall loosely on 17ms intervals (or 8ms for 120 Hz) depending on how many frames are buffered. For PC monitors, this is typically 0-2 frames. As far as I know, this is done mostly for temporal processing effects like more advanced overdrive algorithms and intermediate frame generation/insertion. When all the features are left enabled on a modern LCD TV, things can get unusable (for games and even for browsing) with input lags of 140+ ms.

Again as others have said, this is entirely unrelated to actual/advertised pixel response time, which is a disturbingly common misconception. No manufacturers advertise input lag.

I wish PRAD would test more LCD TVs!
 
As for TN having less lag than IPS or VA, that's got to due with pixel response times mostly but *VA panels tend to have stuff like overdrive and other stuff like that with the possibility of adding input lag.
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Response time and input lag are not the same thing, but they are frequently confused, so are the people who say TN has less input lag than IPS and VA just confusing input lag with response time? Do TN panels have about the same input lag as IPS and VA (ignoring the overdrive that you mentioned), but they only seem faster because of a faster response time? I guess since people say TN is faster than VA, that means there aren't any TN panels with low response times but high input lags, and VA panels with high response times but low input lags?

As others have said, it mostly comes down to frame buffering.
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So this frame buffering is independent of the panel type? If there's a particular model of TV that has a panel lottery, the frame buffering would be exactly the same regardless if it's the TN, VA, or IPS version?

Also, does the lag due to frame buffering include the lag from any scaling? I mean if you watched 1080p material on a 1080p TV, there will be lag from the frame buffering. But what if you watched 480p material on that same 1080p TV - would there be additional lag since it also has to upscale the material? Or would the upscaling happen in parallel with the frame buffering so that there's no additional lag?
 
delerious said:
1) So this frame buffering is independent of the panel type? If there's a particular model of TV that has a panel lottery, the frame buffering would be exactly the same regardless if it's the TN, VA, or IPS version?

2) Also, does the lag due to frame buffering include the lag from any scaling? I mean if you watched 1080p material on a 1080p TV, there will be lag from the frame buffering. But what if you watched 480p material on that same 1080p TV - would there be additional lag since it also has to upscale the material? Or would the upscaling happen in parallel with the frame buffering so that there's no additional lag?
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1) Very good question imho. I would suspect the answer is yes, unless the substituted lottery panel needed a different/altered video board to drive it.

2) Lag stacks , mouse/keyboard lag (up to 8ms) + PC/videocard/game lag + Display lag + Internet lag combines together to create a potentially bad gaming experience. And yes, running non-native resolution introduces even more display lag.

Input lag is nowhere near as noticeable with TV/video watching as it is with twitch gaming.
 
delerious said:
Response time and input lag are not the same thing, but they are frequently confused, so are the people who say TN has less input lag than IPS and VA just confusing input lag with response time? Do TN panels have about the same input lag as IPS and VA (ignoring the overdrive that you mentioned), but they only seem faster because of a faster response time? I guess since people say TN is faster than VA, that means there aren't any TN panels with low response times but high input lags, and VA panels with high response times but low input lags?



So this frame buffering is independent of the panel type? If there's a particular model of TV that has a panel lottery, the frame buffering would be exactly the same regardless if it's the TN, VA, or IPS version?

Also, does the lag due to frame buffering include the lag from any scaling? I mean if you watched 1080p material on a 1080p TV, there will be lag from the frame buffering. But what if you watched 480p material on that same 1080p TV - would there be additional lag since it also has to upscale the material? Or would the upscaling happen in parallel with the frame buffering so that there's no additional lag?
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1. Yes that's true, I do believe that people with only a laypersons knowledge of LCDs (or none at all) probably confuse the 2 leading to that rule. Another thing that tends to exacerbate that stereotype is the fact that TN panels are usually cheap and thus have very basic scalers, adding little input lag while IPS and *VA panels tend to have nicer scalers with more features but adding more lag.

And as always there are exceptions to the rule, there are certainly *VAs that have been great for gaming as well as TNs that have sucked.

2. That is a great question and I think it has to do with particular costs in relation to the product (if perhaps a newer revision comes along with a different panel later, they may use a different scaler to reduce cost).

3. As was said above, lag stacks but is really only a problem with interactive media due to the need to time responses to the image on the screen.
 
Don't forget that not all 120hz TVs are true 120hz. They actually upscale a 60hz signal. AVS has a really good thread about the input lag on different tvs.
 
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