Do high refresh rates prevent motion blur on TVs?

Question

Several TV options are available and most sellers tend to convince you that the higher frequency the TV is the better it is. For example this site says:

LCD TV manufacturers have tried doubling their refresh rates to 120Hz, and then doubling them again to 240Hz. As a result, modern LCD’s can update their images very quickly, drastically reducing motion blur. For watching sports, or even playing fast-moving video games, higher refresh rates will make a big difference on an LCD TV.

Given that the perceptible rate of change for the eye is 1/60th of a second per image or 60Hz I am dubious that any difference can be perceived.

Is there any proof that TVs with frequency greater than 60Hz are better than 60Hz?

Answer 1

Refresh rate: First of all, in case of modern flat screens, be it LCD or OLED, refresh rate is misnomer. It's actually frame rate — rate at which the TV is able to change image from one frame to another. Pixels on these panels are constantly on, thus there is no flicker on LCD or OLED. On the other hand, CRTs had refresh rates, since pixels would glow fully only after the refresh and then gradually go black. Hence flicker in case of low refresh rate on CRT. However, on CRT refresh rate did not mean frame rate. In fact even 100Hz CRT TV would still have frame rate of SDTV - 25Hz.

HDTV resolution and frame rate: In the future HDTV will go beyond 60Hz, H.264/MPEG-4 AVC — code used in modern HDTV defines frame-rates up to 172Hz. However, at the moment it's purely experimental. Highest standard currently in commercial use is 1080p60 (1920×1080, progressive, 60Hz). ESPN HD transmits sports in 720p60 (1280×720, progressive, 60Hz), they argue that lower resolution progressive (ie. non-interlaced) gives smoother experience for sports, than 1080i (1920×1080, interlaced, half-frames at 60Hz, roughly equivalent to full frames at 30Hz).

Ok, thus far it seem like having TV with more than 60Hz doesn't help you much. But there is a catch. Actual parameter for LCD (similarly for OLED) is response time/pixel latency. Higher pixel latency leads to ghosting (motion blur). In case of TVs LCDs with lower response time are marketed as TVs with higher "refresh rate".

(source: TFTCentral)

Conclusion:

• Can faster displays result in less motion blur? — yes, they can.
• Is it because of "refresh rate"? — Not really, it's LCD's response time that matters, which for marketing purposes is translated to higher "refresh rate". However with current HDTV signal, there are only 60 frames per second.

Some TVs use motion interpolation, in which case they "fake" intermediate frames by interpolating consecutive real frames. This reduces "jumpiness" known as judder. It's more significant at lower frame rates as 24Hz of movies or 25Hz of SDTV. Motion interpolation is completely unrelated to motion blur.

Answer 2

Two separate concepts are being confused here:

The Flicker Fusion Threshold (a.k.a. the Critical Flicker Fusion or CFF) is the minimum rate that a screen must flash before the human eye stops seeing it flicker and starts seeing it as a steady dot. It is affected by many factors, and it is dependent on individuals.

Under some conditions 60 Hz is entirely flicker free; however. under other conditions even 75 Hz may produce flicker. - Source

If you have ever been in a situation where some people in the room find the flickering of the fluorescent lights annoying, while others can't understand what the problem is, you have experienced that.

The US FAA recommendations agree:

Non-interlaced CRT displays shall be refreshed at a rate of at least 80 Hz, preferably 100 Hz or higher, to avoid the perception of flicker for photosensitive users.
[Source: Bauer & Cavonius, 1980; Cardosi & Murphy, 1995; DOE, 1992; NASA, 1995; Vanderheiden & Vanderheiden, 1991]

Some experiments that demonstrate that higher than 60Hz rates can make a different to perception:

• Visual discomfort reduced as frequency increased to 180 Hz.

• Differences in brain action between watching 60 and 72Hz screens detected on EEG

• Reading speed increased by 3% on 500Hz over 60Hz

However, this Flicker Fusion Threshold is a different measure to the minimum refresh rate to observe smooth movement by objects. Ref

I don't want to argue whether a higher refresh rate is better in the case of television, because it is a subjective call. Some people argue that the algorithms used to interpolate movement when the input rate is slower than the refresh rate leads to an alien or artificial feel to the movement.

Answer 3

The advantages of higher frequencies depend a lot on what you are viewing. In most resolutions, as per ATSC standards, the signal your TV will receive is likely to be either 24 or 30 frames per seconds, as the 60 frames per seconds format is not yet standardised to all display resolutions (as per ATSC standards, 60fps is supported on 720p resolutions, but not yet on 1080p).

If your television has a 60hz frequency (which is the standard in North America to be in sync with the AC current provided), this means that a signal sent at 30fps will display each frame for exactly 2 60th of a second. This is very good, as each image will be displayed for the same duration. However, if the signal sent to your television is set at 24fps, this means that half of the images will be displayed 2 60th of a second while the other half will be displayed 3 60th of a second. This is what is called a 2:3 pulldown. In slow shots with a steady camera, this can result in telecine judder. In fast moving shots, this judder might be le ss visible, but it can reduce the overall sharpness of the image, leading to motion blur.

Most motion pictures are still shot using 35mm film at 24 frames per second, as this film type has a very good definition. In older televisions, this lead to a large use of the 2:3 pulldown. However, modern HDTVs also use what is known as motion interpolation to generate additional images in between the frames to artificially raise the footage to 60fps. On a 24fps footage, the 2:3 pulldown is first used to raise the signal to 30hz, then interpolation is used to raise it further to 60hz. This reduces the judder slightly, but it does little to alleviate other sources of motion blur, such as your TV's pixel response time, resolution resampling or compression algorithms used in some video formats.

To alleviate those other sources of motion blur, TV manufacturers have begun making TVs with higher refresh rates. As 120hz is a multiple of both 24 and 30, it can eliminate the need for the 2:3 pulldown. In effect, each frame of a 24fps footage would be displayed 5 times before switching to the next, eliminating judder. However, a lot of 120hz TVs are currently using their faster frame rates only to reduce motion blur on regular 60hz signals by using more motion interpolation. On such TVs, the 2:3 pulldown is used to convert the 24fps footage to 60hz, and only then does the motion interpolation take place. This double conversion of the original signal can lead to visual artefacts in the end signal, reducing the quality of the image in a way that is probably assimilated to motion blur although it does not stem from the same source.

This HDTV Magazine article gives good complementory information on the subject.

My personal conclusion would be that a good 120hz TV can in fact be better than a 60hz one. However, I see no need for a 240hz TV as it does nothing more than a good 120hz one to reduce the need for the 2:3 pull down. The only thing it would do is add more interpolation to the footage, which can result in unnaturally smooth motion and an equally unnatural cardboard cutout look (characters appear as cardboard cutouts in front of the background).

Answer 4

The term "motion blur" appears to be a misnomer. When taking the answer literally, the answer is no. In fact, higher refresh rates can increase motion blur.

We've discussed perceptual flicker and other perceptual differences between low and high refresh rate, but we hardly discussed motion blur.

Motion blur is not necessarily a bad thing. There are (at least) three relevant meanings.

(1) Motion blur in the video source. This is the same as motion blur in a still image. When a short shutter time is used, little motion blur is visible. For example, for sports photography one often uses 1/500 sec or less.

http://photo.tutsplus.com/tutorials/shooting/8-tips-for-taking-sports-photos-like-a-pro/

The same goes for video. While a video camera may record 50-60 images per second, the shutter time used for each image is actually often much less. With a lot of light, 1/500 sec is pretty normal for a video camera. When you look at a movie, you can see the difference between a 50 Hz video with 1/500 second shutter time, and a 50Hz video with an 1/50 second shutter time (which is obviously the maximum). So, this type of motion blur has nothing to do with the refresh rate, except that the maximum shutter time is given by the frame rate. My own experience is that a blurry video actually looks better, as very short shutter times produce a "choppy" animation.

http://alexschwindt.typepad.com/alexschwindtcom/2012/02/10-common-mistakes-dslr-video-shooters-make.html

Motion blur is also generated artifically in video games for this reason.

http://http.developer.nvidia.com/GPUGems3/gpugems3_ch27.html

http://gamingbolt.com/ten-games-with-beautiful-motion-blur

(2) Motion blur as an artifact of the display (=TV). When the latency of the display pixels is too high for the frame rate, you get a blurry image. So, increasing frame rate without appropriately shortening the display latency may cause an increase in motion blur.

Display latency is usually specified as part of product specifications. See also: http://www.tftcentral.co.uk/speccontent.htm

(3) Perceptual motion blur, caused neither by video source or display. The human eye has (complex) limitations so as to what speed animation it can keep up with. Very fast changes will result in a blurry perception. Low light will increase perceptual latency and hence will affect blurriness (as is demonstrated by the Pulfrich effect). A higher frame rate may increase perceptual motion blur, as there is simply more temporal information for the eyes to digest. Again, more motion blur will likely look better here.