Refresh Rate

Closely related to the concept of temporal rate is refresh rate, which is the number of pictures a display shows per second. Although temporal rate is often measured in frames per second (fps) or hertz (Hz), refresh rate is typically measured only in Hz.

As discussed in the TV Broadcast Formats article, television is usually transmitted at either 50Hz or 59.4Hz depending on what part of the world you are talking about. So, for many years, TVs simply had a refresh rate of 50Hz for the 50Hz countries and 59.94Hz for the 59.94Hz countries. However, this is no longer the case.

The 3-3-3 Plan for Understanding Refresh Rate

There are three general principles that are crucial to understanding refresh rate. These are:

A computer monitor or plasma can have multiple refresh rates. LCD HDTVs in 50Hz countries have multiple refresh rates as well, but LCD HDTVs in 59.94Hz countries usually (though not always) only have one refresh rate that is 60 or a multiple thereof [1].
The lower the refresh rate used by a film projector, CRT, or plasma, the more display flickering will be noticeable. Conversely, the higher the refresh rate used by a film projector, CRT, or plasma, the less display flickering will be noticeable. Display flickering is normally caused by showing blackness between frames. Conventional 50/60Hz LCDs don't show blackness between frames and therefore shouldn't flicker [2]. If such an LCD does flicker, it is probably the result of fluctuations in the brightness of the displayed frames [3].
With the exceptions of CRT TVs in 59.94Hz countries and CRT computer monitors, the input fps and the refresh rate don't have to match. However, for the content in question to be shown with total accuracy, the refresh rate should be a multiple of the fps.

Given these principles, a refresh rate other than the default 50Hz or 59.94/60 Hz offers the following possible benefits:

If the display has a refresh rate that is a multiple of 24, then 24p content - which is mainly movies - can potentially be viewed without resorting to the 4% speedup or 3-2 pulldown methods described in the TV Broadcast Formats article. LCDs typically accomplish this by having a 120 or 240 Hz refresh rate, since both 120 and 240 are common multiples of 24 and 60. On the other hand, plasmas will typically accomplish this with an additional 72Hz or 96Hz refresh rate. Some plasmas, notably those from Panasonic, have a 48Hz viewing mode for 24p material. However, since CRTs and plasmas are unpleasant to view when the refresh rate goes too low, viewing 24p material with a 48Hz refresh rate on a plasma creates, in the words of CNET Senior Editor David Katzmaier, "unwatchable flicker" [4]. See the Display Buying Tips article for more information on correct 24p viewing.
Some CRTs [5] and plasmas [6] in 50Hz countries double the refresh rate for 50Hz video from 50Hz to 100Hz to remove flicker.
The last benefit is perhaps the most obvious - a higher refresh rate means you can show more frames. CRT computer monitors could accept 160fps, LCD computer monitors have a max input of 144fps, and HDTVs have a max input of 60fps. Additional frames shown by an HDTV are essentially guessed frames generated by the HDTV. Unfortunately, these guessed frames can have visual oddities, since they are, after all, just a guess [7 page 2]. These visual oddities, known as artifacts, are more noticeable for 24p video than for 50Hz or 59.94/60Hz video since more frames are being guessed. Therefore, as a compromise, some HDTVs allow you to turn off this guessing, a.k.a. motion interpolation or smoothing, for 24p video but leave it enabled for non-24p video [8 page 2].

Finally, there are a few times when the "refresh rate" of a display is not really the refresh rate, at least in the traditional sense. The reality is that, traditionally speaking, 240Hz is the highest refresh rate available in displays today. Instances of this "fudging" are:

Many LCD computer monitors that accept video at 75Hz or some other number that is not a multiple of 50 or 60 will actually remove frames from the incoming video to get it to match the REAL refresh rate of 50Hz or 60Hz depending on location [9] [10]. This is why even though an LCD computer monitor may be able to accept video from a computer at 75Hz, the manufacturer will usually recommend sending it at 50/60Hz instead [11 page 6].
As already mentioned, LCDs normally don't flicker. This actually a bit of a double-edged sword. On the one hand, it means that an LCD can have a refresh rate of 50Hz or 60Hz without flicker. On the other hand, the lack of blackness in between frames can heighten the perception of motion blur, which is when moving object may appear to "smear" across the screen. One way of combating this is to use the already mentioned technique of generating new frames between existing frames. However, current HDTVs can only show a max of 240fps. Beyond that, the only way to further combat blur is to actually add blackness in between frames in the form of black frame insertion or a scanning backlight [12]. This added blackness between frames will usually be counted by manufacturers as additional frames, meaning that manufacturers will double the refresh rate from its original number [13] [14]. Somewhat in line with this is Samsung's "Clear Motion Rate" and Sony's "Motionflow XR", although their highest rating - 960 - quadruples rather than doubles the actual refresh rate [15] [16] [17 pages 3-4].
Plasma manufacturers will often advertise the rate of a plasma's "sub-field drive", which was usually 480Hz but is now typically 600Hz. This is NOT, however, the refresh rate of the display. Basically, a plasma shows each frame in parts, and the number of frames times the number of parts per frame gets you the rate of the sub-field drive. So, to take an example, those 600Hz plasmas you see are really 60Hz plasmas that show each frame in 10 parts, but each of these parts must be showing the same frame [18].

Judder & Cadence

As already mentioned, some HDTVs will let you turn off smoothing for 24p video but leave it enabled for non-24p material. But how, exactly, would you go about this? Although it varies somewhat between the different TVs, take a look at the picture below [8]:

The above picture shows the "Auto Motion Plus 120Hz" menu for the Samsung UNB6000 series of HDTVs. The "Blur Reduction" option controls how much smoothing is used for non-24p video, while the "Judder Reduction" option controls how much smoothing is used for 24p video. Setting the "Judder Reduction" to 0 causes each frame of 24p material to be shown 5 times (120/24 = 5), thus allowing you to view movies without that pesky 4% speedup or 3-2 pulldown [8 page 2].

But wait a minute - what the heck is judder, anyway? Believe it or not, there are actually two very different definitions of judder. On the one hand, judder can refer to the unevenness caused by the 3-2 pulldown process. On the other hand, judder can refer to the stuttering and choppiness that is inherently part of anything recorded at a mere 24fps [19]. If someone is talking about dejudder or judder reduction, that person is most likely discussing a smoothing system in a display designed to reduce the 2nd type of judder. This is particularly true if the term judder reduction is used since 3-2 pulldown cannot be "reduced" - it is either there or it isn't.

Another term you might find thrown around is cadence. Basically, if someone says that a display preserves the cadence of film, it is just a fancy way of saying that a display can show movies without using a 4% speedup or 3-2 pulldown.

Flicker-Free Refresh Rates

The refresh rate needed to no longer see flicker depends not only on the refresh rate used but a variety of other factors as well [19] [20].

For film projectors, which run at either 48Hz or 72Hz, 48Hz is adequate for removing flicker, although 72Hz may be needed for some people [21].

For CRT TVs and plasmas, 59.94/60Hz is, for the most part, good enough [19].

For CRT computer monitors, it is generally agreed that at least an 85Hz refresh rate is needed to remove flicker [19], although some like myself may need a higher refresh rate such as 90Hz. This means that for both flicker-free and cadence-correct viewing on a CRT computer monitor, 96Hz would be required for 24p, 100Hz would be required for 50i/p, and 120Hz would be required for 60i/p.

Response Time

The response time of a display is how long it takes any given pixel to change from one color to another color. It is not really an issue for CRTs or plasmas, but it is for LCDs.

The response time for a display is measured in milliseconds (ms), which is one-thousandth of a second. At a bare minimum, the response time of a display should be the duration of each frame, i.e. a 60Hz display should have a response time of at least 16ms (1000/60 = 16.6). However, an LCD will still benefit from a lower response time. This is because the change from one pixel color to another pixel color will produce colors that were never in the original video, i.e. a pixel will be gray during the time it changes from white to black or black to white. A lower response time minimizes the appearance of these changes.

Panel Self-Refresh (PSR) & Selective Refresh (SR)

On Feb. 7, 2011, the Video Electronics Standards Association (VESA) announced the approval of Embedded DisplayPort (eDP) 1.3, which is used in laptops and other products with embedded displays [22].

One interesting feature of eDP 1.3 is PSR, which allows the computer to stop sending frames to the display if the video doesn't change. This is turn reduces power consumption and can save on battery life [23 page 25].

Although PSR is a step in the right direction, the ideal would be to use SR. In SR, only changed information is sent from the computer to the display. This would further reduce power consumption and could also improve game performance. For instance, consider the example of the computer game StarCraft II. When used at high frame rates, this game caused problems for some computer users. The solution to this problem that was posted by Blizzard, the company that makes the game, was to limit the frame rate [24] [25]. However, if there was a video interface in use that used SR, the need to limit the frame rate would be greatly reduced if not entirely eliminated, thus giving better game performance. Additionally, the game would be able to generate more frames with SR than without SR due to freed up power.

So, is there any plug that uses SR? Although the creators of the Digital Visual Interface (DVI) plug, which was approved on Apr. 2, 1999 [26 page 1], commented that SR would be possible over DVI [26 page 8], this never came to be. On Apr. 18, 2004, VESA approved the Digital Packet Video Link (DPVL) standard that could provide SR "utilizing conventional digital monitor interfaces" [27 page 1], but this standard has yet to see widespread use.

However, there may still be hope for the use of SR. On Sept. 10, 2012, VESA announced the upcoming release of eDP 1.4, which allows for "a new partial-frame update capability for Panel Self Refresh (PSR)" among other things. This is basically SR. Embedded DisplayPort 1.4 "is anticipated to be released in October 2012, and utilized in commercial products as early as 2014" [28].

NVIDIA G-SYNC

The most recent advance in the area of refresh rate has been NVIDIA's G-SYNC technology. The main benefit of G-SYNC is that it allows for displays to have a variable refresh rate [29]. As a bonus, G-SYNC displays will also have a strobing backlight mode to reduce motion blur [30]. You can read more about G-SYNC here.

Sources

21. Thread started by SofaKing on May 25, 2011 3:04 PM. What's the purpose of a 24 Hz TV that just converts to 60 Hz?. ©Ars Technica 1998-2013.

23. Craig Wiley. eDP™/Embedded DisplayPort™: The New Generation Digital Display Interface for Embedded Applications. Dec. 6, 2010. VESA.

30. Chief Blur Buster. CONFIRMED: nVidia G-SYNC includes a strobe backlight upgrade!. Oct. 19, 2013. Blur Busters.