Refresh rates explained: 60Hz, 120Hz & more

A refresh rate is the number of times per second that a monitor draws data. This is not to be confused with frame rate, which is the rate at which a device produces a unique image. Rather, a refresh rate is how many times per second an image is displayed. Refresh rates are usually measured by hertz (cycles per second). So, a monitor that produces an image 60 times per second has a 60Hz refresh rate.

Refresh rates (or “flicker rates”) of film projectors

What understandably confuses people is why content is refreshed at a different speed than its native frame rate. If a movie is filmed at 24 frames per second, why is it projected at 48 or 72 frames per second? There are a few reasons for this.

Like motion picture cameras, film projectors have a shutter because the film must advance whilst hidden. If the film were to advance while the image was projected, audiences would see the “smear” of the film moving. Unlike a motion picture camera, which uses a one-bladed shutter, a film projector uses (at the very least) a two-bladed shutter, which flashes each frame twice, resulting in a 48 hertz refresh rate. The film could advance with a one-bladed shutter, but the human eye can perceive slow flashes of light, which causes a flicker. So, to reduce flicker, the film is shown “faster” at 48 hertz, with each of the 24 frames flashed twice. In terms of film projection (rather than TV or digital), refresh rate is more commonly referred to as flicker rate; but it essentially means the same thing.

Many film projectors have a three-bladed shutter, which flashes each frame three times, resulting in a 72 hertz refresh rate. These film projectors are more desirable as the higher refresh rate produces less flicker and allows for a brighter lamp, resulting in a brighter image. Since the advent of digital cinema, film projectors are almost entirely obsolete and replaced by digital projectors. Like TVs, digital projectors do not require a shutter to close the light source, since no film is advancing.

Why 60Hz was the standard refresh rate for TVs

Given the limitations of vacuum tube technology in the 1950s, televisions were required to refresh at AC line frequency. AC (alternating current) is the flow of electric power running through our walls. AC line frequency is 60Hz in the U.S. and 50Hz in Europe. Since CRT (cathode ray tube) televisions generate an image by scanning one line of data at a time, a lower multiple of 60, like 30Hz would produce too much flicker. Similar to the problem projectionists had with 24fps footage, in which displaying the frame rate natively induced too much flicker, 30fps footage is shown faster, (although interlaced) at 60Hz.

Displaying a 24fps source on a 60Hz television requires a telecine process known as 3:2 pulldown. Using interlacing, 3:2 pulldown combines lower and upper fields from different frames in order to create whole new frames, which converts the footage from 24fps to 30fps; compatible with a 60Hz refresh rate. This process can produce some unwanted telecine “judder” artifacts. Progressive scan technology has improved this process. Rather than combining lower and upper fields of frames to create new frames, 3:2 pulldown in progressive mode simply repeat whole frames.

Plasmas, LCDs, and high refresh rates

The current market of plasmas and LCDs (including LEDs and OLEDs) offer refresh rates as high as 960Hz. The most common of these high-speed refresh rates are 120Hz and 240Hz. How plasmas and LCDs achieve higher refresh rates than 240Hz, or if said refresh rates are true, is a topic for another article. However, high refresh rates in LCD monitors help to eliminate motion blur generated by LCDs (not to be confused with motion blur generated by a motion picture camera and recorded to an image).

A refresh rate of 120Hz, since it’s divisible by 24, is ideal for removing the judder effect of 3:2 pulldown associated with a 60Hz refresh rate. Instead, a 5:5 pulldown is used, where each frame of the 24 frames is shown 5 times. Although this refresh rate is effective in removing judder, many film enthusiasts still prefer the slower refresh rate of 60Hz, as it’s closer to the speed of a film projector. Some TVs even offer refresh rates of 48Hz and 72Hz.

An important thing to remember when considering a television with a high refresh rate is that most (if not, all) film and TV content is filmed at 24, 25, 30, 50 or 60 frames per second; no higher. In fact, although the current market of TVs offer high refresh rates, those TVs cannot actually ingest sources above 60 frames per second. So many TVs package their high refresh rates with motion interpolation technology to “fake” in-between frames.

Motion interpolation technology creates new frames

Motion interpolation is a process by which intermediate animated frames are generated in-between existing frames. Manufacturers have different names for this technology; anything from Auto Motion Plus, TruMotion, ClearScan, MotionFlow, SmoothMotion and more. Since the frames in-between the existing frames don’t actually exist, the TV “paints” these frames using the surrounding frames as a reference. This can create some very displeasing artifacts.

Because of the added frames, motion interpolation creates a “Soap Opera Effect” in which everything looks as though it was filmed with higher frame-rate cameras. Some people like this “faster” look, others don’t.