DTV Refresh Rates & Frame Rates
When shopping for a DTV, you can easily be overwhelmed by all the facts and figures surrounding today’s televisions. Two of the most misunderstood features of DTVs are screen (video) refresh rates and frame rates (film). How they work together to provide smooth as glass movement on the screen, even during fast-moving action and horizontal panning, is one of the awesome specifications of DTV.
Similar to still photography, studio movies are shot using film which consists of individual frames vertically connected together. When those frames are displayed before the human eye in succession and at a specific speed (e.g., 24 frames per second), it gives the appearance of uninterrupted movement-hence the name motion picture. The viewer cannot detect the change from frame to frame due to an effect known as persistence of vision. The eye temporarily retains a visual image of each frame for a fraction of a second after it has been removed. The illusion of motion from frame to frame is caused by a psychological effect known as the phi phenomenon.
Another way to look at frame rates and refresh rates is to think of frame rates as film and refresh rates as video (screen). When the movie for a DVD is made, it is filmed. When you watch a DVD, you enjoy it on a video display (your TV screen). Confused? Keep reading; you’ll figure it out!
But first, let’s understand some definitions and then we’ll compare refresh rates and frame rates.
DEFINITIONS
Frame Rates
As new and improved as DTVs are, they still had to bring a little of the old school along with them-namely 24fps (frames per second). This is the rate at which cameras used by movie studios, even today, film movies. When sound was introduced to the film industry in the late 1920s, the sound head on the camera required a consistent and constant speed. While 16fps were used to record silent films, 24fps was chosen for talkies because it was the slowest speed that would still provide sufficient sound quality.
Refresh Rates
The number of times per second an image is refreshed (a new image is displayed) on the screen is called the refresh rate. The value of the refresh rate is expressed in hertz (Hz), which is the basic unit of frequency for the International System of Units (SI). If a DTV has a refresh rate of 60Hz, it means the frequency of the screen refreshing is 60 times per second. Newer DTVs can have refresh rates at high as 120Hz or 240Hz.
Judder
As long as the refresh rate of your DTV is 60Hz or higher, the refreshing of the screen happens so quickly your eyes won’t even notice it unless there is very fast action on the screen. This might cause the motion around the action to appear to shake, wobble, or vibrate-aka judder.
Judder is only an issue for film and is not apparent when video is used. For movie theaters film is still required but many of today’s television shows are videotaped which is easier to work with. Video frame rates for videotaped programs can be 30fps or 60fps. Since 30 and 60 divide evenly into 60, there is no judder at all and quick movements on the screen appear very smooth.
REFRESH RATES & FRAME RATES – Numbers Don’t Match
Let’s do the math. DVDs and Blu-ray Discs are filmed at 24fps but the standard refresh rate for DTVs is 60Hz. The numbers don’t match up at all-24 per second and 60 per second. There is a solution to help them co-exist using 3:2 pulldown (converting film to video). The first frame of the 24fps will be repeated 3 times, the second frame of 24fps will be repeated 2 times and so on.
60Hz Refresh Rate
The following is a chart showing one second’s worth of film frame made compatible with one second’s worth of video using the 3:2, 3:3, and 5:5 pulldown methods.
72Hz Refresh Rate
The following is a chart showing one second’s worth of film frame made compatible with one second’s worth of video using the 3:3 pulldown method.
A feature of the 72Hz refresh rate is the ability to switch back and forth between 60Hz and 72Hz depending on the incoming source signal. Unlike the 60Hz refresh rate, 72Hz is evenly divisible by the frame rate (72Hz / 3 = 24fps). When the refresh rate is evenly divisible from the frame rate, it is referred to as the native playback. The movement of action appears smoother in native playback because every frame of the film is duplicated the same amount of times-in this case 3 times per frame.
120Hz Refresh Rate
The following is a chart showing one second’s worth of film frame made compatible with one second’s worth of video using the 5:5 pulldown method.
Right now, 120Hz is the crème de la crème of refresh rates. The 120Hz refresh rate can display all video in native playback.
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24fps x 5 = 120 |
30fps x 4 = 120 |
60fps x 2 = 120 |
FINAL THOUGHTS
The frames per second of frame rates and the cycles (frequency) per second of refresh rates work together to help give you the best image the human eye can discern. The interactions between the two work together so your eyes see seamless and continuous movement although the film is made up of individual still frames.
The higher the refresh rate, the smoother the image on the screen will appear to you. Smooth movement with no judder will keep action movies and sports looking incredible.
Still have questions?-feel free to speak with an hhgregg associate. We offer the best-educated associates in the business who are smart enough to make it simple. Price and Advice Guaranteed!
