What is a 120 Hz Frame Rate Conversion and How Does It Impact Video Quality?

For the best cinematic viewing experience, a movie theater is the place to be. Films are displayed at the speed they were shot in, which is 24 frames per second. There isn't a loss of quality when the film is projected onto the large screen from the movie projector. The same, however, can not be said for home theaters.

Most high definition televisions display images on the screen at a rate that is incompatible with the native frame rate of films. As a result, the quality of the picture isn't as great. Newer HDTVs have been getting closer to the quality of movie projectors by increasing the refresh rate. By doing so, there is better compatibility with the frame rate of films.

What is Refresh Rate?

There are several specifications which detail how well a television displays an image. Resolution is the most popular, and the options include 480, 720, and 1080, with interlaced and progressive specs for each one. So is 1080i or 1080p the best possible image quality for a high definition television? Not necessarily. The next step would be to consider the refresh rate.

The refresh rate is the number of times an image is repeated on the screen within a specific period of time. For example, a 60 Hz LCD HDTV would display the same image 60 times per second. A 120 Hz HDTV would draw an image 120 times per second. There are models that repeat the same image 240 times per second (240 Hz).

The refresh rate is measured in hertz, which is the unit for frequency. It is described as one cycle per second. The refresh rate of a television plays an important role in how well it displays film, which is shot at a frame rate that is different than the rate television signals are transmitted.

What is Frame Rate?

Frame rate is the number of frames displayed per second. Each frame is a different image that by itself is just an ordinary motionless picture. By displaying multiple frames in a short period of time, motion picture occurs. The frame rate of film is 24 frames per second. The frame rate of television signals in the NTSC zone (North America, parts of South America, Japan, etc.) is 29.97 frames per second. This discrepancy requires some adjustments in order to properly view films.

Relationship Between Refresh Rate and Frame Rate

In order to view a film shot at 24 frames per second on a television that displays television signals sent at 29.97 frames per second, extra frames have to be added to the film. One way to mitigate the effects of the missing frames is to adjust the rate at which the television displays the same image on the screen to a multiple of 24. For example, by increasing the refresh rate of the television from 60 Hz to 120 Hz, that is displaying the same image 120 times per second, there will be enough repeated frames to fill in the gaps so that the viewer doesn't see the effects.

Downside of Incompatible Refresh Rate

The most common problem seen with films converted to video is motion blur. This occurs during panning sequences in which the camera pans across a landscape. The video may also appear jagged during sudden camera movements. These effects are usually called judder, which is a byproduct of converting film to video in a process called 3:2 pulldown. In this process, three frames of an image are followed by two frames of the next image. Then, three frames of the third image is followed by two frames of the fourth image. The sequence continues.

Benefits of 120 Hz HDTVs

The advantage of a 120 Hz HDTV is that it displays film in its native frame rate, without the effects associated with converting film to video. The 120 Hz frame rate conversion is as follows: Refresh rate of TV / frame rate of film = 120 / 24 = 5. Since the number of times an image is displayed on screen, divided by the number of frames in the film is even, there is no need for 3:2 pulldown or extra frames. Similarly, 240 Hz HDTVs have a refresh rate that is divided evenly by the frame rate of films. 60 Hz HDTVs on the other hand, do not convert frame rates evenly.

Before purchasing your next high definition television, consider the impact of the refresh rate on picture quality. HDTVs with a refresh rate that is a multiple of 24 generally display a smoother picture. 120 Hz and 240 Hz LCD televisions fall in this category. With the improved quality, videos will appear like they would in a movie theater, or close to it. Most of these televisions have a higher energy requirement than standard televisions, so expect an increase in your monthly energy bill.