The Relationship Between HDTV Refresh Rate and Frame Rate

When selecting the right HDTV for your home theater, it is important to understand the relationship between film frame rates and screen refresh rates. There is an interaction between the two that plays an important role in the quality of the video displayed on your HDTV.

The refresh rate is the number of times per second that a screen image is refreshed. This is similar to the frame rate found with film. The refresh rate is expressed in hertz (Hz). Hertz simply refers to the frequency or cycles per second. Most high definition televisions have a refresh rate of 60Hz, which means that a new image is displayed 60 times per second. A few of the newer HDTVs have refresh rates of 72Hz and many high-end models have 120Hz refresh rates.

There is a mismatch between the standard television refresh rate of 60Hz and the frame rate of films. Movies are still filmed with a frame rate of 24 frames per second or 24fps. From what I have read, this goes back to the days of hand crank cameras and when someone determined that 24fps was the speed at which films no longer looked choppy. DVDs, including Blu-ray discs are recorded at this frame rate. The problem is that the standard frame rate of film and DVDs does not match the refresh rate of HDTVs.

The solution used by HDTVs is to use what is called a 3:2 pulldown, which means that one frame is displayed two times, while the next is displayed three times, and the next two times, and the next three times, etc., etc. With 60Hz this happens so fast that your eyes don’t notice it, except when there is a lot of motion on the screen, which can create a phenomenon called judder. Judder occurs when the motion appears shaky. It manifests as a strobing effect. You can sometimes notice this in an action flick when a busy background, such as a jungle scene, is in motion. Judder was very apparent in some of the Coliseum scenes in the move Gladiator. If you wondered why some of the actions scenes on your TV look like they have a strobe effect, blame it on judder.

These issues are not apparent when video is used, rather than film. Film is still required for theaters, but video is much easier to work with for television. Many of the shows on television are video taped, not filmed. Video frame rates can be 30fps or 60fps. Because 30 and 60 divide evenly into 60, there is no judder and action shots appear very smooth.

The 72Hz Refresh Rate

Pioneer has introduced a new plasma technology that uses 72Hz. I understand that it can detect the frame rate and can switch back and forth between 60Hz or 72Hz based upon the incoming signal. Why 72Hz? 24 x 3 = 72. Action therefore appears much smoother. When the frame rate is evenly divisible into the refresh rate, it is referred to as playing at a native playback. The action in native playbacks is always smoother.

120Hz Refresh Rate

120Hz high definition televisions can be found at the higher end of the market. Sony’s XBR4 LCD HDTVs use a 120Hz refresh rate. Why a 120Hz refresh rate when only 24 frames per second can be displayed? 120Hz displays all the frame rates with native playback. 24 x 5 = 120, 30 x 4 = 120, and 60 x 2 = 120. Regardless of the frame rate, the video is always displayed in a native playback.

Sony and other manufacturers has taken this one step further with 120Hz HDTVs and use interpolation to smooth the actions scenes even further. Interpolation compares two adjacent frames and creates “filler” frames in between that basically averages the two and creates a new frame. Rather than displaying a movie frame for five cycles, interpolation modifies the frames so that one blends more smoothly into the next. This tends to make the wildest action scenes silky smooth.

I hope this helps you to understand the issues between frame rates and refresh rates and helps you make a better decision about your HDTV purchases.