Refresh rate is the number of times in a second that data is drawn to be displayed on a screen. For high and improved picture quality, 120 Hz or higher refresh rates are preferred.
As we all know, television screens keep refreshing continuously, so as to display the next image. If this refreshing of the screen doesn't take place, the pictures will be still images. Thus, in order to show moving pictures, the pixels are changed after some specified amount of time.
The electricity in the US runs at 60Hz, while it is 50Hz in some other parts of the world. So, if the electricity needed to run the television is at 60Hz, this means that our TVs also run at this rate. This concept was followed in the CRT days.
However, the launch of LCD showed that higher rates, i.e. 120Hz, can display better quality images. This rate further increased in Plasmas to about 600Hz. The launch of HDTV upped this rate to an even higher level.
Refresh Rate: In Brief
As mentioned earlier, 60Hz is the standard frequency of electricity in the US, and this is the refresh rate for televisions. This refresh rate means that the HDTV will refresh the image 60 times per second. But, what we missed out here is the frames/second rate at which the movies/images are shot.
Movies are shot at 24 frames per second, and the difference between these two rates causes the degradation of image quality. Thus, in order to improve picture quality, the refresh rate needs to improve.
However, the standard refresh rate is 60Hz, which means that the screen will refresh 30 times for odd pixels and 30 times for even pixels. A 3:2 pull-down is done, which is nothing but conversion of the film into interlaced video, so as to reduce the timing difference between the two frames.
The frames are spread across 30 frames, instead of 24, to match the 60 Hz refresh rate. Then these frames (30 each) are interlaced to form 60 frames per second, to match the 60 Hz rate. But, this process is also not perfect, and thus, there are bound to be problems in the image quality. The images may look jerky, and this effect is known as film judder.
➽ Reduction of Judder: When it comes to HDTV, the refresh rate has been increased to 120 Hz or 240 Hz. In either of the two cases, the 24 frames per second will be evenly divided by the refresh rate, thus eliminating the need of a 3:2 pull-down. The absence of this pull-down enhances image quality, and gives an alluring viewing experience.
For interlacing of the frames, the TV creates new frames that go in between the original frames, thus spreading the frames across the refresh rate.
➽ Reduction of Soap Opera Effect: However, the frame rate increase may cause problems in watching movies and TV shows, which are known as 'Soap Opera Effect'. The increased frame rates make the images look ultra-smooth just like soap operas.
To reduce this effect, the refresh rates are increased by flashing backlight (in most cases). This means that, the HDTV backlight flashes at a given rate, which directly increases the refresh rate.
The actual frame rate for these films is 24 FPS. But the 3D content is flashed three times per frame, i.e., the viewer actually sees 144 frames per second. Due to a high refresh rate, these frames are equally distributed, and clear and smooth 3D images can be seen.
Refresh rates are important in HDTV, so as to improve user experience by providing better picture quality. The picture will be blur-less, and the image transition will be clear and smooth. When buying an HDTV, look out for television sets with 120 Hz refresh rates.