Do I Need a 120Hz TV?

One of the recent developments in TV is the advent of televisions that run at 120Hz (short for Hertz, pronounced ‘hurts’).

Normally televisions in North America run at 60Hz, which means that images flash on the screen approximately sixty times per second. New 120Hz TVs double that, which means that the image flashes more often, and should theoretically result in a smoother image. In practical terms, the human brain is not capable of perceiving the difference between 60Hz and 120Hz, so it makes little difference for normal television watching.

Visualizing the Process

Imagine a fan, with a light behind it and slightly to one side, so that when the blades of the fan spin, they cover and uncover the light. Each time a gap in the blades passes in front of the light, the light shines through. If the fan is spinning slowly, then you will be able to see the blades, with flashes of light in between. As the fan spins faster, it becomes harder to see the blades, until they are simply a blur. At this point, the light will still seem to be flashing, but very quickly. If the blades turn fast enough, the flashes of light will come so fast that the light will seem to be a constant, continuous light, instead of a series of discernable flashes. In the same way, a TV that flashes quickly enough will appear to be smooth and continuous, even though it is made up of a series of very quick still images. The threshold at which the human brain begins to perceive motion as smooth is known as ‘flicker fusion’. The point of fusion changes in different lighting conditions, but the point at which the human brain is guaranteed to perceive motion as smooth is around 60 times per second.

If the fan, already spinning fast enough that the light appears to be one solid light was to spin faster, would it make a difference to your perception of the light? No, there comes a point where the light appears smooth, and if the fan spins 10% faster, 100% faster, or 1000% faster, it won’t make any difference to how you perceive that light shining through the blades. Your TV, just like the light behind the fan, only needs to be so fast. Hardcore users might be able to pick up on subtle details which are affected, but for the typical user 120Hz is not going to make a significant improvement in image quality.

NTSC, Film and PAL

‘Frames per second’ indicates how many unique pictures are displayed every second in a video stream. NTSC, which is the standard for broadcast television in North America, displays 29.97 frames per second. So then, if there are only 30 pictures each second, how does the screen flash 60 times each second? Well, remember interlacing (part of an earlier blog post) where only the odd lines are shown, and then the even lines? Cut 30 pictures in half, and you have 60. Progressive video simply displays each of the 30 images twice. Even though you’re seeing 30 doubles every second, the human brain perceives it as smooth motion. Most hand drawn cartoons were 15 frames per second, and they showed each picture four times, which is about the lower end of the limit of this Jedi mind trick that TVs are able to do.

The earliest film images couldn’t get beyond the threshold at which the human brain perceived them as smooth motion, which is why the famous footage you’ve probably seen of a train running down the tracks looks like the image is stuttering or flickering. The image isn’t flashing often enough to seem smooth. Modern movies run at 24 frames per second, which is doubled to 48 frames per second, which is sufficient in the controlled environment of a theatre to appear smooth.

PAL is the standard for video in Eastern Europe and Asia. It displays 25 frames per second, and doubles them for a total of 50 flashes per second. This is a little slow, and can result in a bit of flicker, depending on the conditions. Newer televisions for the Eastern Europe/Asian market also double the Hz for a total of 100 flashes per second, and the difference between 50Hz and 100Hz is noticeable in some situations.

One Good Reason for 120Hz

There is one clear situation in which a 120Hz TV makes real sense. When a video camera is pointed at a TV, a 120Hz display will appear better than a 60Hz display. If the TV you are recording with a video camera is displaying 60Hz, and the camera is recording 60Hz and those two devices are out of synch at all, then the image will look blurry or only partially there. That’s because the exact moment that the camera records a frame is out of synch with the exact moment that the TV is displaying the image to be captured. With a 120Hz TV, it’s putting out way more information than the camera needs to record a smooth image, so it looks better.

The Bottom Line

Should you buy a 120Hz TV? Well, there’s nothing wrong with 120Hz. Only the newest and best TVs available right now have it, because it’s new. When I was working in retail, however, I often had people coming in and asking about 120Hz, and a few who insisted that any TV they purchased had to have it. For typical viewing, 120Hz makes little difference, and you have to be paying very close attention to see the difference. It is technically a better television, but the human brain is not capable of perceiving the difference under normal conditions. If you’re buying a television, this is one of the last criteria that should influence your decision. Nice to have, but certainly not critical for the average consumer.