Tuesday, April 27, 2010

60Hz vs. 120Hz vs. 240Hz vs. 480Hz: LCD Refresh Rates

To understand Refresh Rate specifications, we first have to look at some of the basic elements of such.

Hertz (Hz) is a measurement of frequency. In this situation, it refers to the number of video frames displayed on a TV each second. First, it is important to understand that most video content is sent to the TV at 60Hz. To clarify, a DVD player or cable box is sending 60 video frames to the TV each second. Frame rates, such as 120Hz, 240Hz, and 480Hz, increase the number of frames displayed on the TV each second. The
reason why TV manufacturers try to increase the number of frames in LCD TVs specifically is due to a problem with response time in LCD panels. Response time is how fast the individual pixel goes from active to inactive. Large screen LCD TVs have slow response times compared to other technologies which sometimes manifests as blur and trailing artifacts when there is motion in the image.

So, we already established that video sources only send 60 frames to a TV each second. Then how does an LCD TV come up with 120, 240, or 480 frames per second. The answer depends on the technique used. With 120Hz, the answer is the same for most TVs. The TV simply repeats every frame twice. You see each of the 60 video frames twice in rapid succession. The benefit is esoteric and a bit arguable. One theory is that this creates a stronger impression of the image in your brain forcing your brain to better recognize the differences in the following frames. Keep in mind though, that when the first 120Hz TVs came out, the increased frame rates were coupled with better video processing which contributes greatly to the reduction in motion blur and artifacts.

With 240Hz, the rules changed. LED lighting had started to emerge with 240Hz in the market. The ability to rapidly turn LED lights on and off gave the LCD manufacturers another tool. A tool called 'black frame insertion.' It allowed the manufacturers to turn off the LED lights briefly between each of the video frames. Presumably this allowed the LCD cells more time to refresh. It also allowed them to claim that they were pushing 240 frames per second because there was a 'black' frame between each of the 120 full video frames. Some manufacturers did implement 240Hz by doubling the full video frames again over the 120. This meant that they were duplicating each of the 60 original video frames four times, summing to a total of 240 frames per second. These manufacturers frequently accused their counterparts of not using a true 240Hz.

480Hz is a similar story. Manufacturers are essentially using the 'black frame insertion' with full 240Hz processing. 240 full video frames with a black field between each full frame. Most companies are doing this in some of their 2010 models but with different stories. Sony calls it 240Hz Pro, as they defer to the purist opinion that "black fields don't count." LG and other manufacturers are calling it 480Hz, but the technique is the same regardless. "That which we call a rose by any other name..?"

In terms of performance, critics tend to agree that there is a visible improvement from 60Hz to 120Hz. Albeit, that may be, in large part, due to the new motion processing that most manufacturers implemented coincidentally. The differences from 120Hz to 240Hz are arguably less significant. But such is often the case, when simply improving an already implemented technology. It's never as big as the first iteration. The effects of 'black frame insertion' on 240Hz are still being debated and the jury is still out on the few TV models we have seen using this technique. Again the benefits are speculative and esoteric but we are dealing with human perception which has never been truly quantifiable. I guess we will just have to wait and 'see' how significant the perceived benefits will be.

12 comments:

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