LED TVs Refresh rates – introduction

As a technical concept, LED TVS Refresh rates is one of the most important factors affecting picture quality. With TV brands coming up with new marketing pitches about refresh rates, it is good for consumers to know the definition of how this affects the performance of their LED TVs.

Refresh rate – history

In the era of Cathode Ray Tube (CRT) monitors, refresh rate referred to the number of times the image on screen was drawn. In CRTs, there is an electron gun behind the glass of the screen. This gun shoots light onto the screen and creates an image. This process happens line by line across the width of the screen and then downwards. Once the gun reaches the bottom, it goes back to the top left and the process begins again.

LED TVs Refresh rates - CRT Refresh

When the electron gun is in one place, another part of the screen may be blank as it waits for the new image. Since the process happens very fast, we do not perceive this.

The number of times the image is re-drawn on the screen is the refresh rate of the screen. The refresh rate is measured in Hertz which is a unit of frequency. A refresh rate of 60 Hz implies that the screen refreshes 60 times in a second.

Problems with low refresh rate on CRT

On a CRT display, a low refresh rate leads to an appearance of flickering. This is because the “redrawing” of the image is perceptible to the human eye. This usually happens at refresh rates below 60 Hz. This screen flickering is unpleasant to look at and can cause eye strain and headaches.

LED TVs Refresh rates

An LED TV is different from CRT in how the image is displayed. The display is on a liquid crystal display. This display is illuminated by Light Emitting Diodes (LEDs). The LEDs can be placed to the side or behind the LCD. The LEDs change color in response to the image that is shown on screen. Since they do not need to turned off, they do not need to refreshed like a CRT monitor.

So what then is refresh rate in the context of an LED TV? In LED TVs, refresh rate refers to the number of frames displayed per second on the TV. If your input video is running at a frame rate of 60 Hz and your TVs refresh rate is also 60 Hz, then the video will display exactly as intended. However, if your TV has a refresh rate of 120 Hz and the input video source is 60 Hz, then one of two things happens.

LED TVs Refresh Rates - LED TV construction

  1. The TV guesses the difference between successive frames and displays its version of the frame.
  2. The TV displays each frame twice.

The next question then is, what is the frame rate of most input video? Most video sources today do not go beyond 60 Hz. From cable TV to Netflix and YouTube, the maximum frame rate currently available is 60 Hz.

LED TVs Refresh rates – Motion Smoothing

Many TVs provide an extra technology called motion smoothing or motion estimation. This technlogy is designed for high refresh rate LED TVs. When the refresh rate is 120 Hz and the input video is of 60 Hz, the TV can “guess” what should be displayed on the 60 frames it does not receive from the video source in each second. This works really well for watching video which has a high degree of motion like sports.

LED TVs Refresh Rates - motion estimation

LED TVs Refresh rates – conclusion

  • Most sources of video today operate at 60 Hz. In almost all cases a higher refresh rate of 120 Hz or even higher is not going to improve the image quality on screen.
  • Motion Smoothing or motion estimation is a good technology to have while watching high action video like sports. The utility of this feature is still limited.

LED TVs Refresh rates – references

  1. Wikipedia page on refresh rates