Sub-Field Drive Rate vs. Screen Refresh Rate
Many consumers falsely believe that the sub-field drive rate is comparable to the screen refresh rate, like the screen refresh rates commonly stated for LCD televisions. However, the sub-field drive rate on a plasma TV refers to something different. The screen refresh rate is how many times each frame repeats within a specific time period, such as 1/60th of a second. Although plasma TVs have a 60 Hz screen refresh rate, these TVs do something, in addition, to smooth out the motion response further. In support of the screen refresh rate, plasma TVs send repeated electric pulses to the pixels to keep the pixels lit for the period of time that each frame displays on the screen. The sub-field drive is designed to send these rapid pulses.
Plasma TV Pixels vs. LCD TV Pixels
Pixels behave differently in a plasma TV than on an LCD TV. Pixels in an LCD TV can be turned on or off at any time as a continuous light source is passed through LCD chips. However, LCD chips don’t generate light. Instead, LCD chips require an additional back or edge light source to produce images that you can see on the screen. On the other hand, each pixel in a plasma TV is self-emissive. This means that plasma TV pixels generate light within a cell structure (an additional backlight source isn’t required). However, it can only do so for a brief time measured in milliseconds. Electric pulses must be sent at a rapid rate to plasma TV pixels for the pixels to remain lit. The sub-field drive specification states the rate of how many of these pulses are sent to the pixels each second to keep the frame visible on the screen. If a plasma TV has a 60 Hz screen refresh rate, which is most common, and if the sub-field drive sends 10 pulses to excite the pixels within a 60th of a second, the sub-field drive rate is stated as 600 Hz. Images look better and motion between each video frame looks smoother when more pulses are sent within the 60 HZ refresh rate time period. This is because pixel brightness doesn’t decay as quickly during the time when a frame is displayed, nor when transitioning from frame to frame.
The Bottom Line
Although LCD and Plasma TVs outwardly look the same, there are internal differences in how each displays what you see on the screen. One unique difference in plasma TVs is the implementation of sub-field drive technology to enhance motion response. However, as with LCD TV screen refresh rates, this can be a misleading numbers game. After all, how many pulses must be sent per 1/60th of a second to see an improvement in motion image quality? Can you see a difference in image quality and motion between plasma TVs with sub-field drive rates of 480 Hz, 600 Hz, or 700 Hz? The best way to find out is to do an eyes-on comparison to see what looks best to you. Regardless of the sub-field drive rate, plasma TVs generally have better motion response than LCD TVs.
