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What is a Sub-Field Drive on a Plasma TV?

When a Plasma TV has a Sub-Field Rate of 600Hz, what does that really mean?


Plasma TV Sub-Field Drive Illustration and Logo

Photo Illustration of how Plasma TV Sub-Field Drive Works

Image provided by Panasonic

When shopping for a Plasma Television, just as with most consumer electronics products, consumers are confronted with lots of numbers and tech terms. One specification that is unique to Plasma Television is the Sub-Field Drive rate, which is often times stated as 480Hz, 550Hz, 600Hz, or similar number.

Sub-Field Drive Rate vs Screen Refresh Rate

Many consumers are falsely led to believe that the Sub-Field Drive Rate is analogous to Screen Refresh Rate, such as the screen refresh rates commonly stated for LCD Televisions. However, the Sub-Field Drive Rate actually refers to something slightly different.

Unlike Screen Refresh Rate, which is how many times each frame is repeated within a specific time period (such as 1/60th of a second), Plasma TVs also do something additional. In support of the screen refresh rate, a Plasma TV also sends repeated electric pulses to the pixels to keep them lit for the time period (such as 1/60th of a second) that each frame is displayed on the screen. The Sub-Field Drive is designed to send these rapid pulses.

Plasma TV Pixels vs LCD TV Pixels

In a Plasma TV, pixels behave differently than on an LCD TV. In an LCD TV pixels can be turned on or off at any given time as a continuous light source can be passed through LCD chips (LCD chips do not generate their own light). However, in a Plasma TV, each pixel generates its own light within a cell-structure, but can only do so for a very brief period of time (measured in milliseconds). In order for the pixels in a Plasma TV to remain lit, electric pulses need to be sent at a rapid rate to the pixels.

In other words, what the Sub-Field Drive specification states is the rate of how many of these pulses are sent to the pixels each second in order to keep the frame visible on the screen. For instance, if a Plasma TV has a 60Hz screen refresh rate (which is most common) and the Sub-Field Drive sends 10 pulses to excite the pixels within a 60th of a second time period, then the Sub-Field Drive rate is stated as 600Hz.

The idea is that the more pulses that can be sent within the 60Hz refresh rate time period, the images will look better and motion between each actual frame of video will look smoother, due to the fact that pixel brightness does not decay as quickly during the time when a frame is being displayed nor when transitioning from frame to frame.

Final Take

Of course, the question remains as to how many pulses need to be sent per 1/60th of a second to see an improvement in motion image quality. Just as with screen refresh rate, Sub-Field rates become a numbers game. In other words, can a consumer really see the difference in image quality and motion (all other things being equal) between Plasma TVs that have Sub-Field Drive rates of 480Hz, 600Hz, or 700Hz? You just have to compare it for yourself. However, one thing can be stated objectively, Plasma TVs definitely have better motion response than LCD TVs.

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