What Is a Sub-field Drive on a Plasma TV?

The Refresh Rate and Sub-field Drive on a Plasma TV

Plasma TV Sub-field Drive Example
Plasma TV Sub-field Drive Example. Photo from Amazon

Plasma TVs were discontinued in late 2014, but they had a lot of fans and quite a few of those fans ran out to buy up the last remaining plasmas available in stores. A lot of these TVs are still in use around the world and many still favor them over LCD TVs.

They offer excellent black level and motion tracking performance. The sub-field drive rate is a specification unique to plasma television. It's often stated as 480Hz, 550Hz, 600Hz or a similar number.

If you still have a plasma TV and refuse to part with it, what does this mean?

Sub-field Drive Rate vs. Screen Refresh Rate

Many consumers are falsely led to believe that the sub-field drive rate is comparable to the screen refresh rate, like the screen refresh rates commonly stated for LCD televisions. The sub-field drive rate actually refers to something different. Screen refresh rate is how many times each frame is repeated within a specific time period, such as 1/60th of a second. Plasma TVs do something in addition to this. In support of the screen refresh rate, they also send repeated electric pulses to the pixels to keep them lit for the period of time 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

Pixels behave differently in a plasma TVs than they do on LCD TVs. Pixels in an LCD TV can be turned on or off at any given time as a continuous light source is passed through LCD chips – LCD chips do not generate their own light.

Each pixel in a plasma TV generates its own light within a cell structure, but it can only do so for a very brief period of time measured in milliseconds. Electric pulses must be sent at a rapid rate to the pixels for the pixels in a plasma TV 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 60Hz 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 600Hz.

Images will look better and motion between each actual frame of video will look smoother when more pulses can be sent within the 60HZ refresh rate time period. This is 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: How many pulses must 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. Can a consumer really see a difference in image quality and motion between plasma TVs that have sub-field drive rates of 480Hz, 600Hz or 700Hz? One thing can be stated objectively, however: Plasma TVs definitely have better motion response than LCD TVs.