What does 600Hz Sub Field Drive mean?
You have probably seen Plasma panels marked with cryptic terms such as 600Hz sub field drive or 480Hz sub field drive and you’ve probably wondered – What is sub field drive?
If you have, then this article will answer this question and also teach you the difference between LCD and Plasma refresh rates so that you are better informed when you make your 3D TV purchase.
Sub field drive is a term introduced by Plasma panel manufacturers to combat the growing disinformation being spread by LCD TV manufacturers who were claiming higher LCD Refresh rates and using these claims to bolster false arguments about the superiority of 240Hz or 480Hz LCD panels over 60Hz or 120Hz Plasma panels. While most people in the industry are well aware about the truth behind these numbers, and that they refer to completely different things in the case of LCD or Plasma TVs, the average consumer has been left confused and tends to naively go with the assumption – “Higher numbers must mean better”.
Sub Field Drive – Plasma Panel Basics
A Plasma panel display has near instantaneous response times on the order of 2 milliseconds. What this implies is that a plasma TV subpixel is only alight for a fraction of a second. Typically, most conventional Plasma TVs display video at 60fps (research has shown that the human eye cannot tell any significant difference in motion and smoothness of videos at higher frame rates). This implies that each frame has to be displayed for 1/60 or 17ms. However, the sub pixels of a plasma TV stay alight only for around 2ms when excited. Thus, to display a single frame for 1/60 seconds, the plasma panel excites the sub-pixels in pulses so as to keep all the pixels “bright” so that they can continuously display the desired frame.
Thus, for example, a 60Hz plasma panel can have 10 pulses per frame, to display the image. Effectively, the screen is being refreshed 60Hz times 10 pulses/frame which gives us a value of 600Hz. This is what Plasma manufacturers refer to as the Sub field drive refresh rate. If the Plasma panel performs 8 pulses per frame, it gives a sub field drive refresh rate of 480Hz. Now when the displayed frame has to be changed to the next frame, the ultra-fast response times of the Plasma TV sub pixels enables an almost instantaneous transition to the next frame.
As a result, even though only 60 frames are displayed per second by the Plasma TV, the near instantaneous transition between frames drastically minimizes motion blur and image ghosting. While the subfield drive terminology is a bit misleading as each frame is not being updated 600 times a second, it still portrays the inherent advantage of Plasma TVs when it comes to fast moving content due to its near instantaneous pixel response times.
240Hz and 480Hz LCD 3D TVs – The truth behind the numbers
In the case of LCD panels, things change drastically. One of the fundamental limitations of LCD technology is that the switching speed, or the time it takes to change the pixel color is quite slow – on the order of around 4ms. In addition, this refresh rate is dependent on what color the pixel was showing, and what color it has to update to. The 4ms is only a best case estimate and even on some of the commercial 480Hz panels, many pixels can take much longer to update certain pixels.
As a result, when you are trying to view 60Hz content on an LCD TV, in the case of fast moving scenes where there is a large difference between successive frames. If the LCD panel can’t keep up with this (which is usually the case) it gives rise to motion blurring and image ghosting artifacts. While LCD manufacturers have been trying to push the response times of these displays, they are still nowhere close to producing a true 240Hz or 480Hz display that can display all video content accurately and free of artifacts at these high refresh rates.
Instead, what they have chosen to do is use advanced mathematical techniques to interpolate between the frames of a 60Hz video signal to give rise to a 240Hz signal. Keep in mind that a large number of the 240 frames being displayed every second are ‘fake’ interpolated frames. The idea behind this is that the 240Hz source keeps ‘driving’ the LCD pixels at a much faster rate, thus allowing for a better transition from one frame to the next. While this is certainly an interesting way to improve LCD displays, the 240Hz or 480Hz terms are somewhat misleading since the screen itself is still not capable of true 240Hz/480hz response times.
This motion interpolation does improve the quality of high speed video content, but it also introduces some artifacts of its own. Many people report that when this is turned on for regular content, it makes the video look cheap or fake, and almost ‘too smooth’. It is advisable to only turn this feature on when you are viewing sports or an action flick on your LCD TV.
Subfield drive is a clever terminology devised by Plasma TV manufacturers to prove to consumers the inherent advantage Plasma panels have over LCD TVs when it comes to pixel response times and screen refresh rates. This directly translates into a better viewing experience for fast moving video content and minimized image ghosting artifacts. While 120Hz, 240Hz and 480Hz LCD technology has certainly come a long way in lowering these issues in LCD TVs, Plasma TVs still retain the upper hand over LCD TVs in this area. Be sure to check our Plasma vs LCD 3D TV Guide for more information regarding the pros and cons of Plasma and LCD technology.
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