Faster refresh rates (120Hz, 240Hz and higher) are used by HDTV manufacturers to combat the susceptibility of traditional CCFL fluorescent backlit LCD and LED LCD backlit displays with a 60Hz refresh rate to motion blur. [ motion blur is not a problem for plasma HDTVs ] Motion blur is seen as the streaking of rapidly moving object across a display.

Causes of motion blur can include one or more of; a slow liquid crystal response time, the sample-and-hold nature of LCD displays, motion across the screen not being linear enough and frame rate conversions (pulldown). The human eye perception of moving objects is sharper the easier it can track them that is, when their movements are linear or nearly so. Frame interpolation acts to provide smoother transitions of moving objects between frames (easier tracking) reducing perceived blurring. The smoother transitions are provided by inserting interpolated frames between the existing ones of the source video.

MEMC (Motion Estimation and Motion Compensation) for Interpolation of Extra Frames

To compute the new frames to be placed between existing frames, LCD displays use a system called MEMC (Motion Estimation and Motion Compensation). Basically, the HDTV’s software looks frames ahead in order to estimate the motion of moving objects and create new interpolated frames. To achieve higher refresh rates (eg. 240Hz rather than 120Hz) more interpolated frames are inserted. As one can imagine, computing a new frame for a 1080p resolution with over two million pixels is a considerable feat. The image below pictorially shows the interpolation process for 120Hz by Sony’s MotionFlo.

There are different software approaches to MEMC and if you are intending to purchase a less expensive LCD HDTV you need to ensure that the quality of the motion interpolation is satisfactory to you as imperfect processing can result in extra frames with annoying artifacts.

MPRT for Comparing LCD HDTV Blur Reduction

A useful measurement for comparing the blurring minimization capability of LCD HDTVs is MPRT (Motion Picture Response Time). Testing has shown a strong correlation between the perceived motion blur and MRPT measurements. MPRT values should not be mistaken for LCD display response times (gray-to-gray) as although you need a small response time for blur reduction it does not ensure a good (lower) MPRT value as there are multiple possible causes of LCD blur. In an interview with HDTV Magazine Brian Berkeley, Vice President, LCD Business Technology Development Group, Samsung Electronics stated that an MPRT score of 8ms is about state-of-the-art for mass production LCD-TVs (lower is better). In comparison, LCD HDTV response times of 4ms are not uncommon.

Added Benefit of Motion Interpolation – Reduced Judder

As well as reducing motion blur, higher refresh rates through extra frame interpolation can also reduce the likelihood of judder when viewing film sources shot at 24-frames-per-second. Judder is an annoying effect which can occur when 3:2 pulldown is used to boost a 24 fps film source to 60 fps. This judder reduction is separate from the special video processing modes for judder reduction which are available with many HDTVs.

Scanning Backlight Anti-Blur Technology

Motion blur can also be due to the sample-and-hold nature of LCD displays. An important factor for the human eye’s perception of continuous motion is for image frames to have breaks in between with an interruption of constant light. If this lacking the eye may interpret the motion as being blurred. Unfortunately, with LCD displays sample-and-hold technique, each frame being lit for the full time required for the frame rate frequency followed by the liquid crystals cycling quickly for the next frame which is illuminated immediately. This lack of a sizeable break in the lighting is the reason LCD manufacturers use black frame insertion or backlight scanning to do it artificially.