The Power Of Pixel Response
Analyzing the impact of LCD pixel response on images in motion.
As with most manmade things, no display technology is without its shortcomings. CRTs tend to be large and bulky, plasmas come with burn-in and heat dissipation issues, and LCDs have motion artifacts. In an attempt to address these and other issues, manufacturers continue to develop new technologies. In the midst of this process, as prices for LCD panels come down and sizes go up, many industry observers believe this technology will eventually become the favored display type.
A major difference among display technologies is the way images are displayed. CRTs present images via a scanning beam that energizes red, green, and blue phosphors located near the glass viewing screen. The major benefit of this technology is an ability to display interlaced images from two fields to create a single video frame without difficulty. LCDs on the other hand, function similarly to a film projector and can only display single frames of video. The end result is that LCDs look great with progressive, frame-based images; however, they still have some difficulty with field-based interlaced video — an anomaly most people relate to pixel response.What is pixel response?
LCD/TFT screens have groups of red, green, and blue sub-pixels or dots for every pixel. Sub-pixels are basically very tiny light valves that open and close based upon a digital value. When all three sub-pixels are on, the pixel will generate a white image. When all are off, you get black.
Response time (see Fig. 1 at left) has been traditionally defined as the time it takes a pixel to change from black to white (rising time) and then back to black (falling or decay time). This approach is often called “rise-and-fall.” However, many manufacturers are beginning to use a different approach, instead measuring the time between 10 percent and 90 percent of the total amplitude. This method, which tends to minimize errors in the measurement when the source material is saturated to white or driven into negative black, is often called “gray-to-gray” time. As with any electronic device, when comparing any published specifications for an LCD display, it's important to know which method is being used to derive the numbers.
For most LCD/TFT panels, the rise time is slightly faster than the decay time. In this case, the sum of the two times is used. This value, measured in milliseconds, is used in product specifications.
Panels are specified as “normally white” or “normally black.” This describes the normal display state of the resting, or “off,” pixels. Black screens will usually have a longer decay in the white-to-black transition while white will have longer decay times for black-to-white. The end result is a transition effect, similar to a video dissolve, may occur with each frame update.How does pixel response impact the image on an LCD/TFT screen?
In order to answer this question, you must also take into account the frame rates and screen refresh rates used in video applications. Frame rate is determined by the material to be displayed. For video applications, frame rates range from 24 to 60 frames per second (fps). We can determine the period of the individual frame by the equation 1/fps.
Based on the table at right, the fastest standard video frame rate is 60 fps at 17 milliseconds (ms). Of the formats in use today, the progressive frame rates for 1080-60p, 480-60p, and 720-60p are best displayed on LCD/TFT panels with pixel response rates less than 17 ms. Interlace images of 1080-60i, 480-60i (525 lines) are combined to create a single frame measured as 30 fps, while 1080-50i and 480-50i are measured as 25 fps (625 lines) with a period of 40 ms.
The general rule for larger LCD/TFT screens is to make sure the total pixel response time is less than the frame rate. For example, if you're using an LCD to look at 1080-60i, on a large (12-inch or greater) LCD/TFT screen, the gray-to-gray measured pixel response should be less than 33 ms (see Fig. 2 on page 50). A 720-60p frame rate requires faster pixel response, while 24p can use relatively slower pixel response rates. Why omit smaller screens? Simply put, the smaller the screen, the harder it is to see anomalies. A 1280x768 screen has the same number of pixels no matter what size the screen is. The difference is actual pixel size. Smaller screens have smaller pixels while larger screens have bigger pixels.What about pixel response times?
We often equate pixel response with motion artifacts inherent in LCD/TFT panels sometimes called “blur.” While pixel response has an impact on blur, the reality is that other factors and processes are in play, amplifying the occurrence of this artifact.