Dr Raymond Soneira of DisplayMate breaks down misinformation about displays and specs. Never again will you be fooled by LCD refresh rates, or stymied when trying to explain interlacing!
You can also check out the Displaymate website for more comprehensive analysis and explanation.
1. Frame rate:
Frame rate refers to the number of images or frames transmitted per second (fps). Almost all commercial content in the US runs at 60 fps or Hz, although in some games it may be possible to increase the frame rate for faster and smoother action to 85, 100, or 120 fps or Hz. While this made a lot of sense for CRTs because they could follow whatever video rate they were being fed, all current commercial flat panel displays only operate at a single fixed rate, typically 60 Hz. Although some high-end displays can operate at 120 Hz, or 240 Hz or above, it is very important to drive the display EXACTLY at its native rate, or an exact sub-multiple like 60 Hz for 120 Hz and above. If you supply video at some other rate, like 85 or 100 Hz, the display electronics will convert it to its native internal rate, which will produce images with frame tearing or other artifacts.
2. Refresh Rate:
Refresh rate is a term that refers to the number of times per second that a display updates the screen image. This is fixed by the manufacturer, and is generally 60 Hz as described above. With CRTs 60 Hz often produced visible flicker, but essentially all modern commercial flat panel displays and projectors do not product any flicker. For LCDs this is accomplished by using an Active Matrix of transistors that hold the image steady between screen updates. Many CRTs also operated with interlacing (discussed below), however, all commercial modern displays operate internally in non-interlaced progressive scan mode, even when you supply an interlaced video signal.
3. Interlacing:
All broadcast video is transmitted using interlaced scanning – where the odd lines are transmitted in one frame (which is called a field), and then a 60th of a second later the even lines are transmitted in a second field. A complete image is actually transmitted only once every 30th of a second. This cuts the bandwidth needed to transmit the signal by a factor of 2.
While CRTs can display interlaced even then odd line video signals, all modern commercial displays need the screen to be updated in a linear progressive top to bottom fashion. This means that all broadcast video needs to be deinterlaced and converted to progressive images. These signals are referred to as “i” and “p” video respectively, so HD broadcast video (over the air, satellite, or cable) is 1080i and needs to be converted to 1080p. One easy way to do this is to use a frame buffer and just store the two most recent fields and drive the screen progressively that way. The problem with this is that the odd and even lines are taken a 60th of a second apart in time, so a moving object appears with alternating field content, which is particularly noticeable on leading edges – it’s called “combing” because of the repeating in-out appearance of the image but this can be fixed with motion interpolation.
4. Deinterlacing:
Sophisticated computer processing can be used to compare the sequence of video images and quantitatively analyse how all of the content within the image is moving. One application is deinterlacing – the processor can time the sequence of interlaced fields and reconstruct with good accuracy what the image would look like at a single instant of time – this gets rid of the “combing” and effectively produces a proper progressive 1080p image. To get good motion interpolation, the processor may look at up to 6 successive fields before generating an interpolated image which means the displayed image can be delayed up to one tenth of a second before appearing on-screen. That is normally not a problem unless you are involved in an action game where reaction time and hand-eye coordination matters. In that case you’ll need to reduce or eliminate motion interpolation processing.
5. Motion Interpolation for Film:
All content shot on film is recorded at 24 fps – which is a really low rate, especially for objects moving quickly across the screen -which will cause the eye to detect discontinuous jumps or “Hopping” in the image. Another problem is that the 24 fps must be converted to 60 fps (or higher) for video. That is a factor of 2.5 in the frame rate. One easy way to do this is to show one film frame twice and the next frame 3 times – this is called 3:2 Pull Down processing. This 3 then 2 progression produces a visually irregularity in motion called Judder. It isn’t always noticeable but it can be annoying. There are two ways to fix this: increase the refresh rate to 120 Hz and repeat every film frame 5 times – this gets rid of the Judder but still leaves the Hopping. Better yet, use motion interpolation to analyse the motion and build intermediate frames at the native refresh rate of the display. This will get rid of both the Hopping and the Judder.
6. Motion Interpolation to Reduce Motion Blur:
In LCDs, the optical liquid (the L in LCD) may not respond quickly enough when there is fast motion in a video image. This produces Motion Blur that is sometimes noticeable in fast moving objects. One approach that manufacturers have taken is to use motion interpolation to drive the panel at much higher refresh rates, which is claimed to improve the Response Time and reduce Motion Blur. It’s a fabulous marketing concept that has convinced lots of consumers to buy more expensive 120 Hz, 240 Hz, and even 480 Hz refresh rate LCDs. But predictive Motion Interpolation has only a minor effect on motion blur.
By far the best way to improve Response Time is with signal over driving of the display panel, and this is not improved by increasing the refresh rate. In objective side-by-side objective testing documented with high-speed screen shots I have demonstrated that there is no noticeable visual difference between 60 Hz and 120 Hz LCDs. This effect is explained in exactly the same way as the classic story of “The Emperor’s New Cloths” – the principle is that people frequently see what that have been told they should see. For an objective explanation and analysis see my article on LCD Response Time and Motion Blur, with lots of high-speed screen shots to prove my point.
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