Digital TV - Refresh Rate Explained

Published: 04th February 2011
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Refresh Rate Explained

Trying to choose a TV today can be difficult. Very difficult. One of the most widely debated features of flat panel TVs is the ever confusing Refresh Rate. There are quite a number of different refresh rates being featured on todays Digital TVs. 50Hz, 100Hz, 200Hz, and 600Hz. These Refresh Rates can easily divide opinion.

What is Refresh Rate, and why is it important?

Every video we watch is made up of thousands of "frames". The refresh rate, or "Hertz" (Hz) is a measure of the number of frames shown on your TV screen each second. Remember in your younger years, when you were bored in school and used to draw a stick figure in the top corner of every page in your book, and then flick through it quickly to make it look like it was moving? This is basically what your TV is doing to produce motion on the screen.

It is refreshing (i.e. flicking) through frames (i.e. pages) to produce motion. Now the quicker you can flick through pages in your book - the faster and smoother the stick figures motion seems to be. Secondly, the more stages of movement you draw the stick figure in on each page - the more natural it will seem to move.

So let's go back to our TV - if the same principal is applied, then the TV is able to show you a smoother motion if it can flick through its frames quicker. And by adding extra frames in between other frames - this causes the TV to produce a smoother transition from one frame to the other. So, the more times the screen can be "refreshed" every second, the smoother the image's motion will seem to be.

The standard broadcast signal we receive in Australia is 50Hz. That means that the image on your Digital TV is refreshed 50 times each second. 50Hz is a fairly slow rate, and it is possible to see blurring or judder in the picture when watching fast moving sporting events or action movies, or any scenes where the camera pans across very fast. So the way the TV fixes this issue is to create intermediate frames and then insert them between the original frames. This is referred to as interpolation, or 100Hz Technology.

"100Hz technology doubles the number of frames shown per second which helps to reduce the blurring and is ideal for watching action movies and sports."

To some people, the 100Hz motion can seem to be a bit "too" smooth initially, but once you adjust, it can be very difficult to return to a 50Hz TV, especially on a large screen. Today, larger and larger TVs have become very accessible in terms of price. Now because the picture is bigger, you will notice things like judder even more.

The majority of these large Flat Screen TVs (if not all) are 1080P Full HD. That means that there is a lot of information being displayed on the screen, and all of that information needs to be moved around very fast for it to look smooth. On a large Flat Screen TV running at 1080P and 50Hz, fast panning shots can show a lot of judder, because there is just so much image information being shifted around the screen, our eyes will pick up a lot of motion artefacts. So essentially, by doubling the number of frames, 100Hz removes the majority of motion judder.

So if 100Hz gets rid of judder, why do I need 200Hz?

Interpolation at 100Hz is good, but again, with a lot of Full HD, high resolution content like Blu-Ray, Gaming and more recently 3D Technology, now available to every home, and 46" - 65" LED screens surging in popularity due to plummeting prices - there's more benefit to be had from your TV doubling its refresh efforts. While 100Hz will certainly remove a lot of motion judder from LED TVs and LCD TVs, the bigger the screen, the more apparent the remaining artefacts and judder become. So 200Hz was born...

By inserting 3 additional frames between each existing frame, instead of one as 100hZ does, 200Hz processors improve motion smoothness further and succeed in making objects look more "solid". The aim of 200Hz is to keep objects such as Soccer Balls, or any small objects moving very fast across the screen, from losing their shape, blurring or breaking up when being propelled across the screen. However, it's not as simple as just adding an extra three frames - that's not quite the extent of the technology. A processor inside the TV looks at two sequential frames and then actually creates three NEW additional frames in between them.

Going back to our stick figure analogy, that means drawing four times as many stick figures by using four times as many pages, and flicking through those pages at the same speed. This fact, however, is where some 200Hz TVs succeed - or fail miserably!

Imagine you've drawn a stick figure on fifty pages, and can flick through those fifty pages in one second, without missing a page, and at a constant speed. Now double those pages, and you will find it harder to flick through the whole hundred pages without missing a few - let alone the same speed throughout. So your stick figure may seem to move smoothly, then judder, then smoothly again. Now, imagine trying to do that with two hundred pages. It's a struggle, and you are sure to miss a number of pages along the way. To put it one way - some people will do a better job than others.

This also rings very true for TVs. There are some processors out there which do a much better job than others do. There are some 200Hz processors which don't quite cut the mustard, and you can see odd frames skipping, lots of inaccurate and seemingly unnatural movement. So beware - not all 200Hz processors are created equal!

A lot of Plasma screens seem to highlight 600Hz - they must be at least three times faster, right?

Well in short - not exactly! What 600Hz Sub-Field Driving does, with a 50Hz source like how our Australian TV is broadcast, is split each frame into 12 separate frames or "sub-fields", and then show them individually on the screen. What it does not do is create NEW frames. That means 12 sub-fields per frame in 50Hz (frames per second) creates 600 frames per second (50x12=600). The more sub-fields you have per frame, the more accurate the colour reproduction and less picture noise.

So technically, 600Hz sub-field drive is not solely linked to, nor is its primary purpose, eliminating motion judder.

Why are the manufacturers causing all this confusion?

Because Plasma vs. LCD technology is still the biggest debate when it comes to choosing a Flat Screen TV. The difference in technology is causing manufacturers like Panasonic, Samsung, LG and Sony to heavily promote features and benefits of their panels to attract customers.

As far as LCD, and now LED Digital TV Panels have advanced - the fact of the matter is that for smooth, natural motion in fast scenes - a Plasma is still superior.

With the way Plasma technology works, 100Hz or 200Hz isn't a useful feature to have. But with LCD and LED heavily marketing 100Hz/200Hz, from a marketing perspective they needed a number to combat that. Some added 100Hz before, but now 200Hz LED and LCD have become more common, so they've started quoting the sub-field number - 600Hz.

The same thing happened with 1080p and 'Full HD'. LCD manufacturers introduced terms on their TVs - purely from a marketing perspective - to try and deflect attention from the fact that a good Plasma TV produced a more "natural" picture.

In reality, the plasma manufacturers are being slightly misleading in how they market 600Hz sub field driving. In one way they are saying to customers that our 600Hz is a feature that helps remove motion judder three times as much as a 200Hz TV.

While this is not directly true - they are using it to promote a very true feature of plasma technology - superior motion handling. There is a little bit of "the pot calling the kettle black" when it comes to the LCD manufacturers though. While it is true that 200Hz was designed to aid motion on a large screen TV - the fact that the resulting image can often seem unnatural during fast scenes puts a big question mark over whether this aids or hinders what the customer is seeing. The insertion of frames that are not meant to be there can make it all look a little fake, in many peoples opinion. If you do encounter that, it's best to disable it for that particular movie.

So, what is best? 100Hz, 200Hz or 600Hz?

In summary, for fast paced natural movement, Plasma screens are still superior. They do not need to create extra frames to eliminate motion judder, so the resulting image seen is not fake or unnatural. There are pros and cons for both Plasma and LCD technology, and opinion will always be divided one way or another, but if you prefer a smooth, natural picture - even though LCD and LED technology has improved immensely - a Plasma screen is still the way to go.

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