The frame game

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Many TVs now include technology to enhance the frame rate of moving images and reduce flicker, jerkiness and blur. 100Hz and 200Hz rates are common, while some quote 400Hz, 600Hz and beyond. The Hz (Hertz) measurement is inherited from cathode ray tube TVs (CRTs) and refers to impulse-driven cycles per second. This doesn’t really apply to LCD and plasma but the notion of frames per second remains important along with any additional frames generated to modify motion handling.

In most of Europe, the TV signal equals 25 full frames per second, divided into two ‘fields’ of alternate lines, resulting in 50Hz. Frame interpolation or motion compensation are overall names for extra video frames to smooth motion (as opposed to merely repeating frames), which helps with fast material such as sport. These are added into the frame sequence by rapid real-time digital processing. 

“The change from 50Hz to 100Hz gave an obvious improvement in picture quality and, in removing large area flicker, could help reduce eye strain,” explains Sam Johnson, product manager for Loewe UK. “Increasing from 100Hz to higher rates brings an improvement in motion performance but generally has advantages only with fast moving video content.”

Chris Bradshaw, product trainer at Sony, adds: “With 100Hz processing, one additional frame is being created. Within the Sony technologies we look at the frame either side and create one frame from scratch. When it comes to 200Hz, we take the frame being created in the middle as a benchmark to create two more.” 

Steve Lucas, product specialist at Panasonic, says: “LCD rates, ie 100, 200 or 400Hz, refer to the rate the picture is displayed on the screen. Rates described for plasma, ie 400 or 600Hz refer to the processing speed of the video signal, which is used to create brightness levels within each plasma cell.” 

Chris Bradshaw, notes, “With the higher numbers what we start to see actually isn’t to do with motion at all. Let’s take a 600Hz plasma. You get 50 frames per second. Pixels in plasma suffer from phosphor decay. It’s very bright but it dissipates. Therefore a large charge of voltage is put through that pixel 12 times per second. Twelve times 50Hz gives you 600Hz.” 

Blackout

In LCD-based TVs, backlighting (or LED side-lighting) can be manipulated with similar results. “Techniques increasing the frame rate from 100Hz can involve inserting new pictures which are blank or black rather than containing actual picture content,” Sam Johnson says. “This can also improve motion.” 

When quoting the Hz value, it’s important to differentiate between the number of actual interpolated frames and how backlighting is involved, if at all. Sometimes, both are combined to push Hz values higher. 

For Chris Moseley, AV product manager at Samsung, “The way that we’re rating screens now is called CMR – clear motion rating. It’s measured in Hertz but plasma and LCD don’t work using the same system. The CMR basically says 200 is better than 100, 400 is better than 200, 800 is better than 400. The 8000 series has a CMR of 800, because it has other technologies, not just the panel rate but local dimming and frame interpolation.” 

According to Sam Johnson, there can be side effects: “Interpolation techniques estimate what the picture content should be and if inaccurate can cause unwanted distortions. Higher frame rates can improve fast motion but have to be judged against any detrimental effect, which may be dependant on the viewer’s personal preference.” 

Moving pictures

Feature films use 24 frames per second (fps) and therefore differ slightly from the usual TV rate of 25fps. Here motion interpolation can create a rather artificial look. Increasingly, media players such as Blu-ray hardware, can output movies at an unaltered 24fps.  

“Smooth motion technology has been devised to balance the look of film so it appears in the way that most people are used to seeing TV images [at 50Hz],” says Steve Lucas. “However, some film enthusiasts are happy to see the movie displayed at cinema frame rate.” 

Even this could be set to evolve. Peter Jackson’s forthcoming The Hobbit is being shot at 48fps, however, current Blu-ray players and TVs may be unable to take full advantage of that. 

Ultimately the modes that a viewer uses depends largely on taste. “Manufacturers are setting the picture to the shop environment where the picture is optimised for that particular backdrop,” says Steve Lucas. “Due to the short viewing time customers have to evaluate the picture quality in-store, many settings on the TV are not ideal for home use. This is a good point to mention during discussions with the customer, that TVs can be set to their own preferred setting to suit their own home surroundings.”

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