The frame ga
According to the numbers on spec sheets, TV frame rates are getting greater all the time but what do they actually mean? Ian Calcutt reports.
any 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
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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
Sony Monolitic range 20 The Independent Electrical Retailer October 2011
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