Plasma TVs: the 600Hz feature explained

There are nights when I wake sweating from a terrible nightmare in which I’m a signal-processing engineer stuck in the middle of the 20th Century, trying to work out the standards for CRT TVs. NTSC, PAL, 60Hz, 50Hz, 29.97 frames a second… the horror, the horror.

The worst part of the dream is knowing that the choices I make back there in the ’50s and ’60s are going to affect a whole new generation of completely different TV technologies in 2009.

We have entered a new and dangerous phase in the Battle of the Hertz. Some pundits might say the age of the plasma TV is past, and LCD (and eventually OLED, maybe) is destined to completely replace it. But it’s not going without a fight. A big numbers fight.

Panasonic, Samsung and LG now have plasma displays boasting a ‘600Hz subfield motion’. Yes you read that right: 600Hz. Why, the best the competitors have been able to do so far is 200Hz! These TVs must be at least three times better, right?

Well, that’s not an entirely straightforward question to answer, but it’s definitely the catalyst for some of my more disturbed tech-based dreams of late.

Math wonks will of course immediately recognise that 600Hz is a ‘magic number’ when it comes to TVs. Here’s why: modern flat panel TVs have faced a constant struggle to display a variety of sources in a perfect frame-by-frame ratio.

This is because NTSC TV runs at 60Hz (or more accurately 60 fields per second interlaced… or 30 frames a second… or 29.97 frames a… never mind) and PAL runs at 50Hz. Blu-ray movies, as we’ve had hammered into our brains by now, run at 24Hz – though this is usually referred to as 24 frames a second.

A quick break-out note on 50Hz here: there’s a difference between the frame rate of the moving image on the display, and the frequency at which the display runs as an appliance. Because our AC power alternates 50 times a second, that’s why Aussie TVs are 50Hz – and to match this, TV in Australia is broadcast at 50 frames a second. You can run your TV in 60Hz mode – your NTSC DVDs will look good, but your Aussie free-to-air broadcasts may exhibit some oddities when scrolling text, due to the 50Hz signal being converted to 60Hz by your TV.

Confused? Welcome to the nightmare.

The problem with these three numbers – 60, 50 and 24 – is that none of them divide into each other. The best fit is 24fps and NTSC – a ratio of 2.5:1 is neat enough to use a system called 3:2 pulldown. Hit up Wikipedia for a long and not slightly tedious explanation of how this works.

So why is 600 a magic number? Because 60, 50 and 24 all divide equally into it. No matter what kind of signal you give a 600Hz display, it can simply multiply individual frames to create a smooth, flicker-free image that runs at the correct speed.

Explanation finished, everyone back to their own beds? Not exactly. It never gets to be that simple.

These 600Hz TVs don’t actually refresh their image 600 times a second. In fact, they’re still 50Hz or 60Hz sets, depending on the country they’re sold in.

Plasmas use a system called the ‘subfield’ where the same image is repeated. In traditional displays, this is done eight times a second. Why? To eliminate ‘phosphor lag’, or colour artefacts caused by the individual cells in the plasma grid switching on and off.

Until now, most plasmas have had a 400Hz subfield frequency: which is why each image is flashed eight times a second. A 600Hz TV ups this to 12 times a second, for a 50Hz signal.

Where 600Hz becomes useful is with those other frequencies: 60Hz from NTSC, and 24 frames a second from Blu-ray. When playing an NTSC signal, the subfield flashes each frame 10 times a second. And when playing BD, 25.

Does this result in a smoother, more realistic image? Does it do the same thing as 100Hz, 120Hz and 200Hz image interpolation circuitry? Not really. But with Blu-ray in particular, more field updates (instead of forcing the TV to only update the image 24 times a second) should provide a more solid image on plasma. But if you were to cock a cynical eyebrow at this exciteable marketing speak (more numbers, more better!) why, we probably wouldn’t chide.

A final note: the subfield applies only to plasma displays: you won’t be seeing 600Hz LCDs – at least, not in 2009. Next year? Who knows. Kilohertz? Gigahertz? The nightmare continues…

Plasma and LCD tech: related reading

When TV tech hertz: be wary when buying budget LCD TVs – buying a flat panel TV can be a tricky experience, with some budget brands presenting a unique trap for unwary buyers.

200Hz TV technology – what, why and how? – In 2009, the tech du jour is 200Hz motion interpolation on HDTVs. Sony makes a big deal of it in its latest Bravia TVs, and Samsung too has 200Hz tech. But what exactly does it mean? And how can it make your TV experience better?