Plasma TV: what is 600Hz?

Plasma TV specifications hype "600Hz" refresh, but what does this mean compared to LCD's 100Hz or 200Hz?

(Credit: Geoffrey Morrison/CNET)

If you read my "What is refresh rate?" post, you'll know that plasma TV manufacturers (Panasonic, Samsung and LG) make a point in claiming a "600Hz" refresh rate on their TVs. As we discussed in that article, it's not exactly comparable to LCD's 120Hz and 240Hz refresh.

So what is 600Hz, and how does it work?

By its nature, this is a pretty complex subject, so bear with me, and forgive some over-simplifications.

Before we get to 600Hz, we have to discuss how plasma TVs create an image in the first place. Like all modern TVs, they have more than 2 million individual pixels: 1920x1080 = 2,073,600. (Some lower-priced models are 1024x768, so they have 786,432 pixels.)

Each of these pixels has three sub-pixels, one each for the primary colours of red, green and blue. If you were to stick your face right up to the screen, the pixels would look something like this:

A close-up of the pixels from a modern HDTV. Each red, green and blue sub-pixel is collectively referred to as a single pixel.
(Credit: Geoffrey Morrison/CNET)

There are a few variations, like certain Samsung plasmas having a PenTile layout, but the basic functionality works the same. For simplicity, we'll just talk about traditional RGB sub-pixels.

If all three sub-pixels are lit, and your face isn't right against the screen, the pixel will appear white. If just the red and blue sub-pixels are lit, you'll get magenta. Just red and green, and you'll get yellow. Mix and match these, and you'll get more colours than the paint aisle at Home Depot (well, maybe, but we'll get to that).

It's the mixing and matching that is actually the tricky part.

Each sub-pixel is actually a little bucket of air, a noble gas to be specific. If you send a jolt of electricity though this gas, it and the electronics within the bucket emit UV light. This UV light, all excited from its recent magical conjuring (note: it's not magic), wants to get everything else in on the action. In the case of the sub-pixel, the only "else" that's there is a lining with a coloured phosphor. When excited by the UV light, this phosphor emits its own light, either red, green or blue, depending on the sub-pixel.

Got that? In short, each sub-pixel creates light by energising a phosphor, which glows a single, specific colour.

Not only does it glow a specific colour, though, it also glows one specific brightness. Each sub-pixel has only two states: on or off. Like a light switch.

As you can imagine, this makes creating a normal-looking image rather tricky. After all, how do you get grey when all you can create is black or white? Or, to be specific, how do you create "purple" when all you can create is blue, magenta, red or black? The trick is called "pulse-width modulation", which is a fancy way of saying that you turn it on and off a lot, really fast.

Say you flip a light switch 10 times in a minute (kids, don't try this at home). Evenly spaced, the light will be on for six seconds, then off for six seconds. For the whole minute, it's on for a total of 30 seconds, and off for 30 seconds, right? Something like this:

Cycle a light switch 10 times in a minute, and it will be on half the time and off half the time. If you want to make the room brighter, you can leave the switch on for additional cycles, instead of turning it off.
(Credit: Geoffrey Morrison/CNET)

On average, the total amount of light in the room is 50 per cent of what's possible. You have a maximum (light on) and a minimum (light off) each half the time. If you increase how often you switch the light, so fast that you no longer see the light turn on and off, instead of a flashing light bulb, the room will just appear dimly lit.

This is, in essence, how a plasma works. As a plasma TV engineer once told us (as we mentioned in the refresh article), plasmas create light with time. To create different shades of gray, the pixel is on (or off) more often.

Because our TV system is 50Hz (or, in some OS places, 60Hz), and each one of those cycles has to be a complete image — for example, 50 images per second (or 60) — a plasma has to switch the sub-pixels on and off very rapidly. For many years, plasmas had eight of these potential on/off cycles for every frame of the image. You could call these, to steal the modern parlance, 480Hz. Most plasmas now have 10.

While it's not completely accurate, you can sort of think of it as the TV creates 10 dark images that your brain combines into one full-brightness image, 60 times a second. Something like this:

While a little over-simplistic, you can think of a plasma as creating 10 "dark" images that your brain combines into a single, correct brightness frame.
(Credit: Geoffrey Morrison)

Bottom line

As we discussed in "What is refresh rate?", LCDs use a higher-than-normal refresh rate to combat motion blur. Plasmas, by how they work, don't have an issue with motion blur. The 600Hz claim by all the current plasma TV manufacturers has to do with how the TVs create an image, but is not 600 images per second.

So, yes, 600Hz is a marketing term. However, because plasmas don't require faster refresh rates to produce a clear, sharp image, and this is technically how they work, it's fair to give Panasonic, LG and Samsung a pass ... for this one.


More on: Plasma, Refresh Rate, Tv, Lcd

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