Ask Dan: My brain Hertz

Publication date: 26 February 2009
Last modified 03-Dec-2011.


I'm going to set up a HTPC. The viewing portal will be a Sony LCD, 1920 by 1080, 200Hz refresh rate/flicker/whatever thing. The video card is a GeForce 9800GTX+.


Does the video card driver need to be able to "output" 1920 x 1080 @ 200 Hz? The 200 Hz is supposed to be a feature of Sony's Fluidmotion fluff. I am concerned that I wont be getting my full dose of fluff if the video card can't come to the fluff party.


Quick answer: You'll be fine.

This can be a bit confusing, though, because most LCD monitors have two components which each have a "refresh rate". First, there's the LCD panel itself - the sandwich of polarisers and liquid-crystal elements and thin-film transistors - which usually displays 60 frames per second. The panel needs to be backlit, though, for you to be able to see what it's doing. Normal LCD-monitor backlights are fluorescent tubes; fancy recent monitors use LEDs.

The cold-cathode fluoro that illuminates most monitors is driven by high frequency AC power - much too high for any flicker to be visible to the human eye. But if you turn the brightness down from its eye-searing maximum, the lamp driver will probably use pulse-width modulation to make the light dimmer, and the PWM operates at a much lower frequency, often around 200Hz.

In a CRT monitor the "display" and "illumination" functions are both done by the electron beams and phosphor dots, so there's one simple refresh rate for everything. And, because the monitor's painted from top to bottom in each refresh and only a quite small horizontal band of it is actually brightly illuminated in any given thousandth of a second, you can readily see the flicker if you, for instance, wave your hand back and forth in front of the screen.

You can see faint flicker effects if you wave your hand in front of a 200Hz-backlit LCD, but it's not nearly as noticeable. This is partly because of the higher flicker frequency, and partly because the fluorescent backlight lights the whole screen at once, rather than painting it with a tiny bright raster dot.

I don't think the new-fangled LCDs that're backlit with LEDs have this sort of flicker at all, but I'm not certain. If they adjust brightness by pulse-width-modulating the LEDs, they may retain a similar faint 200Hz flicker when they're not running at full brightness.

LCD televisions with "200Hz!" (or an even bigger number) printed on a big sticker on the front are a special case. They take the same inputs as other flat-panel TVs, but have a genuine 200Hz frame-rate, at least in theory. What they do to get that frame rate from inputs that have lower frame rates is take the incoming video - let's say, 50Hz interlaced standard-definition PAL video from a TV tuner - and fiddle with it to, in theory, make it look better.

So, for instance, they'll get rid of the jagged "tearing" effect of simple deinterlacing, and generally try to make the video look as if it's got more resolution and/or frame-rate than it actually has, with any luck without introducing new artefacts of their own. Sony call their version of this idea "MotionFlow" (not "Fluidmotion"), and the TVs that use it apparently work perfectly well.

This is the same sort of thing that "upconverting" DVD players do. They taking ordinary DVD video (maximum vertical resolution 576 pixels) and adding more pixels to bring it up to 720 or 1080 lines. The result usually looks pretty good. Not as good as proper HD video from a Blu-Ray disc or HD digital TV tuner, but a worthwhile improvement.

Just as upconverting DVD players play ordinary DVDs, you can plug ordinary video sources, into a "200Hz" TV set. (Including computers, if the TV has suitable inputs.) As far as the things sending video to the TV are concerned, nothing unusual is happening.

In the last few weeks, however, there's been a sudden burst of interest in "200Hz" computer LCD displays, not TVs. This is because Nvidia have released their "GeForce 3D Vision" system.

3D Vision is conceptually much the same as the shutter-glasses system I reviewed a million years ago, except, you know, way better.

To use shutter-glasses 3D, you put on a pair of LCD spectacles that black out your left eye, then your right eye, then your left, very very fast. If the computer changes the image on the screen so that the left eye sees one view of the scene and the right sees another, you get realistic 3D.

To make this pleasant to view, though, the monitor needs a very high refresh rate. Since each eye only gets to see the screen half of the time, the effective refresh rate for each eye is half of the total rate. 100Hz - giving 50Hz for each eye - is the minimum spec for 3D Vision.

3D Vision nominally supports graphics cards down to the GeForce 8800 series (ATI cards aren't supported), but because the two viewpoints mean the graphics card has to do a lot more work, you're likely to need at least a GeForce 200-series card to get a decent frame rate.

("Frame rate" is distinct from the monitor refresh rate, here. I think 3D Vision works like ordinary graphics with vsync turned on; if your graphics card isn't fast enough to have a new pair of frames ready each time it's gotten through showing the last pair, then you'll just see the last pair out of the buffer again. That last pair of frames will look just as 3D the second, or third, or ninth, time around as they did the first time, but you'd probably prefer some fresh frames, if you're not just standing there admiring the landscape of Liberty City.)

There are plenty of old CRT monitors that'll work with 3D Vision, if you can tolerate 100Hz-or-50Hz-for-each-eye. It's not even hard to find CRTs that can do 120Hz. If you'd rather have an LCD, though - and, today, you obviously probably would - then you'll have to buy one that's capable of at least 120Hz real refresh rate, and preferably 200Hz. There are only a few of those on the market at the moment, all around the 20-inch mark and all more expensive than otherwise-identical 60Hz models.

As I write this, Nvidia themselves are offering a 3D Vision bundle pack including the glasses and a 22-inch Samsung 2233RZ monitor. The 2233RZ spec sheet contains the brilliant line "120Hz Dual Engine", which is Samsung's way of saying that this monitor can do 120Hz.

Nvidia seem to have got themselves an early-distribution deal, here; that monitor is not going to be on retail shelves until April, whereupon it'll list for $US399. But the combo pack can apparently be yours right now for $US598. (Note that this doesn't include a video card - as I mentioned above, you're probably going to need one of the big GeForce 2x0 boards.)

For comparison purposes, here in Australia m'verygoodfriends at Aus PC Market have Samsung's 2243BW, which is the same size as the 2233RZ and has the same resolution, but a normal refresh rate. It's $AU390.50 from Aus PC, including delivery in the Sydney metro area (delivery elsewhere in the country will cost a bit, but not a lot, more). At current exchange rates, that's around the $US250 mark. The $US399 eventual-list-price for the 120Hz 2233RZ adds up to about $AU620, as I write this; that'll buy you a nice 24-incher, delivered to anywhere in the country, with change left over. Readers in the USA can expect to pay a little less again.

This isn't to say that I think 3D Vision is a rip-off. By all accounts it works quite well with all sorts of 3D games, and outstandingly well with others. The old 3D-glasses systems needed careful configuration for different games, special drivers and who knew what else; 3D Vision is pretty much just plug-and-go.

And six-hundred-odd US dollars may seem a bit pricy for a 3D system based around a mere 22-inch monitor, but it's all relative. I remember when poxy little 15-inch LCDs cost a couple of thousand bucks and an 18-incher would set you back $AU6500.

If you've already got a 120Hz-capable CRT sitting around, and you've also got a GeForce 8800 or better, you can pick up just the 3D Vision glasses kit for $US199, which isn't peanuts but isn't a fortune either, and see how you get on. The $598 glasses-plus-monitor bundle pack has a precisely zero per cent combined-purchase discount, after all, so it'll be no biggie if you decide you want a new monitor after all.

But do bear in mind that "200Hz" televisions are not the same as "200Hz" computer monitors. Any minute now, we'll be seeing complaints from poor schmoes who've spent all week trying to make 3D Vision work with their new $US2000 "200Hz" LCD TV.

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