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240 Hz and Beyond

November 20, 2009 By Geoffrey Morrison



Click the images below for bigger versions:
Figure 1: A standard video camera takes a single still image 60 times each second (60 Hz). Here Norman is moving quickly from the lower left to the upper right. All the movement between the frames is done "unseen" by the camera.
Figure 2: With no motion interpolation, 120 Hz TVs just double the frames. Frame 2 is identical to Frame 1 and so on. Frame 3 and 4 are the same as Frame 2 from the original video.
Figure 3: With mild motion interpolation, the TV creates a frame only slightly different to the original. Here, the created frame (Frame 2) shows Norman about 15 percent toward where he will be in Frame 3 (which is the same as the original Frame 2).
Figure 4: With strong motion interpolation, the created frame (Frame 2) is very different from either original frame, and shows Norman halfway between where he is/was in the original frames.
Figure 5: In some 240 Hz models, the backlight switches on and off, effectively showing each frame twice and creating a sort of faux 240 Hz. Norman's eyes are peeking out of "Frame" 4 for no reason other than it’s just so darn cute.

With claims of decreased motion blur, better detail, world peace, and smoother motion, LCD manufacturers are drastically increasing refresh rates. But why?

First it was 120 Hz. Now it’s 240 Hz. Already manufacturers are talking about 480 Hz. I mean, 60 Hz is so 20th century. Are there really any advantages to these faster refresh rates? If so, how do they work?

Well, to find out, I’m gonna bring back Norman.

To 120!

You remember Norman. Norman was our friendly visual aid for my modestly titled Definitive Guide to Video Processing. Sit, Norman, sit. Good dog.

Let’s start with the simplest scenario. Norman was recorded on video at 60 Hz (Hertz, as in the number of times per second). In Figure 1, you can see how he was recorded: moving from the lower left of the screen to the upper right. He’s moving rather quickly (a good trick for a sitting dog).

Figure 1: A standard video camera takes a single still image 60 times each second (60 Hz). Here Norman is moving quickly from the lower left to the upper right. All the movement between the frames is done

Figure 1: A standard video camera takes a single still image 60 times each second (60 Hz). Here Norman is moving quickly from the lower left to the upper right. All the movement between the frames is done "unseen" by the camera.

A video camera is just like a regular camera: It takes a series of still images. These are played back by your TV fast enough so they seem like motion to your brain. At 60 times per second, this is above your flicker-fusion threshold, and as such appears smooth. The “persistence of vision” you were probably told about in school is, as far as TVs are concerned, nonsense. More on this later.

So as the camera cycles (60 pictures per second), Norman has moved. He’s moving faster than the camera can capture him, so when you slow down the series of images, he appears to jump from frame to frame. Not a big deal, as it all gets kind of muddled up in your brain and it appears to be smooth.

There are multiple ways of dealing with twice the number of frames with 120 Hz TVs. The simplest solution is to just double each frame (Figure 2). All 120 Hz TVs, though, have motion interpolation.

Figure 2: With no motion interpolation, 120 Hz TVs just double the frames. Frame 2 is identical to Frame 1 and so on. Frame 3 and 4 are the same as Frame 2 from the original video.

Figure 2: With no motion interpolation, 120 Hz TVs just double the frames. Frame 2 is identical to Frame 1 and so on. Frame 3 and 4 are the same as Frame 2 from the original video.

 

This can go by different names (MotionFlow, Auto Motion Plus and so on), but they all basically do the same thing: Create new frames to insert between the old ones. They look at the original Frame 1 and Frame 2 and create a brand new Frame 1+2.

What varies is how many frames each system looks at and how “intrusive” the interpolation is (this can be adjusted on many TVs). For example, Figure 3 illustrates a mild version of the interpolation; the TV creates a frame only a little different than Frame A. Stronger interpolation (Figure 4) has a frame very different from either—a more exact hybrid of the two original frames.

Figure 3: With mild motion interpolation, the TV creates a frame only slightly different to the original. Here, the created frame (Frame 2) shows Norman about 15 percent toward where he will be in Frame 3 (which is the same as the original Frame 2).

Figure 3: With mild motion interpolation, the TV creates a frame only slightly different to the original. Here, the created frame (Frame 2) shows Norman about 15 percent toward where he will be in Frame 3 (which is the same as the original Frame 2 "source").

 

 Figure 4: With strong motion interpolation, the created frame (Frame 2) is very different from either original frame, and shows Norman halfway between where he is/was in the original frames.

Figure 4: With strong motion interpolation, the created frame (Frame 2) is very different from either original frame, and shows Norman halfway between where he is/was in the original frames.

Not So Fast

At first glance, this may seem like a great thing, and in one way it is. The smoother motion, as far as video is concerned, is actually a happy byproduct of the real reason LCDs moved to 120 Hz (We’ll get there in a bit). Strong motion interpolation, on video at least, isn’t that big of a deal. For most people, the added smoothness won’t look very artificial. In fact, it may be pleasing. The problem is with film.

Comments

I agree with this article and actually learned a couple of new things regarding the processing technique. The only argument I have is against the comment about "more expensive cables". I agree that you don't need special cables in regards to 120Hz certified or higher but you do need cables with certain capabilities, or that have been tested for a certain aspect. I am referring to 1080p, bit color etc. I do work at a big retailer and it does tick me off when sales associates take advantage (some actually don't know better) of customers. Since the boom of LCD HDTVs, I see more HDMI cables with 120Hz certification being sold. But onto my point, if the author of this article is saying that I won't see a visual difference from a $30 HDMI compared to a $100 HDMI when connected to a Pioneer plasma and running off a Pioneer Bluray player, he is sadly mistaken. And that also goes into the audio aspect of surround sound. Might as well say, you don't hear a difference between fiber optic and HDMI. This is just my opinion, take it lightly or heavily. It is open for discussion, and I mean friendly discussion. Again, very well written article on video processing and easily understandable.

Sorry, but you seem to have bought into the hype. It isn't possible for there to be a picture quality difference between a $30 HDMI and $100 (for that matter, $30 is WAY too expensive).

With HDMI, you either get the entire signal, and it's perfect, or you don't get a signal/it has artifacts. If it's the latter, the cable is defective and should be returned.

Your analogy between optical and HDMI is a false one, as you're talking about different audio formats (Dolby Digital/DTS vs. Dolby TrueHD/DTS-MA).

Over short runs there will be no difference in picture quality over HDMI. Over long runs, it's a different story, as the quality of the cable (regardless of price) is important to be able to get a signal at all.

I mention all this in the last page of this very article (direct link) and also we talk about why this is the case in this podcast.

Finally...a well written, easily digestable "manual" to the falsehoods of LCD TV tech, regarding their "ability" to render a natural image without overprocessing. Long Live Plasma! (I miss you, Pioneer.)

what about plasma vs lcd vs led as far as refresh rates are concerned?

First of all, there's no such thing as LED TVs.

Plasmas create a picture in a completely different way than LCDs. They create an image with many pulses of light per second. Increasing refresh rates with LCDs is one of the only ways to combat an inherent flaw with the LCD technology: motion blur. Plasmas don't have motion blur problems, so they don't need different refresh rates.

This is a vast over simplification, but the way to look at it is that LCDs need 240 Hz to have the motion resolution that plasmas inherently have at 60Hz.

what about local dimming, i.e. LED backlights?

As far as 120 and 240 Hz goes, the type of backlight doesn't matter. If a company is "flashing" or "scanning" the backlight to create a faux 240 Hz, that can be done with LEDs or CCFLs.

You can read about LED backlighting in this feature: LED Lighting: Little Lights, Big Light

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