Black frame insertion (BFI) is, to put it simply, inserting full-screen black images between the original frames of a source.
Hmm, OK, that didn't really put it simply, and I'm pretty sure I defined a term with the term.
How about this: black frame insertion is a way to reduce motion blur on LED LCD and OLED devices, which is definitely a good thing.
Here's what it does.
The problem is blur
All LED LCDs, and the current generation of OLED TVs, suffer from motion blur. This is where anything that moves, including the whole image because of a camera pan, blurs slightly compared to when nothing was moving. It's sort of like the moving parts of the image lose focus briefly, then come back into focus when they stop moving (it actually has nothing to do with focus, but the visual analogy works).
In the early days of LED LCD, motion blur was caused predominantly by the response time of the panel. This was how quickly the liquid crystal could change from one state to another, which is necessary to create an image. These days, this is less of an issue -- and not at all an issue for the much faster OLED displays.
Instead, the problem is called "sample and hold." The way LED LCD and current OLED TVs create an image is they "paint" the screen with a single frame, then hold it there until it's time to "paint" the next image. With 60 Hz TVs, this is 16.67 milliseconds per frame.
The problem is your eye/brain expects to see that object move, and your eye tracks where it should go. Except, the image isn't moving, it's just sitting there. The stationary image blurs across your moving retina.
If you want to dive into even more detail on this, check out BlurBusters' great article Why do some OLEDs have motion blur. Another great resource is actually from Microsoft. It's a little old, but the basics are still applicable.
The solution is to paint it black
CRT and plasma TVs don't have motion blur problems (at least not to the same extent) because they don't use sample and hold. CRTs, not that you can find any anymore, would draw the screen line by line. So by the time the electron gun reached the bottom corner, it was time to start at the top opposite corner. There was a lot of blank time, like a shadow chasing the sun.
Plasma TVs, which are disappearing fast, create each image with multiple flashes. Which arguably is sort of sample and hold, but doesn't have the same sort of motion blur problems. Check What is 600Hz for more.
What black frame insertion does is mimic how your brain reacts to CRT, plasma, and traditional film presentations. By creating the image, then taking it away -- by inserting a black, or much darker, full-screen image between the lit frames -- the LCD is better able to fool your brain into thinking it's seeing smooth motion.
The problems with the solution
Black frame insertion has been around for a while, but it never caught on the way other motion blur-reducing tech did. Largely this is because it reduces light output. With black (or a much darker version of the original image) on the screen for a significant portion of the time, the overall apparent brightness goes down. Typically light output is slashed in half, although some better examples, like the "LED Clear Motion" setting on select 2013 Samsung TVs, reduce it by much 30 percent or even less.
Another reason is that, depending on how it's implemented, it can cause visible flicker. The Impulse mode on some Sony TVs is one example where we noticed flicker.
But the main alternative method for reducing blur is the dreaded (loved?) Soap Opera Effect.
However, as these things do, BFI has improved since its early days. On the Samsung KN55S9C OLED that David Katzmaier reviewed (I did too, but *cough* for another Web site), the BFI worked brilliantly to nearly eliminate motion blur, but not dimming the image appreciably. Since the OLED is so bright anyway, a drop in light doesn't really matter.
At CES 2014, we saw a number of TVs with BFI, including a tech demo with Samsung where they showed an LCD with BFI that was still perfectly bright, but had little to no motion blur. Most LCD companies use something like BFI, to varying success -- often by "scanning" the backlight to darken different parts in sequence. This is one of the main technologies used to artificially "boost" refresh rate numbers. What's a 60Hz TV with backlight that flashes between frames? Many manufacturers would claim that makes it 120Hz, or at least 120Hz equivalent.
Just shut up about TVs and tell me about Oculus!
Aside from LCD and OLED TVs, one other technology using BFI is the Oculus Crystal Cove, a prototype virtual reality 3D gaming headset. Its black frames are employed for slightly different reasons.
The Oculus developers found that while their head tracking and processing was fast, it wasn't quite fast enough to avoid a slight smearing effect due to the lag. In other words, if you moved your head, the Oculus would sense that, tell the computer to update what you were looking at, the video card would render that, and you'd see it.
In the prototype I saw at last year's CES, this smearing effect was very slight, but they wanted to fix it. Moving to a OLED screen, faster than the original LCD, was one step, and adding a period of black was the other. The final result is a lot less lag: 30 milliseconds, versus the original dev kit's 50 milliseconds.
They explained that the black "frame" actually persists for a majority of the time, while the screen is lit for a much briefer period. Since brightness isn't an issue (your eyes are in a sealed, black container), the resulting drop in light output doesn't matter. Personally, I'd argue that this unintended consequence probably helped, since I find most smarphone screens (which they're presumably using) are still too bright on their lowest settings in a dark room. They give meeye fatigue.
While it seems a bit weird, adding a black frame, or a partially black image for a period of time, is a fantastic way to reduce motion blur without resorting to the Soap Opera Effect. Sure, it can drop the light output a bit, but these days TVs are so bright this is rarely a concern (though flicker might be, depending how it's implemented). Since it's looks like we're going to be stuck (pun intended) with sample and hold for a while, it's good that this method exits for those of us bothered by both motion blur and SOE.
Got a question for Geoff? First, check out all the other articles he's written on topics like why all HDMI cables are the same, LED LCD vs. plasma, active versus passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.