Refresh Rate

When a moving image is displayed, it is a sequence of still images. The speed at which these images is shown is the refresh rate. It can me represented in a number of different ways.

• fps - Frames per second. Normally used to discuss content.
• Hz - hertz, a measure of events hapening per second, used to specify devices.
• ms - milliseconds, the time to display a single image. Hot topic for gamers.

Split between horizontal and vertical measurements for screens. We are interested in the vertical component.

Origins

Historically, when movies were first made, camera's were hand cranked. This led to various different refresh rates for the same movie. A standard of 24fps was chosen to replay all the fluctuating recordings. It produced a smooth image, and didnt use too much expensive film.

This has remained the case for the majority of movie production. One main exception to this was Peter Jacksons, "The Hobit" shot at 48Hz in 2012.

Movie camera - 1910, hand cranked

Television

Since the first television, displayed images have been constructed in rows accross the screen. The time to complete a full image is the refresh rate. Original TV's refresh rate was 50hz (50 frames per second, 20ms). But TV shows are recorded at 30 or 60fps. FHD TV's were introduced with 100hz then 200hz, when 4K entered the market the increase in image data reduced the max refresh rate to 120Hz. 3D also requires a minimum 120hz and gaming requires as high as possible to reduce latency.

60Hz refresh, 16ms

120Hz refresh, 8ms

Gaming

A higher frame rate can, at most times be the difference between a winner and a loser in competitive gaming. Advantages include:

Clarity

It ensures that the subsequent images produced from the graphics card don’t get blurred.

Lower latency:

So you see things faster. A 240Hz screen will display 4 times faster than a standard 60Hz screen. So you have more data loading in less time. So you can be, ever so slightly ahead of the pack.

Less eye fatigue:

Less eye strain because you don’t have to focus a lot on the images, since they’re significantly clearer and smoother. The reflected ligh from a projector reduces strain too.

The importance of Refresh Rate for gaming

Motion Blur

Its all in your head. Motion blur, is your brain processing the moving image. In slow scenes enough data is supplied to your brain for it to see smooth motion. As the action increases, the object moves accross the screen in larger increments, spacing increases. This manifests in your brain recognising what the motion actually is, still images. Increasing image sizes with bigger TV's and large screen projectors can exacerbate the issue, as they increase the distance the moving object moves between frames.

Motion Blur

So when we look at the solutions for motion blur, we can see ajusting the projection speed can help. If we look at movies, they are recorded at 24fps, but we are projecting at much faster speeds. In order to project the available 24 images at a higher speed. Extra images are needed to bridge the gaps. There are a number of different solutions for this.

3:2 Pulldown

The 3:2 pull down process is used to transfer film to video. It converts 24fps into 29.97fps. Only a slight increase.

3:2 Pulldown

Image Interpolation

So the obvious solution is to increase the number of images from 24. Interpolation creates frames between the existing frames. This process tries to predict the motion frame by frame, creating a frame with movement that bridges the gap between the original frames. Interpolation can effectively increase the frame rate to 30 or 60fps. This creates an image more in line with a TV production. Movie perfeccionardo's call this "the soap opera effect".

Creating ultrasmooth motion can look artificial, some people feel it takes the movie feel out of the movie. Hollywood creators hate it, too, because it isn't what the director intended for his or her creative vision. If they wanted to record at 48fps, they'd have recorded at 48fps. Fortunately, devices allow it to be turned off, or adjust the frame interpolation intensity.

Simplistic Interpolation process

Black Frame Insertion

If you dont like the soap opera effect, Black Frame Insersion (BFI) can mitigate it. As the name implies a black frame is inserted between the real frames, doubling the refresh rate.

BFI process - used for TV's and gaming consols

De-interlacer

Standard TV broadcasts signals are sent in an "Interlaced Scan" format. A TV screen first draws the image's odd lines, one at a time sequentially from top to bottom (which takes 1/60 of a second), and then fills in the even lines (taking another 1/60 of a second). That is, the full picture (top to bottom) is first drawn with half its information hollowed out, and then the other half is filled in -the entire process taking 1/30 of a second.

A newer and superior scanning method called "Progressive" permits the entire picture to be drawn sequentially from top to bottom without the odd/even interlacing. Some newer DVD players now have outputs for both an interlaced and progressive scan image. And HDTV signals are now being broadcast in both progressive and interlaced formats: 720p (720 lines of resolution in progressive scan format) and 1080i (interlaced).

Most projectors contain deinterlacer or "Line Doubler" circuitry that changes the interlaced signal into a progressive EDTV format. This is accomplished by waiting a full 1/30 of a second to receive both the odd and even lines before projecting them together onto the screen. During this split-second wait, the previous image frame continues to be projected a second time, so there is a fully formed image being displayed at all times.

Despite its name, there is not actually a "doubling" in the number of lines of resolution. But there is a doubling of the amount of time that each image frame is displayed, resulting in a picture that not only is devoid of "flicker," but which is also brighter.

With a high-quality line-doubler (and not all of them are), the resulting picture quality from an "interlaced" source is absolutely superb.

Progressive Scan:

Progressive scan - whole image one hit

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Interlaced Scan:

1st Scan of even lines

2nd Scan of odd lines