Sat. Sep 23rd, 2023

Key Takeaways

  • Interlaced scanning is still used in some broadcasts because it saves bandwidth and allows for more visual information to be displayed without increasing transmission size.
  • Interlaced video scanning can result in visual errors, like interline twitter, but progressive scanning is more stable and less likely to have issues.
  • While streaming has become popular, broadcast TV still exists for viewers who prefer it, and interlaced video scanning is still useful for transmitting signals efficiently.

The TV industry has gone through innumerable changes over the years. One such change is the decreased use of interlaced video scanning. Interlaced scanning used to save on bandwidth—but that’s not much of an issue anymore in the digital era. So why do some broadcasts still use this scanning method?

What Are Interlaced and Progressive Video Scanning?

When video images are displayed on a monitor, each frame is created one line of pixels at a time. Starting from the top of your monitor, lines of pixels are drawn across your screen from left to right. Your TV keeps creating these lines of pixels from top to bottom until the full picture of each frame is shown. Even though this is a thorough and precise process, the human eye can’t track what’s happening because of how fast it occurs. For example, if your monitor has a refresh rate of 60Hz, this entire process happens 60 times in a single second.

What is Progressive Video Scanning?

Progressive video scanning is when a complete frame is displayed on a monitor before the following full frame is shown. In slower terms, every frame is drawn from top-left to bottom-right—one line at a time—until the entire frame of video is on-screen. Think of this like you’re fully coloring in a square or rectangle with a marker, but are doing so in horizontal lines starting from the top and moving across the page from left to right.

Black lines run horizontally to represent how progressive scanning works.
Reyadh Rahaman / How-To Geek

What Is Interlaced Video Scanning?

Interlaced video scanning is when a frame is separated into two half-frames. The first half contains all the odd-numbered lines of pixels needed to generate an image while the second half is composed of the even-numbered lines of pixels. Instead of an entire frame being shown at once (like with progressive video scanning), your monitor displays the first half of a frame and then, after it leaves the screen, is replaced by the second half.

Going back to the coloring analogy above, if you were to replicate how interlaced video scanning works by filling in a square or rectangle with a marker, you’d still be drawing horizontal lines—but skipping every other line.

Black lines run horizontally with large white spaces between them to represent how interlaced scanning works.
Reyadh Rahaman / How-To Geek

Keep in mind that this is only the first half of the full frame of the video. After these odd-numbered lines of pixels are displayed, they leave the screen entirely before the even-numbered lines of pixels (in the second half of the full frame of the video) are displayed. Both halves of a full frame are shown so swiftly back-to-back that you won’t notice it…in most cases.

When You See Interlaced Scanning Lines

One case where you’ll most likely notice the separate halves of interlaced video frames is when a video recorded in an interlaced format (720i, 1080i, etc.) is displayed on a progressive monitor (like a 1080p one). Since progressive monitors aren’t built to use interlaced video scanning, the two halves of the full frame don’t get displayed properly. Such a monitor would show each half of a frame as if it’s a full frame, which leaves each half on-screen long enough to be noticeable.

Why Are Most Blu-rays and Streaming Video Progressive?

Video files tend to get manipulated, transmitted, and altered quite a lot from when they’re created to when they’re displayed. Progressive video scanning is more stable, and it’s less likely to have issues due to every bit of information for each frame being stored in one place. When displayed, even if it has file damage (like artifacts), it’s not as noticeable as what happens with interlaced video.

One type of visual error that can happen with interlaced video files is called “interline twitter,” which—despite how it sounds—has nothing to do with the social media platform. Interline twitter is when you can see parts of an interlaced video vibrating as if something is constantly shaking your monitor. This can occur when there is a delay between frames being displayed on a monitor. As a result, there’s almost a strobe effect.

Errors like this don’t happen with progressive video, so, as a result, it’s the type of scanning that’s the most stable to use. To cut down on the need to fix, adjust, and remove bugs, glitches, and errors from media, progressive scanning is simply less troublesome to use for our massive digital media needs.

“But There Are Interlaced Blu-rays”

Yes, and they’re meant for interlaced monitors! If you play a Blu-ray that contains an interlaced video file on a progressive monitor, you’ll see the scan lines—which most certainly does not add to your viewing experience. That is unless you’re using such methods as some sort of hypnotherapy. To each their own.

Why Are Some Broadcast TV Channels Still Interlaced?

Interlaced video uses way less bandwidth. As such, it’s much cheaper to produce, transmit, and display. Using interlaced video scanning instead of progressive video scanning is a way to fake having twice as much visual information. Since human eyes can’t track the separate halves of each interlaced video frame (in most cases), it looks like there are two frames instead of one that’s being displayed one half at a time.

Keep in mind that we’re talking about broadcasting here—which is quite different from the modern methods of transmitting video files through streaming services. Broadcasting involves sending out signals via some sort of device (like a satellite dish) so that it can be received from another device (like an antenna). Cutting down on how many signals you need to transmit, as well as decreasing the size of said signals, drastically cuts down on how much work needs to be done by both the output and input devices. On top of that, the time needed also decreases proportionally.

If you think streaming killed TV, you’re not entirely wrong; however, many TV channels out there still persist…for now. There are still at least a couple of generations of TV watchers who are determined to enjoy their preferred form of media. It’s for these people that broadcast TV still exists, and why interlaced video scanning is still useful. Some dedicated viewers have even been able to watch some of these “terrestrial TV” channels for free. Don’t let anyone tell you what to watch or how to watch it! If you want to try out this method of free media, all you need is a solid antenna and the right know-how.

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By John P.

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