1. What Is Interlaced Video?
Interlaced video is a technique that was originally created and made popular before the advent of digital televised content. First developed over 70 years ago, it was primarily for television video formats like NTSC and PAL.
At its root, interlacing was an early form of video compression that was used to make the video look smoother while sending less data. This was achieved through breaking up each full frame of video into alternating lines taken from two separate fields that were captured at slightly different times. After this process, one set of lines would be delivered to the viewer before 1/60th of a second later the second set of lines would be sent.
In contrast to other possible methods of the time, this process granted what appeared to be smooth movement, at least to the human eye, while being able to send less data related to the broadcast. Interlacing can cause issues, though, trying to deliver that feed to a progressive source due to the differences in presentation between the two.
1.1. Progressive Video Fand How It Differs From Interlaced Video
Unlike interlaced content, progressive video is a video track that consists of complete frames. There is a slight asterix to this statement as techniques like interframe can be used to compress video content to remove redundancies from frame to frame (read more about the interframe process). Even including this technique, progressive video content will not alternate fields and will present a full keyframe that you will never find in interlaced content. This means it won’t serve odd or even lines at different time intervals from each other.
Consumers will be familiar with this terminology due to its proliferation in HD content. For example, 1080p content means it has a vertical resolution of 1080 lines while the “p” relates that this is progressive content.
1.2. Which Method Is Better: Progressive Or Interlacing?
To be blunt, the answer is it actually doesn’t matter which is better. Many playback methods, like computer monitors or modern HD TVs, do not support interlacing. So even if interlacing provided better looking content, a broadcaster would still want to go with progressive delivery due to support for this method. Otherwise, the broadcaster would be displaying interlaced video in a progressive format.
Assuming both methods were supported equally, the human eye can’t keep up and the motion should look smooth regardless.
1.3. What It Looks Like: Interlaced Content As Progressive Video
Sometimes a broadcaster needs to use an interlaced source for streaming. In other words, taking an interlaced source and make it progressive or watching it in a progressive medium, like a computer monitor. This need can range from wanting to use an older broadcast to using an analogue camera that supports interlacing.
Converting the video involves combining the two fields, that were created as part of the interlacing process, into a single frame. By default, this process creates a rather ugly artifact on high motion in the video track. The motion between fields can cause visible tearing when displayed as progressive video. Essentially, the video track shows two different line fields where the fast motion is occurring, creating a staggered line appearance as seen in the image below on the figure to the left.
1.4. How To Tell If Your Camera Captures Interlaced Video
A lot of this article has talked about interlacing as a legacy component, but that doesn’t give a fair representation. A lot of analogue cameras, for example, are setup to deliver video in an interlaced manner. Even some modern digital cameras still offer interlaced mode. Reasoning behind this is partially for compatibility and also 1080, even 1080i, is a strong selling point and it’s cheaper to do 1080i. Because of that, even though interlacing might be sometimes associated with older, televised broadcasts, it’s still very possible to use an analogue camera with a capture card or another setup and still run into interlacing.
One method to tell if your camera was setup for interlaced content or not is in the specs. While some will be overt, describing that the camera outputs in interlaced mode, others will state it in their mentioned resolution. For example, we already discussed that 1080p is an HD feed that is progressive. If that stated 1080i, though, it would mean it’s HD, interlaced content. Chances are good that someone has seen 1080p content much more frequently than the interlaced version. Most modern analogue cameras, if they are interlaced, should mention it either directly or with the resolution. If it’s an older analogue camera, from before 2003, it outputs interlaced content as the first consumer-affordable progressive camera was the Panasonic AG-DVX100 that was released in 2002.
1.5. What Is Deinterlacing Video: When You Have To Use Interlaced Sources
Thankfully, there is a process called deinterlacing which can solve issues created from presenting interlaced content in a progressive medium. Deinterlacing uses every other line from one field and interpolates new in-between lines without tearing, applying an algorithm to minimize the resulting artifacts.
