Video Capturing Concepts: Interlaced vs. Progressive, IVTC and Deinterlaced
the Frequently Asked Questions…
- Why do I have comb lines in my captured video?
- Why does my video capture stutter, jerk and strobe?
- Why are the pans and motion not smooth on my converted videotape?
- Why are there zigzag lines and stair-steps shapes in my DVD conversion?
- Why is my video recording soft, blocky, and/or lacking in detail?
- Why is the image blurry and ghosted on my video capture?
and The Digital FAQ answers…
When capturing and recording video, it is essential to know when video is interlaced, how it is interlaced, and possibly even why it is interlaced. The wrong capture settings can create a video that is anything from low quality to completely unwatchable. Since most people plan to view the final video transfer on a television, a capture must be interlaced to retain full quality.
Many novices make the mistake of deinterlacing video intended for DVD or Blu-ray, thereby reducing quality in the process.
What is Interlacing?
Interlacing is a method used to both store and display video. For example, a standard 30 frames-per-second video (30fps, or more accurately 29.97fps) is composed of 60 images weaved together in alternating lines. Or 25fps for PAL and SECAM, with 50 images shown.
These “images” are technically referred to as fields. A 29.97fps video has 59.94 fields, and a 25fps video has 50 fields.
All televisions properly display interlaced video, and a viewer will only see one field at a time.
Computer monitors, however, are not capable of natively displaying interlaced video, being progressive-only display devices. The computer CRT and LCD monitors will show the raw video with interlacing comb lines. And for this reason, many inexperienced videographers feel the need to “remove” or “fix” what is actually supposed to be left as-is.
Which Video Formats are Interlaced?
Most video is interlaced. This includes –
- All analog broadcasts: cable, satellite, over-the-air antenna
- Most digital broadcasts: 1080i HDTV and 480i SDTV)
- All analog videotape formats: VHS, S-VHS, Betamax, Betacam SP, Video8 (8mm), Hi8, U-matic
- All analog disc formats: Laserdisc, CED
- Most digital videotape formats: MiniDV, Digital8, HDV, DVCam, DVCPro50, Digital Betacam
- Most digital disc formats: some DVD-Video, Blu-ray and HD-DVD
Unless the footage was created on a computer, or is an official release of a film, then odds are that the source is interlaced.
In the digital world, only some video codecs support interlacing. This includes most MPEG-2 and MPEG-4 variants (XviD, Divx, AVC, AVCHD, H.264, etc), as well as non-distribution intermediary codecs used solely for editing workflows (Huffyuv, Apple ProRe, DNxHD, Cineform, and others). Not all resolutions, frame rates and advanced settings support interlacing. For example, the interlace barrier for MPEG-2 is at the approximate x280 resolution, meaning VCD and other lower-end formats do not have the luxury of interlace. And this is the primary reason VCDs are such a low quality format.
Notice that mainstream disc-based video formats — DVD, HD-DVD, Blu-ray — only use MPEG-2 or MPEG-4. (Note: Technically, DVD-Video supports low-resolution CIF MPEG-1, but it’s rarely used.)
Why is Video Interlaced?
Without interlacing, 20th century televisions would have flickered. Although mainstream films are 24fps, projectors actually show it at 48 frames per second — double the speed to reduce flicker.
NTSC television is based on a 60 hertz (60Hz) signal, which is where 29.97 frames is derived. For PAL, it’s 50Hz and 25fps. Like film, 25-30 frames is slow enough to flicker between frames. Unlike film, however, doubling the speed was not possible, because it would have required double the bandwidth to broadcast. And at that time, in the early 20th century, the bandwidth was literally pulled from the sky — VHF radio frequency. Unlike digital bandwidth, radio waves are limited by the physics of the our universe.
The solution was to split the signal in a way that showed double the images, but within the confines of 30fps (60Hz). And that is where interlacing comes from. On a CRT tube television (the basis for interlacing theory), as one image is being drawn, the other is fading away, awaiting the redraw cycle by the electron beam. Two images (fields) are shown for every one frame, albeit temporally offset to make the illusion work.
Without interlacing, television would have looked like a cheap projector from the silent movie era.
What is Progressive? IVTC? Deinterlaced?
Progressive video is a series of separate images shown in progression, in order to give the feel of motion. Think of cartoons and flip books. All film source is progressive — film is nothing more than a series of still images shown one after the other.
Inverse telecine (IVTC) is a method used to reverse the process used to convert film to interlaced broadcasting and home video formats. All video must be stored, transmitted and viewed at 29.97 or 25 frames per second, and progressive frames are duplicated and weaved together to create it. IVTC undoes this process, with varying degrees of success. IVTC is not the same as de-interlacing, and is usually non-destructive. Note that IVTC can only be used when the ultimate original source was film.
For the purpose of capturing video, progressive is not possible/available, and on-the-fly IVTC is best avoided. Capture-time IVTC is generally very poor quality compared to slower methods available in dedicated editing software, and is therefore never suggested.
Deinterlacing is typically destructive process that converts interlaced video to progressive. For the purpose of capturing video, you should never deinterlace on the fly. If the ultimate end-goal of a project is to have progressive video, there are much better deinterlacing methods available in freeware editing software such as VirtualDub, Avidemux or Avisynth.
Examples of Interlaced and Non-Interlaced Video
Looking for examples? You’ll find them here: Video Capturing Concepts: Interlacing Examples
Be Careful Working With Interlaced Video!
One of my biggest pet peeves is people who mindlessly deinterlace videos — especially when the videos are distributed to others. Too many users just hit the “Go” button in their software, and never consider the source, the capture method or the actions of the software. Recording in the wrong format will look bad and lose quality.
Even professionals are not immune to making interlacing mistakes! The most blatant example of interlace butchery I’ve ever seen was an independent film production from John Schneider (Bo Duke, of Dukes of Hazzard fame). His 2006 indy movie Collier & Co. — Hot Pursuit! is so vastly screwed up as to be completely unwatchable; a complete waste of $25.
There are only a few situations where deinterlacing is even necessary, and most Blu-ray or DVD-making home users will never find themselves in those situations.
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