Two ways to draw a picture
Every video frame is made of horizontal lines of pixels. The question is the order in which those lines are scanned and displayed.
In progressive scan, every line of a frame is drawn in sequence, from top to bottom, and each frame is a complete image. Computer monitors, phones and nearly all modern online video work this way.
In interlaced scan, each frame is split into two fields. One field holds the odd-numbered lines and the other the even-numbered ones. The display draws one field, then the other, and the eye blends them into one picture. Because each field contains half the lines, the picture is refreshed twice as often per frame, which reduces visible flicker without needing the full data of twice as many complete frames.
The broadcast origin
Interlacing was a solution to early television’s limits. Bandwidth for a broadcast signal was scarce, and displays based on cathode ray tubes would flicker visibly if refreshed too slowly. Sending half of the lines at a time allowed a higher refresh rate for a given amount of data. For decades this was the standard, and it shaped the video systems of broadcast television in most countries.
Standard-definition formats are described in lines and scan type: 480i in regions with 60 Hz traditions and 576i in regions with 50 Hz ones, where the letter i marks interlaced. High-definition television then included both 720p and 1080i, with the p meaning progressive. A 1080i signal at 60 fields per second is roughly 30 full frames per second, and in 50 Hz regions the equivalent is 50 fields, or 25 frames.
Labels such as 1080i and 1080p
The number gives the vertical line count and the letter gives the scan type. 1080p and 1080i both have 1080 lines, but 1080p delivers whole frames, while 1080i delivers alternating fields. They are not the same quality in motion, even though the line count is identical. Resolution is discussed further in the article on video resolution.
What goes wrong with interlacing
The two fields of an interlaced frame are captured at slightly different moments. When something moves, the odd and even lines no longer line up, and showing both together on a progressive display produces combing, jagged horizontal teeth along moving edges. This is easy to see in old sports or news footage played on a modern screen.
Other drawbacks include flicker on fine horizontal lines, such as thin stripes or text, and more complicated editing and processing. Slow motion and freeze frames are also harder, since a single frame mixes two moments in time.
Deinterlacing
To show interlaced material on a progressive display, the video must be deinterlaced. Software can do this in a few ways:
- Combine fields (weave): keeps full vertical detail but shows combing on motion.
- Interpolate (bob): converts each field to a frame by filling in missing lines, so motion stays smooth, usually with some loss of detail.
- Adaptive or motion-compensated methods: analyse movement and choose the best approach by region, which often gives better results at a greater computing cost.
Quality varies by tool, and deinterlacing is lossy in the sense that it estimates information that was never recorded. For this reason, it is best to do it once, with a good method, and to avoid repeated conversions. Telecined film, where 24 fps film was converted for 60 Hz TV, is a separate problem that calls for inverse telecine, covered under frame rates.
Why modern video is mostly progressive
Several developments moved the industry toward progressive scan:
- Computer and phone displays are progressive, and modern flat panels have no need for interlacing’s flicker advantage.
- Streaming and online delivery use progressive video. Players and platforms are built around complete frames.
- Compression works more efficiently on progressive frames, and later video codecs such as HEVC and AV1 were designed with progressive material in mind. They have limited or no support for interlaced coding.
- Progressive video is simpler to scale, edit and combine with graphics.
- Higher resolutions such as 4K are specified as progressive.
Interlaced broadcast has not vanished. Some television systems, cable feeds and archives still use it, and older camcorders and tapes recorded it. But new productions are generally shot progressive.
Practical advice
- Shoot in progressive modes whenever the camera allows.
- When working with interlaced archive footage, identify the field order (top field first or bottom field first) and deinterlace once, early, at the project’s frame rate.
- Do not mix interlaced and progressive material on one timeline without conversion. Many editors flag this in the clip properties.
- Check exports. Online platforms expect progressive files, so confirm that the export settings do not carry an interlaced flag.
Understanding the difference is mostly useful for rescuing older material and making sense of labels. For new work, progressive video is the default.