Fundamentals
Interlaced and Progressive: What the i and the p Actually Mean
1080i and 1080p are not two grades of the same thing. One sends half a picture twice as often, and knowing which you have explains combing, weird motion and a surprising amount of bad conversion.
3 min read · Published 18 August 2026
The letter after the resolution is not a quality grade. 1080i and 1080p describe two genuinely different ways of building a picture, and mistaking one for the other is behind a whole family of odd looking video.
Progressive: the obvious way
In progressive video every frame is a complete picture. Thirty frames a second means thirty whole images a second. Pause it and you get one clean still. This is what phones, computers and every modern camera produce, and it is what every flat panel display actually shows.
Interlaced: half a picture, twice as often
Interlaced video sends only the odd numbered lines, then only the even numbered lines, alternating. Each half is called a field. Two fields make a frame, so 1080i at 30 frames a second is really 60 fields a second, each holding half the lines.
This was a clever answer to a real constraint. Television had limited bandwidth, and interlacing bought smoother motion at half the data, because the eye notices update rate more than it notices missing lines on a moving subject. On a cathode ray tube, which drew line by line and had phosphors that faded, it worked genuinely well.
The awkward part is that the two fields are captured at different moments, one sixtieth of a second apart. They are not two halves of one image. They are two halves of two images.
| 1080p at 30 | 1080i at 30 | |
|---|---|---|
| What is sent | 30 complete frames | 60 half frames |
| Lines in each | All 1080 | 540, alternating |
| Captured at | 30 distinct moments | 60 distinct moments |
| Pauses cleanly | Yes | No, unless deinterlaced |
| Native on a flat panel | Yes | No |
Combing, and why it appears
Those horizontal teeth along the edges of anything moving are the two fields showing at once. The subject moved between the two captures, so alternating lines show it in two different positions and the edge looks like a comb.
It is most visible on fast horizontal movement and on vertical edges, and it disappears entirely when nothing is moving, which is why a paused shot can look perfect and a pan can look shredded.
Deinterlacing, and why it is a compromise
Flat panels cannot display fields, so interlaced video has to be converted to progressive before it can be shown. Every television does this silently. The question is only how well.
There are three approaches and they trade different things away.
| Method | What it does | What it costs |
|---|---|---|
| Weave | Combines both fields into one frame | Combing on anything that moved |
| Bob | Stretches each field to full height | Half the vertical detail, and visible flicker |
| Motion adaptive | Weaves still areas, bobs moving ones | Processing effort, and the occasional wrong guess |
Good software uses the third. Cheap or fast conversions use the first two, which is why a badly converted old recording can look worse than it did on the television it was made for.
Where you still meet it
- Broadcast television, which still uses 1080i in many countries
- DVDs, and anything captured from tape
- Older camcorder footage, especially anything labelled 60i
- Archive material of almost any kind
You will not produce it by accident on a phone. You will absolutely receive it if you work with anything older than about 2010.
What to do with an interlaced file
- Read the file and check whether the scan type says interlaced. Guessing from combing alone is unreliable, because a previous bad conversion leaves combing baked into progressive video permanently.
- Deinterlace once, with a motion adaptive method, and keep that as your working copy.
- Decide the frame rate deliberately: 60i can become 30p by combining fields or 60p by interpolating them. The second keeps the motion smoothness the format was built for.
- Never deinterlace twice. The second pass has no fields to work with and only softens the picture.
If combing is visible in a file that reports itself as progressive, it was deinterlaced badly by someone else and the damage is now part of the picture. Filters can reduce how obvious it is; nothing can recover the lines.
Common questions
Is 1080p better than 1080i?
For anything watched on a modern display, yes, because the display has to convert interlaced video anyway and that conversion always costs something. The letters are not quality grades though, they describe how the picture is assembled.
What causes those horizontal lines on movement?
Combing: two fields captured a sixtieth of a second apart being shown at the same time, so the moving subject appears in two positions on alternating lines.
Should I convert 60i to 30p or 60p?
60p keeps the motion smoothness that made interlacing worth using, at the cost of interpolating detail into each field. 30p is cleaner per frame and discards half the motion information. Which is right depends on whether the footage is action or talking heads.
Can I remove combing that is already baked in?
Not properly. Once a bad deinterlace has been encoded, the alternating lines are simply what the picture contains. Filters can soften the appearance but the original lines are gone.