Interlaced video is a scanning method in which each frame is divided into two fields, one containing the odd-numbered lines and the other the even-numbered lines, captured and displayed one after the other. In video fundamentals, it is the counterpart of progressive scan, and it was designed for analog television to deliver smooth motion at half the bandwidth that full progressive frames would require.
Interlacing is marked with a lowercase i after the line count, as in 1080i or 576i, while progressive formats use p. Although modern cameras and displays are natively progressive, interlaced video remains present in broadcast television, archives and legacy formats, so understanding it is essential for anyone who edits, converts or delivers video.
How interlaced scanning works
Interlaced scanning works by sampling the image twice per frame period, each time on half the lines. The first field carries lines 1, 3, 5 and onward; the second carries lines 2, 4, 6 and onward. Because each field is captured at a different moment, the system records motion at the field rate, which is double the frame rate.
On a cathode-ray tube, the phosphor glow and the persistence of human vision blended the two fields into one apparently complete picture. This allowed early television to show 50 or nearly 60 motion samples per second while transmitting only 25 or 30 full frames, keeping large-area flicker low within the channel bandwidth available at the time.
The order in which fields are captured is called field dominance or field order. It is either upper field first (top field first) or lower field first (bottom field first), and a mismatch during editing or conversion makes motion judder back and forth.
| Format | Active lines | Field rate | Typical region and use |
|---|---|---|---|
| 480i | 480 | 59.94 fields/s | NTSC standard definition |
| 576i | 576 | 50 fields/s | PAL and SECAM standard definition |
| 1080i50 | 1080 | 50 fields/s | HD broadcast in 50 Hz regions |
| 1080i59.94 | 1080 | 59.94 fields/s | HD broadcast in 60 Hz regions |
Interlaced video compared with progressive scan
The essential difference is when the lines are captured. Progressive scan records all lines of a frame at the same moment, which gives full vertical resolution for every image, while interlaced video trades vertical detail on moving subjects for higher temporal sampling.
| Property | Interlaced | Progressive |
|---|---|---|
| Motion samples | Two per frame period | One per frame |
| Vertical resolution in motion | Effectively reduced | Full |
| Bandwidth for same motion rate | About half | Full |
| Artifacts | Combing, line twitter | Possible judder at low frame rates |
| Native display | CRT televisions | LCD, OLED, projectors, computer screens |
| Editing and effects | Field-aware handling required | Straightforward |
Static images in interlaced video still show nearly full detail, because both fields describe the same scene. However, broadcasters apply vertical filtering to reduce interline twitter, which makes the practical vertical resolution lower than the nominal line count. This effect is often expressed through the Kell factor and a separate interlace factor.
Deinterlacing for modern displays
Flat-panel displays are inherently progressive, so interlaced video must be deinterlaced before it is shown. Displaying both fields woven together produces combing, the saw-tooth edges visible on anything moving horizontally. Deinterlacers rebuild full frames using different strategies, with clear trade-offs between sharpness, motion and processing cost.
| Method | How it works | Strength | Weakness |
|---|---|---|---|
| Weave | Combines both fields into one frame | Full detail on static scenes | Combing on motion |
| Bob | Line-doubles each field to a frame | Smooth motion at field rate | Lower detail, line flicker |
| Blend | Averages both fields | Simple, no combing | Ghosting and softness |
| Motion adaptive | Weaves static areas, interpolates moving ones | Good general balance | Errors at motion boundaries |
| Motion compensated | Tracks motion vectors between fields | Highest quality | Heavy computation, rare artifacts |
For best results, deinterlace to double rate, for example converting 1080i50 to 1080p50, so that each field becomes a frame and the original motion cadence is preserved. Deinterlacing to the frame rate, such as 1080p25, discards half of the temporal information.
Interlacing, frame rate and shutter
Interlaced video interacts with neighboring concepts in specific ways. Its effective motion rate equals the field rate, so 50i looks as fluid as 50p rather than 25p. Each field typically uses an exposure related to the field period, so the shutter angle and resulting motion blur are defined per field rather than per frame.
Film-originated material complicates the picture. Telecine pulldown, such as the 2:3 pattern that maps 24 frames per second onto 59.94 fields per second, mixes fields from different film frames. Correct processing requires inverse telecine, which restores the original progressive frames instead of deinterlacing them.
Common mistakes when working with interlaced video
Most interlace problems arise when footage is treated as progressive somewhere in the workflow. The following errors are the most frequent in editing and delivery.
- Wrong field order after conversion causes stuttering motion; verify upper or lower field first in sequence settings.
- Scaling interlaced frames directly mixes fields; deinterlace before resizing or scale each field separately.
- Slow motion without field handling produces combing in every frame.
- Fine horizontal patterns and thin text flicker on interlaced output; soften them or use larger graphics.
- Uploading interlaced files to streaming platforms leaves combing visible; deliver progressive files instead.
Put simply
Imagine drawing a picture made of horizontal stripes. Instead of drawing every stripe at once, you draw every other stripe first and fill in the gaps a split second later. That is interlaced video.
Old television used this trick to show smooth movement without sending too much information. The screen glowed long enough that viewers saw one complete picture. Modern flat screens cannot show it that way. They must rebuild full pictures first, or fast motion shows jagged comb-like edges.
In practice
A regional sports channel delivers live football in 1080i50, the format its transmission chain requires. A production company receives the match recording to create highlights for a website and social media, where every player expects progressive video.
The editor sets the project to upper field first, confirms that the source flags match, and uses a motion-adaptive deinterlacer at double rate to produce 1080p50. Fast passes keep their fluid motion, the ball no longer shows comb edges, and the scoreboard graphics remain sharp. A test export converted only to 1080p25 looked noticeably more jerky, so the double-rate version becomes the delivery standard for all future highlight packages.