Interlaced is a method of displaying video where each frame is split into two halves, drawn on screen one after the other. Instead of painting every line of the image at once, an interlaced display first draws all the odd-numbered lines (1, 3, 5, 7…), then goes back and fills in the even-numbered lines (2, 4, 6, 8…). Each of these half-images is called a “field,” and two fields combine to make one complete frame.
You’ll most often encounter the term in video settings, TV specs, or camera options. If you’ve seen labels like 1080i or 480i, the “i” stands for interlaced. Understanding what it means helps you choose the right settings for recording, streaming, or watching video.
How Interlaced Video Works
A standard interlaced TV signal, like the kind used in broadcast television for decades, sends 60 fields per second. Since it takes two fields to make a full frame, that works out to 30 complete frames per second. The first field contains every odd line of the image, and the second field contains every even line. These are sometimes called the “top field” and “bottom field.”
Your brain merges these rapidly alternating half-images into what looks like smooth, continuous motion. This works because of a principle called critical flicker fusion: the human eye generally stops perceiving flicker somewhere around 50 to 90 Hz, depending on brightness and contrast. At 60 fields per second, interlaced video sits comfortably above that threshold for most viewers, so the image appears stable rather than flickery.
Why Interlacing Was Invented
Interlacing was essentially a compression trick that made television possible with 1930s technology. Early broadcast systems didn’t have enough bandwidth to send a full image 60 times per second. Sending 30 full frames per second was technically feasible, but 30 Hz produced visible flicker that made the picture uncomfortable to watch.
The solution was clever: rather than draw 30 whole images per second, the system drew 60 half-images per second. This doubled the apparent refresh rate without doubling the amount of data transmitted. Viewers got smooth, flicker-free motion using only the bandwidth required for 30 frames. It was such an effective trick that interlaced scanning remained the standard for analog and early digital television for roughly 70 years.
Interlaced vs. Progressive Scanning
The alternative to interlaced is progressive scanning, where every line of the image is drawn in a single pass, top to bottom. Progressive video captures and displays the entire frame in one exposure. You’ll see it labeled with a “p” in formats like 720p, 1080p, or 4K (which is progressive by default).
The practical difference comes down to motion clarity. Because interlaced video captures its two fields at slightly different moments in time, fast-moving objects can appear in slightly different positions in each field. When both fields are combined into a single frame, this creates visible artifacts: a comb-like pattern of horizontal lines along the edges of anything that moved between the two exposures. Progressive video doesn’t have this problem because the entire frame is captured at once.
For still or slow-moving content, interlaced video holds up well. News broadcasts, talk shows, and studio-based programming look perfectly fine in 1080i. But for sports, action sequences, or video games with fast motion, progressive formats produce noticeably cleaner results.
Where You Still See Interlaced Video
Most major broadcast TV networks in the United States still transmit in either 1080i or 720p using the current ATSC 1.0 standard. While the industry is gradually transitioning to ATSC 3.0 (Next Gen TV), FCC rules require broadcasters to continue simulcasting in the older standard during the transition so that existing TV sets and receivers remain compatible. That means 1080i signals will be part of over-the-air broadcasting for years to come.
Outside of live broadcasting, interlaced video is fading. Streaming services, Blu-ray discs, computer monitors, and phone screens all use progressive formats. If you’re recording video for YouTube or social media, progressive is almost always the better choice.
What Deinterlacing Does
When interlaced video needs to be displayed on a progressive screen (which includes virtually every modern TV, monitor, and phone), it has to be converted. This process is called deinterlacing, and the quality of the conversion matters more than most people realize.
There are three common approaches:
- Bob: Each field is scaled up to fill the full frame, essentially doubling the lines. This eliminates combing artifacts but cuts the actual vertical resolution in half, since each displayed frame only contains information from one field.
- Weave: Both fields are simply merged together into a single frame. This preserves full resolution for still content, but any motion between the two fields creates visible combing, where moving objects show a double-image effect with horizontal lines through them.
- Motion-adaptive: This is the smart approach. The algorithm analyzes the image on a pixel-by-pixel basis, using field merging for areas that aren’t moving (preserving full detail where your eye is most sensitive to it) and interpolation for areas with motion (avoiding combing artifacts). Transitions between the two methods are crossfaded rather than switched abruptly, so you don’t see jarring shifts in quality.
Most modern TVs and media players use some form of motion-adaptive deinterlacing. If you’re converting old interlaced footage in video editing software, look for the motion-adaptive or “smart” deinterlacing option for the best results.
Interlacing Outside of Video
The concept of interlacing shows up in other fields too. In MRI brain scanning, for instance, technicians sometimes use interleaved slice acquisition, where they scan alternating slices of the brain rather than moving sequentially from top to bottom. This reduces a problem called cross-talk, where the magnetic signal from one slice bleeds into the neighboring slice and degrades the image. By scanning slices in a non-adjacent order, each slice has more time to settle before its neighbor is scanned.
In digital imaging, interlaced file formats like interlaced PNG and GIF load in multiple passes. The first pass shows a low-resolution version of the entire image, and subsequent passes fill in more detail. This lets you see a rough preview of the image before it finishes downloading, which was especially useful in the era of slow internet connections.