Interlace and Progressive Scanning

Printer-friendly version

Moviola's Resource Center: 

  Tips, Tutorials, Articles & More »

Interlace and Progressive Scanning

Interlace or progressive scanning refers to how the video image is captured and displayed. Every video format, whether standard definition (SD) or high definition (HD) is either interlace or progressive.

The Short Answer

Interlace: With any interlaced video format, each video frame is divided into two fields. The fields of each frame are displayed one after the other, giving the appearance of a complete frame of video.

Progressive: With progressive video, each video frame is drawn in one complete pass. This would be similar to a frame of film, where all the information of the image is contained within the single pass.

More Info

For more information, and the impact of these methods on quality control, keep reading.

Some Basic Terms

Frame: Indicates a single photographed moment in time in film or video. In U.S. film production, there are 24 frames a second, and in Europe 25 frames a second. In NTSC video, there are almost 30 frames displayed a second (29.97). In PAL, there are 25 frames a second. Persistence of vision creates the illusion of continuous motion.

Frame Rate: The amount of frames displayed during one second of time. Often displayed as frames per second, fps, or frames/sec. For example: 24 fps indicates 24 frames per second. Sometimes the frame rate is referred to or displayed in Hertz, which refers to how many times an event (a cycle) happens in one second. For example: 24 fps is also referred to as 24 Hz.

Scan Line: Video is comprised of a number of horizontal scan lines, each line representing a row of video information. In NTSC, there are approximately 480 active (visible) scan lines. In HD, there are either 1080 or 720 active scan lines.

Interlaced Scanning

Every video frame is divided into two fields, each field containing half of the video frame. They are displayed in quick succession. Each field contains every other line of video, one containing all odd lines, the other containing the even lines.

In NTSC video, the lower (even) fields are displayed first, then the upper (odd) fields. In HD video, no matter what the specific format (i.e. HDCAM, HDV), the field order is always upper field first (then lower field).

Interlace scanning was necessary in the early days of television, used as a method to reduce or eliminate noticeable flicker with the available bandwidth, but technology has since improved, and progressive scanning is inherently a better method.

An interlaced video format will probably be referred to or labeled with an "i" somewhere in the name. For example: 480i (NTSC), 576i (PAL), and 1080i (an HD format). The number refers to the amount of total active (viewable) scan lines.

Real world issues with interlace scanning:

• Still (freeze) frames- when taken from a moment of action, you will see motion artifacts in the freeze frame. The jagged lines in this image show the displacement from one field to the other.

This is caused by both fields being captured from slightly different moments in time- the first drawn field is earlier in time than the second drawn field.

To an external interlace monitor like NTSC, this frame would appear to flicker heavily, as it is basically jumping back and forth between these two very close moments in time. One method used to eliminate this interlace flicker is to strip one of the fields and replace it with a duplicate of the other field. This is called de-interlacing. The following image has been de-interlaced.

Another de-interlacing method involves creating a solid image by interpolation of the two fields. The end result with any de-interlacing is less vertical resolution in the freeze frame than in the original video image. If the original captured video was progressive, these artifacts would not be present, as the entire image is not comprised of two different fields, instead just one complete image.

When 24 frame film is transferred to NTSC video (29.97 fps), then these interlace issues are only present on 2 of every 5 frames of video, due to how film is transferred to video. See Pulldown for more detail.

• Display on progressive monitors (including web outputs): Similar to the previous freeze frame example, when an interlaced image is displayed on a progressive monitor, which includes computer monitors, both fields are displayed at once, resulting in a jagged looking image. For the web, you should consider de-interlacing the video output for the web for a cleaner looking picture on computer monitors.

• There are many other artifacts due to interlace scanning in standard definition video, including title rolls where the titles appear to flicker their way up the screen. Small adjustments in the timing of the title roll can help if the title roll speed matches the refresh rate (cycling) of the monitor. In addition, fine horizontal detail often displays in an unpleasing flickering type fashion, referred to as a moiré artifact. This is often seen in fine detail like the front grille of a car, Venetian blinds, and tight horizontal stripes in an actor's wardrobe. When producing content for TV, cinematographers and costume designers know to usually avoid tight patterns in wardrobe because of this possible artifact. In HD interlaced video, these artifacts are greatly diminished and in many cases, not visible at all.

Progressive Scanning

A method of scanning all lines of a video frame in one vertical scan. Although the scan lines in a progressive image are scanned one at a time, from left to right, then moving from one row to the next, from top to bottom, the entire frame is created in one pass, instead of interlace scanning, where the frame is comprised of two separate fields. Progressive scanning is used in many of today's digital formats, including many HD formats. You can think of progressive scanning as similar to a film negative, where all the information of an image is in one complete frame.