Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon.
This effectively doubles the time resolution as compared to non-interlaced footage. Interlaced signals require a display that is natively capable of showing the individual fields in a sequential order. CRT displays and ALiS plasma displays are made for displaying interlaced signals.
Interlaced scan refers to one of two common methods for "painting" a video image on an electronic display screen by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame. One field contains all odd-numbered lines in the image; the other contains all even-numbered lines.
A Phase Alternating Line -based television set display, for example, scans 50 fields every second. The two sets of 25 fields work together to create a full frame every 1/25 of a second, but with interlacing create a new half frame every 1/50 of a second. To display interlaced video on progressive scan displays, playback applies deinterlacing to the video signal.
The European Broadcasting Union has argued against interlaced video in production and broadcasting. They recommend 720p 50 fps for the current production format—and are working with the industry to introduce 1080p 50 as a future-proof production standard. 1080p 50 offers higher vertical resolution, better quality at lower bitrates, and easier conversion to other formats, such as 720p 50 and 1080i 50. The main argument is that no matter how complex the deinterlacing algorithm may be, the artifacts in the interlaced signal cannot be completely eliminated because some information is lost between frames.
Despite arguments against it, television standards organizations continue to support interlacing. It is still included in digital video transmission formats such as DV, DVB, and ATSC. New video compression standards like High Efficiency Video Coding are optimized for progressive scan video, but sometimes do support interlaced video.
Descriptioncaptures, transmits, and displays an image in a path similar to text on a page—line by line, top to bottom.
The interlaced scan pattern in a standard definition CRT display also completes such a scan, but in two passes. The first pass displays the first and all odd numbered lines, from the top left corner to the bottom right corner. The second pass displays the second and all even numbered lines, filling in the gaps in the first scan.
This scan of alternate lines is called interlacing. A field is an image that contains only half of the lines needed to make a complete picture. Persistence of vision makes the eye perceive the two fields as a continuous image. In the days of CRT displays, the afterglow of the display's phosphor aided this effect.
Interlacing provides full vertical detail with the same bandwidth that would be required for a full progressive scan, but with twice the perceived frame rate and refresh rate. To prevent flicker, all analog broadcast television systems used interlacing.
Format identifiers like 576i50 and 720p50 specify the frame rate for progressive scan formats, but for interlaced formats they typically specify the field rate. This can lead to confusion, because industry-standard SMPTE timecode formats always deal with frame rate, not field rate. To avoid confusion, SMPTE and EBU always use frame rate to specify interlaced formats, e.g., 480i60 is 480i/30, 576i50 is 576i/25, and 1080i50 is 1080i/25. This convention assumes that one complete frame in an interlaced signal consists of two fields in sequence.
Benefits of interlacingOne of the most important factors in analog television is signal bandwidth, measured in megahertz. The greater the bandwidth, the more expensive and complex the entire production and broadcasting chain. This includes cameras, storage systems, broadcast systems—and reception systems: terrestrial, cable, satellite, Internet, and end-user displays.
For a fixed bandwidth, interlace provides a video signal with twice the display refresh rate for a given line count. The higher refresh rate improves the appearance of an object in motion, because it updates its position on the display more often, and when an object is stationary, human vision combines information from multiple similar half-frames to produce the same perceived resolution as that provided by a progressive full frame. This technique is only useful though, if source material is available in higher refresh rates. Cinema movies are typically recorded at 24fps, and therefore don't benefit from interlacing, a solution which reduces the maximum video bandwidth to 5MHz without reducing the effective picture scan rate of 60 Hz.
Given a fixed bandwidth and high refresh rate, interlaced video can also provide a higher spatial resolution than progressive scan. For instance, 1920×1080 pixel resolution interlaced HDTV with a 60 Hz field rate has a similar bandwidth to 1280×720 pixel progressive scan HDTV with a 60 Hz frame rate, but achieves approximately twice the spatial resolution for low-motion scenes.
However, bandwidth benefits only apply to an analog or uncompressed digital video signal. With digital video compression, as used in all current digital TV standards, interlacing introduces additional inefficiencies. EBU has performed tests that show that the bandwidth savings of interlaced video over progressive video is minimal, even with twice the frame rate. I.e., 1080p50 signal produces roughly the same bit rate as 1080i50 signal, and 1080p50 actually requires less bandwidth to be perceived as subjectively better than its 1080i/25 equivalent when encoding a "sports-type" scene.
The VHS, and most other analogue video recording methods that use a rotary drum to record video on tape, benefit from interlacing. On the VHS, the drum turns a full revolution per frame, and carries two picture heads, each of which sweeps the tape surface once for every revolution. If the device were made to record progressive scanned video, the switchover of the heads would fall in the middle of the picture and appear as a horizontal band. Interlacing allows the switchovers to occur at the top and bottom of the picture, areas which in a standard TV set are invisible to the viewer. The device can also be made more compact than if each sweep recorded a full frame, as this would require a double diameter drum rotating at half the angular velocity and making longer, shallower sweeps on the tape to compensate for the doubled line count per sweep. However, when a still image is produced from an interlaced video tape recording, on most older consumer grade units the tape would be stopped and both heads would just repeatedly read the same field of the picture, essentially halving the vertical resolution until playback proceeds. The other option is to capture a full frame upon pressing the pause button right before actually stopping the tape, and then repetitively reproduce it from a frame buffer. The latter method can produce a sharper image but some degree of deinterlacing would mostly be required to gain notable visual benefit. While the former method will produce horizontal artifacts towards the top and bottom of the picture due to the heads being unable to traverse exactly the same path along the tape surface as when recording on a moving tape, this misalignment would actually be worse with progressive recording.
Interlacing can be exploited to produce 3D TV programming, especially with a CRT display and especially for color filtered glasses by transmitting the color keyed picture for each eye in the alternating fields. This does not require significant alterations to existing equipment. Shutter glasses can be adopted as well, obviously with the requirement of achieving synchronisation. If a progressive scan display is used to view such programming, any attempt to deinterlace the picture will render the effect useless. For color filtered glasses the picture has to be either buffered and shown as if it was progressive with alternating color keyed lines, or each field has to be line-doubled and displayed as discrete frames. The latter procedure is the only way to suit shutter glasses on a progressive display.
Interlacing problemsInterlaced video is designed to be captured, stored, transmitted, and displayed in the same interlaced format. Because each interlaced video frame is two fields captured at different moments in time, interlaced video frames can exhibit motion artifacts known as interlacing effects, or combing, if recorded objects move fast enough to be in different positions when each individual field is captured. These artifacts may be more visible when interlaced video is displayed at a slower speed than it was captured, or in still frames.
While there are simple methods to produce somewhat satisfactory progressive frames from the interlaced image, for example by doubling the lines of one field and omitting the other, or anti-aliasing the image in the vertical axis to hide some of the combing, there are sometimes methods of producing results far superior to these. If there is only sideways motion between the two fields and this motion is even throughout the full frame, it is possible to align the scanlines and crop the left and right ends that exceed the frame area to produce a visually satisfactory image. Minor Y axis motion can be corrected similarly by aligning the scanlines in a different sequence and cropping the excess at the top and bottom. Often the middle of the picture is the most necessary area to put into check, and whether there is only X or Y axis alignment correction, or both are applied, most artifacts will occur towards the edges of the picture. However, even these simple procedures require motion tracking between the fields, and a rotating or tilting object, or one that moves in the Z axis will still produce combing, possibly even looking worse than if the fields were joined in a simpler method.
Some deinterlacing processes can analyze each frame individually and decide the best method. The best and only perfect conversion in these cases is to treat each frame as a separate image, but that may not always be possible. For framerate conversions and zooming it would mostly be ideal to line-double each field to produce a double rate of progressive frames, resample the frames to the desired resolution and then re-scan the stream at the desired rate, either in progressive or interlaced mode.
Interline twitterInterlace introduces a potential problem called interline twitter, a form of moiré. This aliasing effect only shows up under certain circumstances—when the subject contains vertical detail that approaches the horizontal resolution of the video format. For instance, a finely striped jacket on a news anchor may produce a shimmering effect. This is twittering. Television professionals avoid wearing clothing with fine striped patterns for this reason. Professional video cameras or computer-generated imagery systems apply a low-pass filter to the vertical resolution of the signal to prevent interline twitter.
Interline twitter is the primary reason that interlacing is less suited for computer displays. Each scanline on a high-resolution computer monitor typically displays discrete pixels, each of which does not span the scanline above or below. When the overall interlaced framerate is 60 frames per second, a pixel that spans only one scanline in height is visible for the 1/60 of a second that would be expected of a 60 Hz progressive display - but is then followed by 1/60 of a second of darkness, reducing the per-line/per-pixel refresh rate to 30 frames per second with quite obvious flicker.
To avoid this, standard interlaced television sets typically do not display sharp detail. When computer graphics appear on a standard television set, the screen is either treated as if it were half the resolution of what it actually is, or rendered at full resolution and then subjected to a low-pass filter in the vertical direction. If text is displayed, it is large enough so that any horizontal lines are at least two scanlines high. Most fonts for television programming have wide, fat strokes, and do not include fine-detail serifs that would make the twittering more visible; in addition, modern character generators apply a degree of anti-aliasing that has a similar line-spanning effect to the aforementioned full-frame low-pass filter.