The interlace method (also called interlacing designated; (English interlace ) [ ˌɪntəleɪs ]) the reduction of image flicker is used in television technology and was in the late 1920s in Telefunken by Fritz Schröter developed; Patented in 1930 as a "method for scanning television images" ( DRP patent no. 574085). It is still used today in the capture, editing, and display of images around the world.
It was developed with the intention of displaying the signals with the smallest possible bandwidth without flicker. A complete picture (frame) is built up from two different fields (fields; upper field - lower field). When the image is created, only the odd lines (odd field) of the output device are displayed for the first field (upper field); if this is complete, the second field (bottom field, bottom field, lower field, bottom field second) is built up from the straight lines (even field). The human eye perceives the individual lines not as flickering, but as moving. Ideally, it integrates the line jumps into an overall picture.
With the analog television standard B / G used in Germany (colloquially known as PAL ), a full picture consists of 575 visible lines, the fields of 287½ lines each. 25 full or 50 half images are transmitted per second.
Reason for introduction, advantages and disadvantages
Introduction and benefit
The process offers two main advantages:
- Flicker reduction through interlacing during playback: The screen lights up at twice the frequency (50 Hz) of the refresh rate (25 Hz) without the amount of information being transmitted also doubling. This greatly improves the subjective flicker impression with simultaneous bandwidth economy.
- Increase in the temporal resolution . When recording with electronic cameras, the interlacing process achieves an actual motion resolution of 50 Hz without increasing the amount of data transferred. In this way, a smoother impression of movement is achieved with a simultaneous bandwidth economy. The high temporal resolution is particularly noticeable in images with a lot of movement (for example sports).
The disadvantages of the interlace method are artifacts (image errors) in various situations:
- Line flickering : Horizontal edges in the picture may appear to dance up and down if the edge falls exactly between two picture lines, as it is then only visible in one of the two fields.
- Stripe structures : Homogeneous surfaces appear streaky. The stripes move up or down depending on the viewer.
- Still images can either be made from a field and then have a reduced vertical resolution or they can be made from a frame, then moving picture elements have comb-like double structures .
- Double contours if the display type is incorrect (line jump displayed as progressive or reversed).
- High technical effort and increased risk of artifact formation when de-interlacing , which is necessary as the first processing step for almost every post-processing.
- Problems with scaling digital artwork (e.g. from 480i to 576i or vice versa).
- The line flicker is more pronounced the more detailed and sharper the recorded image is. Older TV recordings have few problems with this artifact, while downscaled images and videos from high-resolution sources are hard to bear. Line flicker can be reduced by preprocessing on the sender side or postprocessing on the receiver side, which, however, costs resolution.
- The formation of stripes is more pronounced, the sharper the picture display tube on the receiver side. It is therefore advisable not to use high-resolution picture tubes to reproduce interlaced material.
- The line flicker occurs due to greater differences in brightness between the two fields. A vertical structure becomes a temporal brightness modulation of the image. The effect disappears at higher frame rates (e.g. 100 Hz field rate / 50 Hz frame rate).
Line flicker can be prevented by:
- Post processing of formation and limitation of vertical resolution.
- Converting the images into progressive images and using displays that work without interlacing.
- The streaking occurs through microsaccades of the viewer's eye. As a result, the fields on the retina are no longer mapped between each other, but more or less on top of each other. With sufficiently high-resolution (tube) displays you can see the black stripes between adjacent lines of a field, which are no longer supplemented by the line in between of the next field.
The formation of streaks can be prevented by:
- Using displays with limited vertical resolution.
- Converting the images into progressive images and using displays that work without interlacing.
Other problems: Many problems can now be solved through signal processing. If it weren't for:
- The problem with the real source format: For high-quality signal processing, you have to guess which exact (!) Format the source format is in and which processing history it already has behind it.
- The problem of cumulative image processing: The consensus of video technology is that the format will be converted if it is in a format that is not allowed. This leads to many conversions that are completely unnecessary when viewed along the entire chain. Sports videos are mostly produced with 1080i60, some broadcasters convert them to 720p25, and televisions convert them back to 1080p50.
Beginning of the transition from half-screen to full-screen television
Flicker reduction can be solved differently today
Thanks to digital full-frame memories, there are now other possible solutions for increasing the frame rate , with which the frame can only be reproduced in the playback device. Common uses of this principle are 100 Hz televisions and computer monitors . From today's point of view, it is no longer necessary to double the repetition frequency at the transmitter for this purpose. Some of the new HDTV television standards have therefore now dispensed with interlacing.
Time resolution still relevant
However, increasing the motion resolution is still relevant today and continues to justify the interlace procedure. For this reason, it is still provided as an option in HDTV standards.
Analog television as well as digital standards in standard definition ( digital video broadcasting ) will continue to be broadcast interlaced due to the necessary backward compatibility with existing transmitter networks and end devices. Possibly. the interlaced signal is regenerated in the end device from progressive image material ( e.g. DVD player and receiver for digital television).
In the HDTV sector, transmission modes with and without interlacing are possible, with the broadcaster deciding which mode is used.
The screens are currently a problem. At the present time (beginning of 2008) only displays with cathode ray tubes are able to display interlaced images in the same mode and thereby actually utilize the advantages of the method.
Flat screens based on plasma or LCD technology are currently not able to work in interlaced mode due to their design due to sluggish switching times and must therefore deinterlace and, if necessary, scale the image material before it is displayed . This is problematic because, on the one hand, the advantages of the process are wasted, and on the other hand, artifacts (image errors) are added, since deinterlacing cannot function perfectly. Due to their design, flat screens, compared to cathode ray tubes - in which an electron beam writes the image line by line - also use the technology of parallel image output, for this purpose the flat screen first writes a video signal to a memory chip, the image is then output together with the detected movements via a matrix in parallel to the display pixels. Current flat screens are still too sluggish for such a technology.
The development will show whether future flat screens will be able to cope with interlacing or whether the weaknesses of the display technology will persist and thus suggest that the process should be abandoned.
Areas of application
A distinction must be made between interlacing during recording, transmission and playback. The same term denotes different procedures.
Line skip in recording technology
Whether a picture is taken with or without an interlace depends on the exposure times of the superimposed even and odd lines. If these are identical or displaced only for the duration of a row, no interlace is present (progressive scanning frames, engl. Frames ); if these are offset by the duration of a field, the result is a recording with interlaced (interlaced scanning of fields).
Classic film material works - for physical reasons - always progressive, playback frame rates between 16 full frames / s (normal 8) and 48 full frames / s ( IMAX HD) are common. Normal cinema has been working at 24 full frames / s worldwide since the 1920s.
Video cameras usually work with interlaced (topfield / bottomfield). Most American countries, Japan and some other countries work with 60 fields / s, the rest of the world with 50 fields / s. In the meantime, however, there are also cameras that can optionally master full screen modes with 24, 25 or 30 (rarely even 48, 50 and 60) full frames / s.
However, digitally processed images can also be mixed. There are image elements with interlaced (advertising, news ticker, credits, inserted image parts, CGI ) and those without (base material, CGI).
Line skip in transport
Scanning or transferring images does not change the “interlace” property of the source material. The conversion of interlaced material into progressive material and vice versa is a technically very complex problem that can only be approximated and solved with heuristic methods. See deinterlacing .
If a transmission medium (analog television, VHS video recorder) only allows the transmission of fields with a fixed frame rate, each frame must be divided into two fields (at 50 Hz) or two and a half fields (at 60 Hz, for example with 24 frames / s). so-called NTSC pulldown). Remaining differences must be compensated for by slightly modified playback speeds (→ PAL Speedup ).
The decomposition of full images can take place in different ways - depending on which field is started with (BFF: Bottom Field First , TFF: Top Field First, Topfield, Top-Field).
The resulting signal, however, is not a classic interlaced picture, which can be recognized by artifacts on moving edges (ridges at 50 Hz, jerks at 60 Hz) when displaying these pictures on conventional devices (50/60 Hz television). Extensive post-processing on the display side can solve these problems, see the next section.
Digital storage and transmission processes work in exactly the opposite way here - with interlacing, two fields are always combined and encoded to form a full image. MPEG-1 works more inefficiently on edges than the improved MPEG-2, which provides an alternative coding variant specially intended for interlaced material in moving passages of a picture.
Line skip in the representation / visualization
With the exception of the display of video material on 50/60 Hz television sets, post-processing of the received signal is necessary if a high-quality display is to be achieved.
This post-processing becomes necessary when
- the timing of the recording and playback do not match,
- for interlaced material: if the vertical resolution of recording and playback do not match (different format, anamorphic / non-anamorphic),
d. H. actually mostly.
Source material with interlaced
In camera mode, both fields are scanned with a time delay. If they were put together in the same way, on the one hand there would be an unsightly interwoven double image in the moving areas of the picture, and on the other hand the movements would no longer be so fluid. It therefore makes sense here to view all fields as full images, in which, however, lines are missing. Completing these missing lines is called deinterlacing . In the simplest method, the two adjacent lines are mixed, which, however, leads to blurring and does not reduce the flicker of fine structures. Taking over the line from the last field is sharp and does not flicker, but leads to a comb effect at moving places.
Source material with pseudo-line jump through 2: 2 or 3: 2 pull-down
In film mode, two fields can be seamlessly combined to form a full image by temporarily storing the signal and rearranging the lines . A movie is thus shown with 25 full frames in PAL . It should be noted that this speeds up the film by around 4.166 percent, in contrast to the 3: 2 pull-down in NTSC (→ PAL Speedup ).
Misinformation about HDTV
In a number of publications it is stated that the high-resolution format (1080) always works with interlacing. That is not right. The available modes allow 1080 lines to be displayed both interlaced and without interlacing. With the various standards in 720-line format, however, there are no interlace formats. All standards with an “i” (for interlaced , example 1080i) in their name use the interlaced method, the letter “p” (for progressive scan ) stands for full images.
720 line format:
- 25p, 50p (50 Hz regions)
- 24p (23.976), 30p (29.97), 60p, (59.94) (60 Hz regions)
1080 line format:
- 25p, 50i, 50p (50 Hz regions)
- 24p (23.976), 30p (29.97), 60i (59.94), 60p (59.94) (60 Hz regions)
It is generally recommended that HD productions be recorded in 1080p59.94 or 1080p50, while 1080p 29.97 or 720p59.94 is recommended for broadcasting. The European broadcasters, however, prefer 720p50.
The view that 1080p does not exist with 60 or 50 frames per second is a half-truth. It is correct that a final standardization for 1080p50 / 59.94 / 60 has not yet been adopted [out of date] . However, that does not mean that you have to do without 1080p60 / 59.94 or 1080p50 (since the BD / HD DVD offers enough memory for it).