Moving image formats
An editor has nominated this article for deletion. You are welcome to participate in the deletion discussion, which will decide whether or not to retain it. |
 |
A number of formats have been and are used to transmit and store film and video recordings. These can vary based on aspect ratio, frame rate, resolution, and file format.
Transmission
[edit]Analog broadcasting systems—PAL/SECAM and NTSC—were historically limited in the set of moving image formats they could transmit and present. PAL/SECAM can transmit 25 Hz and 50 Hz material, and NTSC can only transmit 30 Hz and 60 Hz material (later replaced by 30/1.001 and 60/1.001 Hz). Both systems were also limited to an aspect ratio of 4:3 and fixed resolution (limited by the available bandwidth). While the wider aspect ratios were relatively straightforward to adapt to 4:3 frame (for instance by letterboxing), the frame rate conversion is not straightforward, and in many cases degrades the "fluidity" of motion, or quality of individual frames (especially when either the source or the target of the frame rate conversion is interlaced or inter-frame mixing is involved in the rate conversion).
50 Hz television systems
[edit]Material for local TV markets is usually captured at 25 Hz or 50 Hz. Many broadcasters have film archives of 24 frame/s (film speed) content related to news gathering or television production.
Live broadcasts (news, sports, important events) are usually captured at 50 Hz. Using 25 Hz (de-interlacing essentially) for live broadcasts makes them look like they are taken from an archive, so the practice is usually avoided unless there is a motion processor in the transmission chain.
Usually 24 Hz material from film is usually sped up by 4%, when it is of feature film origin. The sound is also raised in pitch slightly as a result of the 4% speedup but pitch correction circuits are typically used.
- Older technology allows an alternative option where every 12th film frame is held for three video fields instead of two mostly fixing the problem.
- More modern film playback technology allows for every 25th frame to be interpolated, with less objectionable results and no need for pitch modification.
- Each of these film oriented content transmission techniques has its own drawbacks. However modern motion compensation processors are considered to produce the least objectionable output.
With roughly 30 or 60 Hz material, imported from 60 Hz systems, is usually adapted for presentation at 50 Hz by adding duplicate frames or dropping excessive frames, sometimes also involving intermixing consecutive frames. Nowadays, digital motion analysis, although complex and expensive, can produce a superior-looking conversion (though not absolutely perfect).
60 Hz television systems
[edit]Because of higher television production budgets in the US, and a preference for the look of film, many prerecorded TV shows were, in fact, captured onto film at 24 Hz.
Source material filmed at 24 Hz is converted to roughly 60 Hz using the technique called 3:2 pulldown, which includes inserting variable number of duplicate frames, with additional slowdown by the factor of 1.001, if needed. Occasionally, inter-frame mixing is used to smooth the judder.
Live programs are captured at roughly 60 Hz. In the last 15 years, 30 Hz has also become a feasible capture rate when a more "film like" look is desired, but ordinary video cameras are used. Capture on video at the film rate of 24 Hz is an even more recent development, and mostly accompanies HDTV production. Unlike 30 Hz capture, 24 Hz cannot be simulated in post production. The camera must be natively capable of capturing at 24 Hz during recording. Because the ~30 Hz material is more "fluid" than 24 Hz material, the choice between ~30 and ~60 rate is not as obvious as that between 25 Hz and 50 Hz. When printing 60 Hz video to film, it has always been necessary to convert it to 24 Hz using the reverse 3:2 pulldown. The look of the finished product can resemble that of film, however it is not as smooth, (particularly if the result is returned to video) and a badly done deinterlacing causes image to noticeably shake in vertical direction and lose detail.
References to "60 Hz" and "30 Hz" in this context are shorthand, and always refer to the 59.94 Hz or 60 x 1000/1001 rate. Only black and white video and certain HDTV prototypes ever ran at true 60.000 Hz. The US HDTV standard supports both true 60 Hz and 59.94 Hz; the latter is almost always used for better compatibility with NTSC.
25 or 50 Hz material, imported from 50 Hz systems, can be adapted to 60 Hz similarly, by dropping or adding frames and intermixing consecutive frames. The best quality for 50 Hz material is provided by digital motion analysis.
Modern digital systems
[edit]Digital video is free of many of the limitations of analog transmission formats and presentation mechanisms (e.g. CRT display) because it decouples the behavior of the capture process from the presentation process. As a result, digital video provides the means to capture, convey and present moving images in their original format, as intended by directors (see article about purists), regardless of variations in video standards.
Frame grabbers that employ MPEG or other compression formats are able to encode moving image sequences in their original aspect ratios, resolution and frame capture rates (24/1.001, 24, 25, 30/1.001, 30, 50, 60/1.001, 60 Hz). MPEG—and other compressed video formats that employ motion analysis—help to mitigate the incompatibilities among the various video formats used around the world.
At the receiving end, a digital display is free to independently present the image sequence at a multiple of its capture rate, thus reducing visible flicker. Most modern displays are "multisync," meaning that they can refresh the image display at a rate most suitable for the image sequence being presented. For example, a multisync display may support a range of vertical refresh rates from 50 to 72 Hz, or from 96 to 120 Hz, so that it can display all standard capture rates by means of an integer rate conversion.
50 vs. 60 Hz
[edit]60 Hz material captures motion a bit more "smoother" than 50 Hz material. The drawback is that it takes approximately 1/5 more bandwidth to transmit, if all other parameters of the image (resolution, aspect ratio) are equal. "Approximately", because interframe compression techniques, such as MPEG, are a bit more efficient with higher frame rates, because the consecutive frames also become a bit more similar.
There are, however, technical and political obstacles for adopting a single worldwide video format. The most important technical problem is that quite often the lighting of the scene is achieved with lamps which flicker at a rate related to the local mains frequency. For instance the mercury lighting used in stadia (twice the mains frequency). Capturing video under such conditions must be done at a matching rate, or the colours will flicker badly on the screen. Even an AC incandescent light may be a problem for a camera if it is underpowered or near the end of its useful life.
The necessity to select a single universal video format (for the sake of the global material interchange) should anyway become irrelevant in the digital age. The director of video production would then be free to select the most appropriate format for the job, and a video camera would become a global instrument (currently the market is very fragmented).
See also
[edit]- 3:2 pulldown, a method for converting film frame rates to television frame rates using interlacing
- Deinterlacing, converting an interlaced video signal into a non-interlaced one
- Frame rate
- Interlaced video
- Oldest television station
- Progressive scan, the opposite of interlacing; the image is displayed line by line
- List of motion picture film formats
References
[edit]External links
[edit]- "Temporal Rate Conversion" – A very detailed guide about the visual interference of TV, video, and PC (Web Archive / Wayback Machine), version of May 20, 2017.