24p
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In video technology, 24p refers to a video format that operates at a true 24 frames per second (or typically, 23.976fps) framerate with progressive scanning (not interlaced). Originally, 24p was used in the non-linear editing of film-originated material. Today, 24p formats are being increasingly used for aesthetic reasons in image acquisition. Capturing video at 24p offers film-like motion, which is arguably more suitable for narrative projects and creates more film-like images. Some vendors market 24p products as a cheaper alternative to film acquisition.
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[edit] Conversion of 24p to NTSC-based frame/field rates
At standard analog NTSC video rates (30/second) a full "interlaced" frame, unlike a progressive frame, is 1/30th of a second and is composed of two separate "fields," each 1/60 per second. The first field containing the odd horizontal scan lines and the second, the even lines. What is seen onscreen is two of these fields, "interlaced" together, to produce a single full 1/30th per second frame. This framerate is often referred to as "60i".
24p cameras do not, as NTSC video cameras do, shoot 60 interlace fields per second; they shoot 24 full frames per second.
24p material can be recorded directly into formats that support the framerate. Some of the emerging HD formats support the 24p framerate in addition to 60i and 50i (PAL). Previously, few formats supported 24p and the industry used workarounds to work with 24p footage with 60i equipment.
To record 24p material onto a 60i format (i.e. any NTSC-based format), pulldown is typically added to 'pad' the 24 frames into 60 fields. This is done by taking every frame and splitting it into two fields. Then, every second frame has one of its fields duplicated, resulting in three fields. The fields are then played back in that pattern -- 2-3-2-3-2-3-2-3-2-3-2-3-2-3 . . . and so on. The resulting video becomes a 60i stream and can be displayed on NTSC monitors. However, the aesthetic of 24p motion is retained and the footage does not have the motion of typical 60i video.
This 3:2 pulldown is the same process that is used when transferring film into video.
[edit] Advanced pulldown
Another pulldown pattern is the "advanced pulldown" ("24pA") pattern, first implemented in the Panasonic AG-DVX100 camcorder. Instead of padding the frames into a repeating 3:2 pattern, the frames are padded into a 2:3:3:2 pattern.
It converts the first frame into two fields, the second into three fields, the third into three fields, and the fourth into two fields. It then repeats this pattern for every group of four frames that follows. This pulldown pattern is used to avoid segmenting a 24p frame into two different 60i fields that exist in two different 60i frames. When a 24p frame is split up and recorded into separate 60i frames, interlacing artifacts can exist in the 60i frames. These artifacts decrease the compression efficiency of DV and can result in cycles of efficiency compression followed by less-efficient compression. The advanced pulldown scheme avoids this as no 24p frame is segmented across two separate 60i frames. The compression efficiency remains the same throughout.
Editing systems need specific support for the 24pA format to be able to detect and remove the pulldown so that the 24p frames can be edited in a 24p timeline. Many but not all prosumer and professional-level non-linear editing systems are able to recognize and remove this advanced pulldown scheme.
[edit] 60i to 24p conversions
Another method of achieving the 24p aesthetic is to capture 60i footage and convert it into 24p. Various techniques can be used to perform this conversion. A simple scheme would blend the fields together. This can result in motion artifacts where comb-like jagged artifacts appear in areas of high motion. De-interlacing can remove these artifacts but will halve the resolution. Adaptive de-interlacing schemes only de-interlace areas of high motion, hence preserving resolution in stationary areas. More advanced techniques can be used to mitigate problems such as aliasing from the temporal displacement between the 60i fields.
[edit] Displaying 24p material
With NTSC equipment, it is impossible to display a 24p signal directly as the monitors only support the 60i framerate. Hence, pulldown must be added to the 24p material to be displayed. Most editing systems will either add 3:2 pulldown or 2:2:2:4 pulldown. In the 2:2:2:4 pulldown scheme, every fourth frame is repeated. This scheme is easier for the hardware to implement as it requires less processing.
In HD production, the HD-SDI interface supports the 24p framerate in addition to the 60i and 50i framerates. Many HD monitors are able to receive a 24p signal (not a 60i signal with pulldown added) and can display the 24p material directly.
For end-user viewing of HD material, many digital formats are offering 24p support. Computer formats such as Windows Media, Quicktime, and Real video can play 24p video directly on a computer monitor.
[edit] Converting 24p to PAL
24p material can be converted to the PAL format with the same methods used to convert film to PAL. One method is to speed up the material by 25 / 24. Each 24p frame will take the place of two 50i fields. This method incurs no motion artifacts other than the slightly increased speed, which is typically not noticeable. The ~4% difference in speed will cause a noticeable pitch shift in the audio. The audio needs to be pitch shifted accordingly. This is the most popular technique.
Another method is to use a 2:2:2:2:2:2:2:2:2:2:2:3 pulldown scheme, whereby one of the 24p frames is held for 3 25p video fields instead of 2. Thus the viewer would see motion stutter twice per second.
24p can be preferable over 30p since performing a standards conversion to 50i PAL has fewer technical complexities. The subtle differences between the 30p and 50i framerates will cause motion artifacts upon conversion[1].
[edit] 24p on DVD
DVDs, however, are capable of storing the native 24p frames. Every Hollywood movie is laid to disc as a 24p (actually 23.976p - see below) stream. With a progressive-scan DVD player and a progressive display, such as an HDTV, only the progressive frames are displayed and there is no conversion to an interlaced format -- eliminating the appearance of any interlace or de-interlacing artifacts. When displayed on a standard NTSC TV (which only display 60i) the DVD player will add 2:3 pulldown to the signal.
In traditional television broadcast and VHS, the video stream has 3:2 pulldown added. This material cannot be displayed progressively without the resolution loss of de-interlacing, unless the de-interlacer has accurate cadence detection.
[edit] 24p video production
Increasingly, 24p is used to acquire video. The most prolific use of this has been with HDTV and digital cinema such as the Star Wars Prequels. In 2002, Panasonic released the Prosumer DV camera AG-DVX100 (followed by the updated models AG-DVX100A in 2003 and AG-DVX100B in 2005). This camera was the first DV camera that could switch between different frame rates. The 24p feature on the camera produces film-like video that is preferred by many narrative filmmakers. Canon soon followed suit with the Canon XL-2.
Following the success of the DVX100, in December, 2005, Panasonic released the Panasonic AG-HVX200, which offers 24p HD at the prosumer level. Basically an HD version of the DVX100A, it will heavily target independent filmmakers, as HD has a much higher resolution than DV and will generally look superior on a film blow-up. It is also noteworthy that the camera will record HD footage, complete with clip information, to static P2 memory cards instead of tape. This could potentially signify a radical change in the video editing workflow.
For recording 24p to tape in formats which typically do not support 24p, such as DV, options include PsF, 3:2 Pulldown, advanced pulldown, and 24-over-60.
Some music videos and television series today are now filmed in 24p video instead of 35mm or Super 16mm film.
[edit] 23.976p
Many 24p productions, especially those that are made only for TV and video distribution, actually have a frame rate of 24 * 29.97 / 30 FPS, or 23.976fps. Many use the term "24p" as a shorthand for this frame rate, since "23.976" does not roll off the tongue as easily. This is because the "30fps" framerate of NTSC is actually 30 * 1000 / 1001, also referred to as 29.97fps - this framerate is matched when video at 23.976fps has a 3:2 pulldown applied. Similarly, 60i is shorthand for 60 * 1000 / 1001 fields per second.
Film productions may be shot at exactly 24.000 FPS. This can be a source of confusion and technical difficulties, since the slightly differing framerates can be problematic for video and audio sync.
[edit] 24p in high definition disc formats
Both HD DVD and Blu-Ray support the 24p frame rate, but technical implementations of this mode are different among the two formats. Blu-Ray supports 24p with its native timing, while HD DVD uses 30p timing for 24p (replacing missing frames with "repeat field flags"). [1]
[edit] Disadvantages
24p video has more trouble with high motion, sometimes showing a "strobe" or "choppy" motion, just like 24 fps film will if shot as if it's video, without careful panning, zooming, and slower camera motion. It is therefore not well-suited for programming requiring spontaneous action or "reality" camerawork. 24p can also hurt the credibility of newscasts by making news footage look too much like staged movie clips -- though many newscasts do incorporate 24p footage.
It should also be noted that while the strobe of 24p is in many ways considered a disadvantage, it's also part of the "film look." 24 fps film strobes in exactly the same way.
Most consumer-level video editors (particularly non-HD ones) are designed for 30 frames per second, and the addition of 24p is sometimes awkwardly implemented. Incorrect user settings can result in a 24p frame at the edge of an edit existing on only one NTSC field, thus cutting its resolution in half. If a non-linear editor is incapable of removing pulldown, the standard 2:3 pulldown pattern should be used when shooting.
[edit] Future
Next generation digital cinema equipment is being designed to also handle the 48p frame rate along with the traditional 24p. 48p has twice better (video-like) motion (temporal resolution) than 24p, but also requires twice more bandwidth and data storage. It is, however, totally unclear when movie makers start adopting the faster frame rate (probably not earlier than digital movie distribution becomes common). But even then many (or even most) movie makers may still prefer the old 24p.
[edit] See also
- Frame rate
- 25p
- Telecine
- Filmizing(a.k.a. Filmlook)
- Progressive scan
