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, 23.976frame/s when using equipment based around NTSC frame rates) frame rate 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, delivering film-like motion characteristics. Some vendors advertise 24p products as a cheaper alternative to film acquisition.

When working entirely within the digital non-linear domain, 24p material is as easily handled as material at any other frame rate. However, 24p material requires care when it is processed using equipment designed for standard video frame rates.

There are two common workflows for processing 24p material using video equipment, one using PAL frame rates, and the other using NTSC frame rates. Of these two, the PAL route is the simpler, but each has its own complications.

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[edit] 24p vs. PAL video

Main article: Telecine

[edit] Converting 24p to PAL

24p material can be converted to the PAL format with the same methods used to convert film to PAL. The most popular 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% increase in speed raises the pitch by 0.7 of a semitone, which again typically is not noticed. Sometimes the audio is pitch shifted to restore the original pitch.

If 24p footage cannot be sped up, (for example if it were coming through a live NTSC or HD feed) it instead can be converted in a pattern where most frames were held on screen for two fields, but every half second a frame would be held for three fields. Thus the viewer would see motion stutter twice per second. This was the common result when programs were shot on film, edited on NTSC, and then shown in PAL countries. NTSC to PAL conversion also tends to blur each film frame into the next, and so is seen as a sub-optimal way to view film footage.

30p can be preferable over 24p since performing a standards conversion to 50i PAL has fewer technical complexities - any NTSC-PAL converter will do. The larger differences between the 30p and 50i framerates will cause less noticeable motion artifacts upon conversion.[1]

[edit] Non-linear editing and 24/25 telecine

The process of transferring 24frame/s video at 25frame/s rates is also the most common method for ingesting 24p film rushes into a non-linear editor. The resulting 25frame/s video can then be transferred into a non-linear editing system at 25 frame/s, maintaining the 1:1 frame correspondence between film frames and video frames. Once in the non-linear editing system, the editing system, knowing that the material actually originated 24frame/s rather than at 25frame/s, will replay it at the correct speed.

The original film Keykode and 24frame/s audio timecode can be then be reconciled with the 25frame/s telecine timecode by the generation of a telecine log file containing this information. Again, once the non-linear editor has this information, editing can be performed entirely in terms of 24frame/s timecode, and the Keykode information preserved for either film cutting or digital intermediate post-production of scanned film images.

Because sound is recorded separately from moving pictures in 24p projects, there are no problems regarding synchronization or audio pitch: the audio material is simply ingested separately from the moving picture material at its natural rate, and synchronized within the non-linear editor.

[edit] 24p vs. NTSC video

[edit] Conversion of 24p to NTSC-based frame/field rates

Main article: Telecine

Working with 24p material via video equipment working at NTSC frame rates has many of the same attributes as the 24frame/s workflow, but is more complicated by the NTSC-rate practice of using telecine pull-down rather than the PAL practice of transferring 24frame/s material at 25frame/s.

At standard analog NTSC video rates (30/1.001 frames per second) a full "interlaced" frame, unlike a progressive frame, is nearly 1/30th of a second and is composed of two separate "fields," each field nearly 1/60 second. The first field (the odd field) contains visible scan lines 21-263 and the second field (the even field) contains visible scan lines 283-525. What is seen onscreen is two of these fields, "interlaced" together, to produce a single full frame. This framerate is often referred to as "60i" or "59.94i"; the exact rate is 60/1.001.

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.

Any editing application which supports NTSC video can be used to edit footage employing the 3:2 pulldown scheme. It can be captured as a standard 60i file and edited like footage from any other camera, while still retaining its 24p aesthetic. There can be issues when editing the footage as 60i, however, including choppiness in short transitions or fades, and also a mismatch in the motion characteristics of the footage and any graphics which may be added to it, such as text or logos. So, while 24p footage can be edited as 60i, it is usually better to edit the footage on a 24p timeline with the pulldown removed.

Most current prosumer-level editing applications which edit native 24p can remove the 3:2 pulldown for editing in native 24p, although some cannot. However, this is not ideal; the removal of the 3:2 pulldown involves reconstruction of every fourth frame from two different field groups, which can cause a generational loss and some banding problems if the application doesn't interpret the footage properly. Therefore, using the 3:2 pulldown scheme is not ideal when planning to edit on a 24p timeline.

Note: "3:2 pulldown" has a cadence of 2-3-2-3-2-3..., but in the industry is called "3:2 pulldown", even though the cadence is 2-3. Some people use the term "2:3 pulldown", which corresponds to the cadence, but is not normally used in the industry for the technique.

[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. This pattern is specific to the NTSC DV format, and would serve no purpose in HDV or native 24p formats.

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 every 24p frame can be found intact within the resulting sequence of 60i frames, yet the compression efficiency remains the same as with 3:2 pulldown.

When editing 24pA footage, conversion from 60i back to the original 24p is very efficient. It only requires blending the fields made from the frames back into full frames. Then, only every fifth frame will be made up of fields from two different frames, and that frame can be discarded, leaving only the other four full frames. In order for this to work properly, the DVX100 camera records video in chunks of five video frames. This ensures that each clip has regular and predictable cadence.[2]

Because the 2:3:3:2 scheme was devised for efficient pulldown removal for editing, and because 24p editing applications more universally support its removal, it should always be used when planning to edit in native 24p.

Editing systems need specific support for the 24pA format to be able to detect and remove the pulldown properly 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. However, among the editing applications able to remove pulldown and edit in native 24p, it is more common for them to have support for 24pA 2:3:3:2 pulldown than for standard 24p 3:2 pulldown removal.

Still other editing applications have the option for editing on a 24p timeline, and will accept footage where the pulldown has already been removed in another application.

Also because the 2:3:3:2 pulldown scheme was devised in order to make pulldown removal for editing in native 24p more efficient, the pulldown arrangement is not ideal for watching footage. There can be exaggerated stutters in motion, because the frames which are split into three fields are not only onscreen for 50% longer than the other frames, they are back-to-back. As such, 2:3:3:2 pulldown should be used only when a native 24p edit is planned, and not for final viewing. This includes when shooting the footage initially, and also when printing back to tape from an NLE.

Diagram here

[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 certain methods will cause up to half the footage's vertical resolution to be lost. 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, used as a choice primarily by Apple's Final Cut Pro, every fourth frame is repeated. This scheme is easier for slower 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] 24p compared to 30p

It has been suggested to record shows for television at 30p rate instead of 24p, because 25% higher frame of 30p allows representing motion better.

As Charles Poynton explains, the 24 frame/s rate is not just a cinema standard, it is also "uniquely suited to conversion to both 50 Hz systems (through 2:2 pulldown, 4% fast) and 59.94 Hz systems (through 2:3 pulldown, 0.1% slow). Choosing a rate other than 24 frame/s would compromise this widely accepted method of conversion, and make it difficult for film producers to access international markets".

[edit] Native 24p

[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 3:2 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 prequel trilogy.

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, including 60i, 30p, and 24p with a choice between the 2:3:3:2 or 3:2 pulldown schemes. 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, offering the same frame rates and pulldown choices as the DVX100.

Following the success of the DVX100, in December, 2005, Panasonic released the Panasonic AG-HVX200, which offers 24p HD at the sub-$10,000 level. Basically an HD version of the DVX100 series, it heavily targets 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 records 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, 2:3 Pulldown and advanced pulldown.

Some music videos and television series today are shot with 24p video instead of 35 mm or Super 16 mm 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 frame/s, or 23.976frame/s (24/100.1% to be exact). 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 "30frame/s" framerate of NTSC is actually 30/100.1%, also referred to as 29.97frame/s – this framerate is matched when video at 23.976frame/s has a 3:2 pulldown applied. Similarly, 60i is shorthand for 60/100.1% fields per second.

Film productions may be shot at exactly 24.000 frame/s. This can be a source of confusion and technical difficulties if material is treated as normal video, since the slightly differing framerates can be problematic for video and audio sync. However, this is not a problem if the video material is merely treated as a carrier for material which is known by the editing system to be "true" 24frame/s, and audio is recorded separately from moving images, as is normal film practice.

[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 60i timing for 24p (replacing missing frames with "repeat field flags").[3]

[edit] Disadvantages of 24p

24p video has more trouble with high motion than other, higher frame rates, sometimes showing a "strobe" or "choppy" motion, just like 24 frame/s 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 frame/s 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 3:2 pulldown pattern should be used when shooting.

[edit] Future

Next generation digital cinema equipment is being designed to also handle the 48p frame rate[citation needed] along with the traditional 24p. 48p has twice the (video-like) motion (temporal resolution) of 24p, but also requires twice the bandwidth and data storage[citation needed]. However, it is unclear when, or indeed if, movie makers will start adopting the faster frame rate (probably not before digital movie distribution becomes common).

[edit] See also

[edit] References

  1. ^ Music Video Shooting At 30p, Cinematography.net
  2. ^ 24PA, 24F, 24P, 25F, 25P, 30F, 30P explained.
  3. ^ High Definition Facts.

[edit] External links

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