24PA, 24F, 24P, 25F, 25P, 30F, 30P explained
After Canon presented the HV20 camcorder a year ago, many a videographer cursed it for not including pulldown flags in 24p output, which turned capturing 24p video into a nightmare. Different schemes have been devised, from those that use dozens of free utilities in sequence, to straighforward but costly ones. Did Canon indeed make a mistake, and what could have been done better?
To answer these questions let us take a look at Canon XH series first. The 60Hz versions shoot traditional interlaced video, as well as 24 fps and 30 fps progressive video. The 50Hz versions shoot 25 fps progressive video in addition to interlaced. Learning the recording format of these cameras will help with understanding of Canon consumer cameras, such as HV20, HV20 and HG10.
Canon XH and 25F
Let us start with a simple one, 25 fps progressive mode for 50Hz markets. The Canon XH is an 1080i HDV camera, it uses interlaced scanning behind the scenes for writing progressive frames to tape. One progressive frame is sliced into two fields, which are written to tape as parts of one interlaced video frame. Unlike native interlaced mode, two fields represent one point in time. Therefore, full reconstruction of progressive frame is possible.
In the world of high definition video this recording scheme is known as progressive segmented frame (PsF or sF). The original PsF specification included only 24 fps video with 1920x1080 frame size, and was intended for digitizing film-based movies. Later, other scanning rates were added.
A similar scheme, called "2:2 pulldown", has been used for decades in television to broadcast movies. Movies intended for television are shot directly with 25 fps, while 24-fps movies are sped up 4%. Unlike PsF, television content processed with 2:2 pulldown is intended for interlaced devices, therefore fine vertical details are usually filtered out to minimise interlaced line twitter. Nevertheless, the principle is the same: one progressive frame is transmitted as two interlaced fields.
Recorded HDV video can be captured from the camera using variety of output ports, like HD-SDI, component or Firewire. Firewire port is generally used for capturing onto a computer for editing, therefore it outputs true 25p video. Other ports are made compatible with consumer equipment like TV sets. Output from these ports is done using traditional interlaced scanning. If the receiving end recognizes video as being progressive, it recovers 25 frames back from 50i stream and displays them progressively. If video is not recognized as progressive, it is displayed as interlaced.
Canon XH and 30F
30F is pretty much the 60Hz analog of 25F, the only difference is the frame/field rate. Do not forget, that for most 60Hz videocameras, including the XH, 30p means 29.97p, while 60i in fact means 59.94i.
Canon XH and 24F
This mode is used only in 60Hz models. 24F is recorded on tape exactly in the same way as 25F and 30F. Video is sent as true 24p over Firewire, and can be captured either in its native progressive form, or as interlaced video. For HD-SDI and component, the camera automatically converts progressive to interlaced.
MPEG-2 standard can be used for encoding of both progressive and interlaced video. Progressive video can contain "flags" that help decoder to render video on interlaced equipment. 24p signal is commonly rendered on 60Hz equipment, so the flags specify how to convert 24p progressive video into 60i video. There are several patterns for progressive-to-interlaced transfer. The Canon XH employs flags for the most common "2:3 pulldown" pattern.
Two MPEG-2 flags are used to control the pulldown: Top Field First (TFF) and Repeat First Field (RFF). You can see these flags spelled out in the picture above. If an editing suite is configured for regular interlaced video, missing fields are build on the fly by a MPEG-2 decoder out of progressive stream according to pulldown flags. On the other hand, tools that are able to work with progressive 24 fps video, can obtain full original frames.
Interlaced video sent over HD-SDI and component ports is generated from original progressive video according to the same pulldown rules.
Canon HV20 and 25p
Now we can return to original subject of the article, the HV20 camera. Since it is a consumer model, Canon chose a format that is compatible with other HDV 1080i camcorders. On the 50Hz version, progressive video is converted to interlaced, and is recorded on tape in traditional 50i format.
To properly capture 25P video one has to know beforehand that it has been recorded as progressive. Video is captured using regular 50i preset, then it can be easily converted to progressive by changing project properties from "interlaced" to "progressive", no additional processing is needed.
Canon HV20 and 24p
The Holy Grail of an American and Japanese videographer, 24p mode of the HV20 camera, is written to tape in interlaced form too, with 2:3 pulldown applied in the camera. This is what caused all sorts of curses from the HV20 owners, but they are not justified.
Those who blame Canon for not including "pulldown flags" in interlaced video just do not understand how MPEG-2 works. As it has been explained, TFF and RFF flags a.k.a. "pulldown flags" are not used for interlaced video, they are used only for progressive video as hints to decoder, helping converting progressive to interlaced.
Is it technically possible for the HV20 camera to output 24p from Firewire? Yes, it would be possible if the camera either recorded true 24p to tape, or if the camera automatically performed automatic pulldown removal a.k.a. inverse pulldown, when data were sent over Firewire. As we know, 24p is not written to tape in its original form. As for the automatic pulldown removal, Canon either was too lazy to implement this feature, or it thought that no one would use it. Remember, 24p mode has been advertised as a film-like effect, not as a full-blown 24 fps mode.
The HG10 is a first camcorder for Canon that does not use tape nor optical disk. It is basically a small computer with a lens, which writes video as computer files onto a built-in hard disk drive (HDD). These files are not intended for immediate broacast, they are dealt with only by the camera itself and by an editing application. Nevertheless, Canon used the same approach as with the HV20, storing video in interlaced form, even if it has been shot as progressive.
This is not a big deal for 50Hz version, 25p video can be converted to progressive by changing one property in a video project. But for 60Hz version this turns into unneeded hassle. The pulldown has to be removed for proper working in 24p timeline. Adding insult to injury, data is duplicated. Unlike native 24p file with pulldown flags, Canon's version is a true 60i interlaced file with repeated fields.
After reading the article you still may have doubts. What about the DVX100, you may wonder. You know for sure that it outputs video with pulldown flags, you've read about it elsewhere on the Net. This disinformation may even have been posted on some high-profile sites, still it is not true.
DVX100 allows shooting beautiful 24 fps cinematic video, yet it records to tape in interlaced fashion, to comply with DV standard. The camera has two 24 fps modes, 24P and 24PA. The former is similar to 24p mode employed on the HV20, the only difference is that the HV20 is a high difinition camera while the DVX100 is a standard definition camera. This mode is supposed to be used for cinematic effect in regular interlaced video.
What makes all the difference, is the 24PA mode, "progressive, advanced". In 24PA mode the DVX100 camera uses 2:3:3:2 cadence to convert progressive frames into interlaced ones, unlike regular 2:3 cadence. The result is five video frames, corresponding to four progressive frames, but unlike 2:3 cadence, original frames can be recovered without loss of information, simply by discarding one video frame out of each 5-frame sequence. Another important feature is that the camera guarantees to record video frames in 5-frame chunks. That is, every clip contains a whole number of 5-frame sequences, and every clip starts with 2:3:3:2 sequence. Pulldown removal can never be easier!
The "advanced progressive" mode is supported by several video editing suites, like Final Cut Pro. It has an option to turn on processing of "advanced progressive" video. The software does not perform analysis of video, neither it relies on pulldown flags, because there are none. Instead, it simply discards a 3rd frame in every 5-frame group. And this is it!