PAL acceleration

from Wikipedia, the free encyclopedia

PAL acceleration ( English PAL speed-up ) refers to a technique of standards conversion , which is applied to films , which are usually produced at 24 frames per second in the PAL convert format, which instead of 25 frames or 50 fields per Second works.

When converting cinema films (24 frames / s) to NTSC format (29.97 frames / s or 59.94 fields / s), the playback speed must also be changed; this is described in the 3: 2 pull-down method.

Media and formats that can save films at 24 frames per second, such as Blu-ray Discs , are of course not affected. In the case of television, on the other hand, the problem remains largely the same with HDTV , since stations that broadcast in PAL (analog) and SD (digital) at 50 Hz continue to do so in HD.

Technical background

Predecessor: standard conversion to NTSC

The historic predecessor of PAL acceleration is the 3: 2 pulldown that is still used today for converting conventional cinema films (24 fps) into the US television standard NTSC (29.97 full frames / s or 59.94 fields / s) .

This conversion from the cinema film to the NTSC standard takes place in that the film frames, after slowing down to a speed of 23.976 fps, alternately take three and two fields (2: 3 pull-up); However, the running speed does not change from 23.976 fps, but images are repeated according to a given rhythm or divided into different fields in order to arrive at the 59.94 fields / s required for the NTSC television standard.

A disadvantage of this method is the occurrence of jerky motion sequences with slow pans - an advantage, however, is the almost correct playback speed.

PAL acceleration

With PAL with its 25 frames per second, the pull-down conversion would be far more impractical, since the numbers 24 and 25 (unlike 23.976 and 59.94) have no common factors; therefore the disadvantage of accelerated playback is accepted in favor of a simple 1: 1 conversion process.

With the so-called PAL acceleration, the source material running at 24 fps for the PAL standard is simply played back at 25 frames per second, i.e. the frame rate is increased to the 25 frames per second prescribed by the standard. The acceleration typically also causes a change in pitch of about half a tone to a semitone , which has recently been compensated for by artificially lowering the pitch to the original pitch frequency after the acceleration (see also the following section on pitch ).

Due to the change in the image speed, which takes place as standard for conversion to the various television standards, a film (assuming that the content is identical) automatically has a lower level of approx. 4% (100/24 ​​* 25) for PAL and 0.1% for NTSC (100/24 ​​* 23.976) longer barrel length.

When comparing the running lengths of a film in its NTSC or PAL variant, 23.976 frames per second or 95.904% are calculated accordingly. Examples:

  • The running time of a movie changes from 120 minutes to 96% of the running time, i.e. 115 minutes and 12 seconds.
  • Conversely, a film on a PAL DVD with a length of 120 minutes in NTSC has a total of 125 minutes.

Limits

A PAL acceleration is out of the question if the source material

  • comes from an NTSC video camera = 59.94 fields per second
  • is from a progressive NTSC source = 29.97 frames per second

In this case, during the standard conversion, the images are weighted and distributed precisely to the 50 fields per second of the PAL format (mostly using line disentanglement either by interpolation, i.e. calculation of the spaces in between, or by blending , i.e. merging of both fields, for both see deinterlacing ).

When converting the standard from NTSC to PAL, a slowdown from 29.97 to 25 fps would definitely be possible, however, in addition to a significantly larger difference in the speed of visible movement sequences, this would require extensive post-processing of the sound, as with a 2-hour film there is almost a 24 min run length difference. With + 19.88% (29.97 / 25), the difference in the actual running speed would be much greater than the 0.1 to 4% under the previous standards of 24 (film), 23.976 (pull-down for NTSC) and 25 fps (PAL). Therefore, there is no such slowdown when converting the standard from NTSC to PAL.

If you convert the NTSC source material to PAL as if it had 30 fps instead of the actual 29.97 fps, you can easily convert the images. For a 2h film, the sound is shortened by 7.2 s, i.e. H. the sound cannot simply be taken over.

Pull down for PAL

There are also times when a TV broadcaster wants the original length of a movie or series without PAL acceleration. Then, similar to NTSC, mixed images are used, but this process is not called 3: 2 pull-up, but 2: 2: 2: 2: 2: 2: 2: 2: 2: 2: 2: 3- Pull-up (11 x 2 + 3 = 25 pull-ups). The field with the odd image lines of frame 12 and the field with the even image lines of frame 24 are repeated as the third field every second (frame 12> fields 23, 24, 23; frame 24> fields 48, 49, 48). The copy of field 23 becomes field 25 and the copy of field 48 becomes field 50. The advantage is that the original speed and pitch are retained without further processing.

A disadvantage is that with this method the picture pauses briefly when the camera is panned twice a second, but the jerking effect is less than with the NTSC 2: 3 pull-up. In the case of video films, there was also the fact that productions before the beginning of the 1990s did not have the technical possibility of reversing the pull-down ( inverse telecine ). Therefore, blending was often the only option.

Problems

pitch

Technically speaking, the film is played back 1/24 (4.17%) faster and the sound also increases by 1/24, which corresponds to about 71 cents . This can be noticed by people who have perfect pitch - such people take e.g. B. the difference of film music compared with z. B. a CD is perceived as "out of tune", since all tones are "off the mark" due to the shift. The acceleration of the image is hardly perceived by humans; The same applies to changing the pitch of the language if the viewer is not familiar with the original sound version.

In order to avoid the pitch problem, a pitch correction is carried out using the digital time stretch method in some series and very rarely in films . A disadvantage here is that when using inferior converter software (e.g. in video editing software) sound artifacts in the form of distortion and tiny audio dropouts occur. Artifacts of this kind occur more intensely when an attempt is made to change the speed or run length while maintaining the pitch (corresponding settings with inferior conversion algorithms can be found in Adobe Premiere Pro , for example ). It is therefore generally advisable to carry out the digital pitch correction in specialized audio programs. Simply changing pitch is not to be equated with the distortion artifacts of an inferior transducer.

Example of poor quality converted film

Movie title distribution
Back to the Future - Trilogy Universal Pictures

A distinction is made between synchronized television productions:

  • Synchronization before the PAL acceleration: The voice of the voice actors is also raised. This is often the case with old productions where a new, qualitatively better standard change was made after the first performance. Usually this happens in connection with a DVD release.
  • Synchronization after PAL acceleration: Only the music and sound tracks are increased. This type of processing is increasingly being used in new productions, recently also partly in the cinema (if there is no playback at 24 frames / s).
  • New dubbing after PAL acceleration: music, noise and synchronized tracks retain their natural sound. This type of processing is increasingly used in (US) series that are shown on TV.

TV series on Blu-Ray Disc

TV series that were produced at 24 frames / second are almost always first accelerated to 25 frames / second and then synchronized for the European market. If the pitch was not corrected during the conversion, the pitch of the music and sound tracks is increased, but the synchronous track is not. On a Blu-Ray disc, however, the synchronized soundtrack is slowed down again to 24 frames / second. The result is that the music and noise tracks sound normal again, but the synchronized track sounds too deep.

While the problem of different pitches can be corrected in retrospect with a pitch correction of the dialogue track alone, a correction of distortion artifacts due to inferior converters is almost impossible.

To avoid this problem, either the synchronized track should always be created with 24 frames / second (see synchronization before PAL acceleration), or a high-quality pitch correction should be made with every speed change to maintain the original pitch.

Special case of music video

Music videos that are produced on film in the US are usually cut after scanning into the NTSC standard; As with television series, a film copy is usually sent to Europe for a PAL scan. In the case of music videos from film soundtracks with excerpts from the corresponding cinema film, however, this results in a legal problem, since there are no copying or scanning rights for a complete copy of the feature film for the music video, and a separate copy and PAL scanning of individual feature film scenes would usually be too costly.

In practice, a standard-compliant film scanning of the pure music video material on PAL is the best solution, which is then cut together with the feature film scenes previously scanned on NTSC and mostly inferiorly converted to PAL. The disadvantage here is an unsightly ~ 10 Hz stuttering effect and (due to the lower line resolution of NTSC) a general blurring of movie scenes in the music video.

PAL acceleration and cut version

When it comes to whether a DVD release is trimmed or uncut, PAL acceleration often creates confusion. If you only compare the running times of the theatrical / NTSC version and the PAL version of a film, the approximately four percent difference quickly gives the impression that the content is shortened. In fact, this may not even be the case. In order not to have to make a detailed direct comparison of the content, the cinema or NTSC runtime can simply be converted to the PAL runtime.

To address (including market-adverse) confusion, for example, the company pointed Kinowelt on the Director's Cut - DVD cover of the movie Saw explicitly point out that its PAL run time of about 99 minutes, about 103-minute NTSC or Theatrical version fully corresponds to the content.

Web links