Pull-Down and Speed Changes

Pull-down is an algorithm for the frame rate conversion of film footage to the faster video frame rate.

When film footage is transferred to video, the frame rate must be converted from 24 fps to either 25 fps (PAL/SECAM) or 29.97 fps (NTSC). This process introduces a slight speed change as a result of the mathematical relationships between the different frame rates. The film to NTSC conversion is done as a 2-3 pull-down and the film is running at 23.98 fps to maintain the exact 2:3 relationship. As a result the film is running ~0.1 % slower in NTSC TV.

These speed changes must also be applied to the audio recorded along with the film, in order for the audio to remain in sync with the picture. Sometimes the speed change is applied at the same time as the film transfer and is recorded directly on the videotape. This allows the video editor to hear the audio along with the transferred video while editing.

However, speed changes also result in pitch changes. Furthermore, this may lead to artifacts in the audio, due to the fact that a direct digital transfer from the field recorder to videotape is not possible without performing either a sample rate conversion or an analog transfer.

Therefore, most audio engineers prefer to use the original source material when working with film audio. Once the original audio has been digitally transferred into Nuendo, the speed change must be compensated for in order to keep the audio in sync with the video. Nuendo has the flexibility to apply these speed changes independently to either the audio or video.