Most TV broadcasts are received with a 1920×1080 resolution but originate from a 720×480 source at the studio. It is a waste of space to compress the entire 1920×1080 if the only resolvable details are 720×480. Unfortunately resizing 1920×1080 video to 720×480 is not as simple as shrinking it.
At the TV station the original 720×480 footage was first converted to fields of 720×240. Each field was then scaled up to 1920×540. The two 1920×540 fields were finally combined with interlacing to form the 1920×1080 image. This technique allows a consumer TV to display the re-sampled image without extra circuitry to handle 720×480 interlacing in a 1920×1080 image.
If you merely deinterlace the 1920×1080 images, you would end up with resolution of 720×240. The 1080 to 480 effect properly extracts two 1920×540 size fields from the image, resizes them separately, and combines them again to restore a 1920×480 interlaced image. The scale effect must then be applied to reduce the horizontal size to 960 or 720 depending on the original aspect ratio.
The tracks to which 1080 to 480 is applied need to be at 1920×1080 resolution. The project settings in settings → format should be at least 720×480 resolution. The effect does not know if the first row in the 1920×1080 image belongs to the first row of the 720×480 original. You have to specify what the first row is in the effect configuration. The output of this effect is a small image in the middle of the original 1920×1080 frame. Use the projector to center the output image in the playback.
Finally, once you have 720×480 interlaced video you can either apply Frames to Fields or Inverse Telecine to further recover original progressive frames.
The CINELERRA-GG Community, 2021