Progressive Scan

Progressive scan is a method of transmitting and displaying a video signal that draws every vertical line of the image with each screen refresh. It contrasts with interlaced video, which updates alternating vertical lines with each refresh (and requires less information at a time). Progressive scan video results in smoother motion and a more detailed picture than an interlaced video signal, particularly in action scenes with a lot of fast motion, and is the standard for streaming online video and DVD / Blu-ray discs.

Interlaced video was originally used for standard television broadcasts since it requires less bandwidth to transmit only half the image at a time, but also due to how older televisions work. CRT televisions refresh the screen at 60 Hz, twice for each video frame filmed at 30 fps — once for odd-numbered lines and once for even-numbered lines. This reduces flickering and makes motion look smoother. However, modern LCD and OLED displays are able to update the screen more frequently and display each video frame for multiple passes, resulting in a more detailed image. When LCD and OLED screens display interlaced video, the television processes the signal to combine both interlaced passes into a single progressive scan image.

Modern television broadcasts may be either progressive scan or interlaced. Whether a video stream is a progressive scan or an interlaced scan is indicated by a letter following the number of vertical lines of resolution in the image. For example, the two most common television broadcast formats for HDTV are 720p and 1080i — 720p has 720 lines of resolution and is progressive scan, and 1080i has 1,080 lines of resolution and is interlaced. Both of these formats require approximately the same amount of bandwidth.