Video Fundamentals

Resolution is the number of pixels in the image. 1080p, the HD format of ITU-R BT.709, is 1920 by 1080 pixels. UHD, often marketed as 4K, is 3840 by 2160 pixels, four times as many, and is defined for television by ITU-R BT.2020; the DCI cinema format called 4K measures 4096 by 2160. Additional pixels improve sharpness only when the display has them natively and the source delivers them at that resolution. A lower-resolution signal sent to a higher-resolution panel is upscaled to fill it, and the scaling adds no detail. Aspect ratio is the shape of the image. 16:9 is the current standard for HD and UHD, so a 4:3 source shown on a 16:9 display appears between black pillarbox bars, or distorted if it is stretched to fill the screen. Frame rate determines how motion is rendered. 24 fps is the cinema rate; 25 and 50 fps are used in countries with 50 Hz television, and 30 and 60 fps (in practice often 29.97 and 59.94) in countries whose television derives from NTSC. At 50 or 60 fps motion looks smoother than at 25 or 30, with twice as many frames to carry.

Each frame is either progressive, drawn in a single pass and noted “p”, or interlaced, drawn as two alternating fields of odd and even lines and noted “i”. Combining the two requires deinterlacing. Every device in the chain (source, switcher, scaler and display) therefore has to share the same resolution, frame rate and scan type. When two devices disagree, one of them converts the signal, and each conversion costs some image quality and adds latency.