Fundamentals of Video
Fundamentals of Video covers how a moving image is captured, scanned, stored and described numerically. The exam checks frame-rate and shutter arithmetic, aspect ratio and resolution literacy, and understanding of what interlacing and chroma subsampling actually discard.
Motion and scanning
- Persistence of vision and the phi phenomenon make discrete stills appear to move above roughly 12 to 16 frames per second.
- Common frame rates: 24 fps for cinema feel, 25 fps for 50 Hz PAL regions, 30/29.97 fps for 60 Hz NTSC regions.
- Interlaced video scans two fields per frame, odd lines then even lines; progressive scans the whole frame at once.
- The raster is written line by line with horizontal and vertical blanking intervals carrying sync information.
Resolution and aspect ratio
- SD PAL is 720 × 576 with a 4:3 and later 16:9 display aspect; HD is 1280 × 720 or 1920 × 1080; UHD is 3840 × 2160; DCI 4K is 4096 × 2160.
- Aspect ratio describes the frame shape; a 4:3 image in a 16:9 frame gives pillarboxing, a 16:9 image in a 4:3 frame gives letterboxing.
- Chroma subsampling such as 4:2:2 or 4:2:0 reduces colour detail because the eye resolves luma more finely than chroma.
Shutter and exposure
- A 180-degree shutter gives natural motion blur; shutter speed is 1 / (2 × frame rate).
Worked example. At 25 fps with a 180-degree shutter: shutter speed = 1 / (2 × 25) = 1/50 s. At 50 fps the same rule gives 1/100 s. Reasoning: the shutter angle is the fraction of the frame period the shutter is open, and 180/360 = one half.
Traps
- Confusing field rate with frame rate for interlaced material (50 fields is 25 frames).
- Quoting 625-line PAL as 625 visible lines; only 576 are active.
- Using 1/50 s shutter at 50 fps and then complaining about stroboscopic motion.
- Loading a 4:3 project into a 16:9 timeline and calling the black bars cropping.