Intro to Digital Video
The Medium
Digital video is an incredibly flexible and forgiving medium. Its files can be moved from one location to the next without losing quality, and they can be modified with effects and transitions and then later restored. If done properly you could transfer a video project from your PC at work to an external harddrive, then to your Mac at a home, upload it on a server, and then download it on a new computer without a loss of quality.
Digital video is comprised of a series of images, called frames, quickly changing from one to the next. This rapidly progressing sequence of images produces the illusion of continuous motion that we perceive and call video. The following outline provides information concerning how individual frames are structured as well as how frames change from one to the next. Keep in mind that these characteristics are first determined by the type of camera with which you record. You should try to preserve these settings throughout the video editing process since changing these characteristics can result in a loss of quality. Fortunately many current video editing applications manage most, if not all, of these settings for the user. This material is fairly advanced and typically takes time to understand fully.
Video Properties
- The resolution is the frame size in terms of its width by height
- Resolutions are most commonly represented by the height of the frame
- NTSC (480p) is the most common standard definition format
- 1080p, 1080i, and 720p are the most common high definition (HD) formats
- The numbers for both represent the height of the frame in pixels
- 1080p video, for example, has a larger resolution than 720p
- 1080p and 1080i video have the same resolutions
Proportional depiction of different video resolutions. Image not to scale.
- The aspect ratio represents the ratio of the width by height of a video frame
- High definition formats are typically 16:9
- HD video is "widescreen" because the frame's width is much larger than its height
- A 720p video, for example, has a resolution of 1280 by 720 pixels.
- This corresponds to a ratio of 16 by 9 (or 16:9).
- A 1080p or 1080i video has a resolution of 1920 by 1080 pixels. Its ratio is also 16 by 9.
- Standard definition formats are generally 4:3 but are sometimes 16:9
- The frame rate determines how fast frames change from one to the next
- The frame rate represents the number of frames per second (fps)
- The most common frame rates are 30, 25, and 24fps
- Broadcast video and consumer camcorders typically record in 30fps
- Motion picture cameras typically record in 24fps onto 35mm film (an analog medium)
- Whether video is progressive or interlaced determines how the frames change from one to the next
- In progressive video (e.g. 1080p), each frame instantly renders in its entirety
- In interlaced video (e.g. 1080i), there is a rendering delay for the even and odd horizontal lines of pixels. This decreases the amount of data but also lowers the sharpness of the image. Interlaced video can look especially choppy or low quality when there is a large amount of motion in the video.
Examples of progressive (left) and interlaced (right) video. The progressive video is slightly sharper because of how each frame renders. This difference can be quite noticeable when the video is full size.
