Understanding digital video

With digital technology, each frame that makes up a sequence of still images (which produce the illusion of movement when displayed in rapid succession at a constant rate) consists of bitmapped digital images (Prime Learning, 2014, p.6). Digital images are rendered electronically on computer device and TV screens.

Each pixel of a screen is illuminated under control of the program that renders pictures. Each pixel has three colour elements; red, green and blue. The picture rendering program will determine the level of intensity each element is to illuminate at and the overall illumination level of the whole pixel (all three elements together). Through this process colour, tone and brightness can be controlled. In video, this process is applied repeatedly to each frame at least 24 times a second.

The rendering program uses bitmapped data originally created by the camera sensor to drive the screen pixels. Much of this original data is stored in the original image or media file and can be interpreted and manipulated by editing technology. Smart editing technology will not alter the original data, but create new data stored separately that can be called to substitute the original data during run time. When the rendering program is closed, the substituted image information is removed leaving only the original data. However the substitute data can replace the original data if the changes are saved. Once this happens the original data is lost from that copy of the media file.

It should be noted that rendering is a process that produces the final picture. In many video editing or production programs rendering is also referred to as the process that converts a production project into a final video product by wrapping it, or putting it into a file container from options available in the editing and production program. The process is referred to as encoding, while the type is referred to as the codecs.

Codecs types are the audio & video packaging types, not the audio or video bit mapping methods. For instance, while video may be produced to MPEG standards it might be encoded into a .mov file format requiring the Apple quicktime codecs reader to view it.

The table further down presents some of the container files available along with some information on them. Image clarity and fidelity improves with image size. Image size is expressed as the number of horizontal pixels (width) multiplied by the number of vertical pixels (height). The horizontal measurement is sometimes referred to as the number of samples and the vertical measurement is sometimes called the number of ‘lines’ (Prime Learning, 2014, p.15).

  • A quarter-screen size is 352 x 288 pixels.
  • A ‘full screen’ standard definition size is 720 x 576 pixels (PAL based technology).
  • A ‘full screen’ high definition size is 1280 x 720 pixels or 1920 x 1080 pixels.

With larger higher resolution frames the amount of data representing each pixel must also be higher. File size is thus bigger. To reduce file size compression is used. File sizes for 15 minutes of digital video material are approximately:

  • Uncompressed: 18 to 25 GB
  • Lossless compressed: 6 to 12 GB
  • Lossy compressed: 2 to 5 GB

File formats Consider video file formats as wrappers or containers which hold the individual frame images, information on how they are to be played back, any audio files associated with the video stream and metadata about the contents of the file. The file format does not necessarily dictate the quality of the video though some file formats do not support uncompressed or lossless encoded image data.

File format Extension Archival Web Notes
AVI .avi yes yes Audio Video Interleave. A standard Windows multimedia container. Semi-open format, well documented and widely used.
flash .flv no yes Used to deliver MPEG-encoded video via a flash player. Widely used for web dissemination.
Motion JPEG 2000 .ml2, .mjp2 yes no Well documented, open standard supporting uncompressed or lossless encoded image data. Not currently widely adopted
MPEG-1, MPEG-2 .mpg, .mpeg no yes Although open, stable and widely used, this format has been eclipsed by MPEG-4.
MPEG-4 .mp4 no yes Open source and widely used, but inherently lossy. This format is used mostly for end user dissemination.
MXF .mxf yes no Material eXchange format container. Open, capable of encompassing uncompressed or lossless encoded image data.
QuickTime .mov yes yes Apple’s proprietary container format. Semi-open, well documented and widely used
Real Time .ram no yes Real media’s proprietary container for delivering streaming media online.
WMV .wmv no yes Windows Media Video. Microsoft’s proprietary container for digital video.

(Prime Learning, 2014, p.16).

References:

Prime Learning, (2014). CUFPOS201A Perform basic vision and sound editing. Evocca College Student Learning Guide, 1-51.

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