A detailed ‘Introduction to Digital Video’, covering its creation and storage has been created by JISC Digital Media. This guide aims to draw upon the main points of this – and other – documents but the user is referred to the JISC Digital Media guide for further detail. In general, this guide aims to provide an overview of desktop video files (i.e. video that exists as a file either on the capture device or on a computer) and does not aim to cover either the digitisation of analogue materials or transfer of digital tape or disc/DVD-based video to desktop formats. As with other data types covered in these Guides, it is assumed that data is best preserved on network-based storage rather than on physical media such as DVD discs and tapes.
As mentioned in the previous section, the sources of data/digital video can often determine the format, quality and final physical size of the video files. When digitising from analogue sources, a range of formats are open to the creator in addition to a number of considerations determining the quality and size of the final file. The digitisation of analogue materials are outlined in detail in the JISC Digital Media documents ‘Selecting a video digitisation system’ and ‘Equipping a video digitisation system’. When creating video files from other datasets (e.g. a ‘fly-through’ of a 3D model), similar considerations exist as those that apply to digitisation although final file formats and quality are likely to be limited by the software application used.
Containers and codecs
As with Digital Images and Digital Audio files, digital video files contain a range of significant properties (discussed in the next section) that determine the quality and size of the video file. Many of these properties are also directly related to the capabilities of the various file formats used for digital video. However, unlike other file formats, many widely used digital video formats are in fact ‘wrapper’ or container formats that simply encapsulate separate video and audio streams. These separate streams can in turn also be present in a number of differing formats (described in terms of the codec used to encode them) and it is therefore of great importance that the data creator is aware of the exact codecs being used, their capabilities and intended use. The wrapper format itself can also vary in terms of capabilities with certain formats e.g MPEG limiting the types of codecs used within the wrapper for each of the streams.
In general, data creators should be aware of the use of:
- Compression, for both video and audio streams. As with various other formats, lossy compression results in the loss of data. It is recommended that no compression (or lossless where possible) is used for the original video file as this will provide a high quality ‘master’ from which other files can be derived..
- Frame Size / Pixels per Frame describes the physical width and length of the video picture and can therefore, as with image resolution, determine the level of detail captured in the file.
- Frame Rate describes the number of frames per second captured/displayed by the file. A higher frame rate creates a smoother video but also a larger file.
- Bit Rates, for both audio and video streams. Bit rates are derived from a combination of frame size, bits per frame, and frame rate and often appear as as a preset option in many digital video applications. In addition to constant standard presets, many applications give the option of variable bitrates which allow the video file to adjust these rate depending on the complexity of the frames.
The table on the File Formats page outlines a number of the common codec and container formats used for digital video files, more detailed comparisons of the capabilities of these can be found in the Wikipedia codec and container format comparison tables.