Digital images, sound and video files

1. Images

Not only can image files have different resolutions but they are saved by different software in different file formats: file name extension may be PCX, BMP, WMF, TIF, etc...

Software exists that can translate, convert a certain number of file formats into others, but not all conversions are possible. Such a conversion necessarily induces a quality loss. Software also exists that handles images in many ways: rotating, resizing, etc., with a possible quality loss, which means that whenever possible, one should directly digitize the quality and size one intends to use, in order to get the best possible result.

Image files may be uncompressed, or compacted or compressed. Compacted images are slightly compressed without quality loss; it is only another way of recording areas of similar colour; the compression ratios are rather poor. Compressed images, with the current state of the compression algorithms, may have a compression ratio as high as 140:1 (about 2:1), but this always represents a loss of quality.

2. Sound

Digital sound also exists in different qualities expressed in sample width (bits) and in sampling rates (samples/sec or Hz); the higher the quality, the bigger the file; compression techniques may be applied, but their results are less spectacular than those for images. Sample width (8 or 16 bits) is combined with the sampling rate (8, 11, 22 or 44 million samples/sec or MHz), which produces files of 8 to 44 Kb/sec for 8-bit mono sound, and 16 to 22 Kb/sec for 16-bit mono sound; stereo sound is represented by files twice as big!

Sound files are also saved in different formats (e.g. AUD, WAV); software exists that can convert them into another format.

3. Video

Digital video data can be saved and played in different sizes (e.g. full screen, 1/4 screen) and resolutions (320x200, etc.); they exist in different formats (e.g. AVIS AVI); they are generally compressed because of their rather huge size, using different compression algorithms (e.g. DVI, MPEG, etc.). As an example, the Digital Video Interactive (DVI) system of INTEL allows the user to compress "live" (if the PC is equipped with the adequate compression adapter) in Real Time Video (RTV) mode, with a low compression ratio, or to get the file compressed by a specialized company in Production Level Video (PLV) with a ratio around 140:1 and a much better quality.

Video decompression needs on every work station a decompression adapter, or a decompression software (much cheaper) provided the PC is equipped with a fast enough microprocessor (a 486 as a minimum).