Computer Multimedia

The Audio Video Interleave format is the windows standard and is o called because it stores the audio and video information as a large file, with the video data and audio being divided into blocks and interleaved in the file. Since both streams of data are stored next to each other in time. It aids the synchronization process where video and audio files are helped separately, the delays in reading both sets of data from disk and interpreting the separate streams leads to severe problems of keep the sound synchronized to the picture. AVI files can be in compressed or uncompressed format depending whether one of the compression algorithms has been applied them.

QuickTime The Macintosh range of computers made by apple has long led the field in innovation and is still the most popular system for use with art, design and DTP. The Mac range was introduced in 1984 and apple were the first to use graphical user interface (in their Lisa computer), colour monitors (in there1977 apple 2 model), built in sound and eventually video in their own QuickTime format. The windows-type environment was embedded in the Mac operating system form its introduction; users never had to struggle with a command-line environment like DOS (Barlow 12).

Unfortunately, the Mac computer range is not compatible with PCs as they use a different processor range that uses different machine code instructions. Apple originally produced the quick time video format for they're range of Macintosh machines. This format also interleaves audio and video information. Quick time and AVI files can be played on a pc that has QuickTime driver installed. MPEG The Motion Picture Experts Group standard MPEG-1 compression forms the core of VideoCD as used in CD-I players, etc. and can store about 70 minutes of video on a single CD. The later version, MPEG-2 is used with DVD players.

Video storage Like animations, full motion video can easily occupy huge amounts of hard disk space. Video has to store and playback between 15 and 30 individual screens per second. The highest demands from video a currently available CD's is an 800*600 screen, although this is bound to increase. The storage capacity per second of video can be calculated thus: File Size = Bit Depth x Screen Resolution * Frames/Sec (Ivins 12) This is divided by 8 to get the answer in bytes and divided again by 1,048,576 to get the answer in MB's. A 256-colour (8 bits) VGA (640x480) at a 15fps screen update would require 640 * 480 * 8 * 15 / 8/ 1048576 = 4.39Mb. (Barlow 14)

If the video occupies a quarter of the screen area, it only needs a quarter of the storage space. This is commonly used on CDs to accommodate the low performance of most disks/CPUs/video cards. • Reducing the colour palette to 256 colours may mean a barely noticeable loss of colour gradation, but would result in a video that is a third of the size of a 16.78m colour clip • Lowering the frame rate at which the picture is displayed. This makes savings but the picture is jerkier. • Compressing the files for storage and decompressing them when they are to be played. Unlike the other three methods, compression need not produce any deterioration in picture quality. The user has the option to make even bigger saving at the expense of some picture quality.

Compression Data compression schemes are mostly based on the following techniques: Intraframe This considers each individual frame and discards any trivial data. This is the method used by both JPEG and M JPEG compression systems. Interframe This considers the differences between successive frames, removes the unchanged parts of the information and applies JPEG type compression on what's left. This is also called 'difference compression' and is used by the MPEG system. Other, less well-used methods involve converting 24-bit colour down to 16-bit colour and using dithering to represent the lost colours. (Ivins 14) This reduces file sizes at the expense of introducing a 'grainy' effect to the clip when played back. CODECS The CODEC (compressor/decompressor) is an engine that shrinks the files on saving and expands them again when they are to be used. CODECs can be implemented in either hardware or software. Hardware CODECs are more expensive but because they use dedicated chips instead of the computers CPU time' they are significantly more efficient. No CODECs were supplied with windows 3.1 and windows 95/98 comes with CODECs preinstalled. (Barlow 18)