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3.1. Characteristics of multimedia

Multimedia can be as simple as a few images with some accompanying text to a multimedia presentation using video clips, sound, images animation and text. Multimedia files to use a lot of data when in a digital format. Video is the most demanding. A PAL signal when digitised can require a data rate of 170 Mbps. Audio is less demanding but still requires 1.3 MByte for a 1 minute clip using a Sound Blaster Pro system at 22 kHz sampling rate. Still images require use more data proportional to their size. Synchronisation of sound and video is important. Sound is likely to breake up if parts of it are lost or delayed in storage or transmission.

Video is less vulnerable to loss ( depending on the application), but still requires all of the picture to be on the screen at the same time and is also vulnerable to jitter. Jitter could be controlled in some applications if the sender of the isochronous video data time stamps each piece of data when it is generated, using a universal time source, and then sends the data to the receiver. The receiver reads a piece of data in as soon as it is received and store it. The receiver processes each piece of data only at the time equal to the data's time stamp plus the maximum transit delay. Thus isochronicity of the video would be restored.

An example estimate of the requirements made by voice and video on an ATM network is given below.

Parameter	      Interactive Voice       Non-Interactive Video @ 30Mbps
Delay 200 msec 1000 msec Jitter 1 msec 5 msec Throughput 8.8 Kbytes/sec 4.1 MBytes/sec Average Throughput 3.9 kbytes/sec 4.1 Mbytes/sec Packet sequencing required Yes Yes Absence of packet duplication Yes Yes Setup time 0.8 sec 15 sec
(Ferrari RFC 1193 Requirements for Real-Time Services November 1990)
To summarise, multimedia data is large, sensitive to delay and loss of data.

To accommodate these characteristics techniques used by the telecommunication networks to carry telephone and television traffic are required. These include compression of data, and methods of timing the transmission and replay of multimedia. Data networks and computers have been built in a different way (they are asynchronous) to telephone and TV networks (which are isochronous).

3.2. Compression

Compression algorithms and techniques are critical to the viability of multimedia networking.Uncompressed digital television requires about 140 Mbps. Since few users have this sort of network access compression is the only hope for the widespread deployment of digital video and multimedia. Compression techniques depend on algorithms implemented in software or hardware. The use of hardware is important still to enable rapid compression, and also speeds de-compression. At this time the cost of hardware is still high, from 200 to 350 for a MPEG video compression PC card. Sound cards can implement proprietary compression, and software only video compression is available in products like Microsoft Video for Windows, or for UNIX workstations.

While compression can ease the demands on networks and storage media there are several trade-offs. Since some compression techniques remove information considered to be less important a loss in resolution may result. Once material is compressed the algorithms may prevent access to single frames of video for viewing or editing. The cost of complex hardware and software and compression and decompression delay are other factors important to users.

Different uses require different compression methods. Video conferencing must be done in real time so fast encoding and decoding is needed. This is the aim of the H.261 standard. Video film distribution via cable networks, radio or CD is essentially a playback process, so encoding is not time critical, and decoding should be easy to implement to reduce consumer costs. The MPEG standards address these applications.

MIDI encoding of audio notes is not really a compression method, but almost another form of media.

Inevitably, successful compression techniques encourage the design of applications which require higher bandwidths still, such as Super Definition TV which will also require appropriate compression.

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