Digital video is typically used in multimedia productions for three different purposes:

• Presentation of moving images which were captured with a camera
• Presentation of animations that can not be reproduced by the playback system in real time, due to performance problems (2D- and 3D computer animation)
• Presentation of virtual reality (based on Quicktime VR for example)

In order to be able to use digital video in multimedia productions at all, it must be compressed heavily. Without compression the amount of data can not be handled by current multimedia playback systems (like Multimedia PCs, etc...).

The digitization of analog video is typically carried out in two steps:

1) Digitalization of the analog video scenes with the aid of special hardware, compression using a M-JPEG codec and storage in the M-JPEG format.
- Fast computer, fast hard disk and special video digitizer board, equipped with DSP chip supporting M-JPEG, required. Process done in real time.

2) Recompressing the M-JPEG files into final (hardware independent) format using another codec (like Cinepak, Indeo, Sorensen, MPEG 1, MPEG 2, MPEG 4 [DivX], Real Video, Windows Media Video, QuickTime...). Images and sound are being interleaved at this step.

- Process mostly not done in real time. Can take very long. Special accelerator hardware can speed up this process considerably (real time possible)

Audio is normally digitized at step one (without compression) along with the video scenes. It can also be digitized separately and later be combined with the video before the second step. Compression of audio is also possible.

The conversion of already compressed video files is always connected with losses of quality and should be avoided if possible (cascading of compression methods). Since video is normally digitized and compressed using the two step method mentioned above, a conversion to another format would already be the third cascade.

-Instead of conversion of a video file in final compressed format (after step two), the second step of recompression should be carried out again, using the appropriate codec to create the desired format directly

The quality of a compressed video always depends heavily from the quality of the original analog source material. Digitization from professional Betacam SP tapes normally results in far better quality than from VHS tapes for example.

In order to achieve best visual quality, MPEG 2 should be used for multimedia productions of very high quality standard. Video acquisition and editing should be carried out on professional high-end systems (Betacam SP or higher). The encoding should be performed using professional technology (software and hardware). DVD or DVD-ROM should be used as distribution medium if possible.

Videos for multimedia productions of semiprofessional level can be acquired satisfactorily using Hi8, S-VHS or MiniDV and be edited and compressed then completely on PC systems. The quality level reached this way is sufficient for QuickTime- or AVI-Movies for CD-ROM as well as for Real Video or Windows Media videos for the Internet.

Today the best possibility to create digital video for the semiprofessional level is the application of the Fire-Wire concept introduced Apple Computers in 1994 (also known as IEEE-1394 or Sony iLink) which meanwhile is supported by all major companies of the consumer electronic market and of computer technology.

At that the video signal is digitized directly inside a digital camcorder, then compressed using a special DV codec and finally recorded as compressed digital data on the tape (DV technology by Sony: for example MiniDV, Digital 8).

Using the IEEE 1394 interface (= FireWire, and/or iLink) the digital data can later be transmitted from the tape to a computerdirectly in compressed form without any change. Step one of digitization can therefore be omitted completely. Fire- Wire interface boards for PCs are obtainable on the market for reasonable prices; all recent Macs come equipped with FireWire interfaces from factory.

Video for multimedia must be acquired differently, than for TV or film:

- lower contrasts
-less fine details (for example close-ups instead of totals)
- Avoid camera motion (pan shots, zooms and runs)
- Short shots rather than long shots

Further tips:

- Videos for CD-ROM should have a data rate of max. 350 KByte/s. Higher rates provoke frequent interruptions of video and audio on slow systems
-Realistic dimensions of image area for playback from CD-ROM is 320x240 or 384 x 288, for WWW approx. 120 x 90 or 160 x 120, only for DVD 768 x 576 (PAL) or 720 x 475 (NTSC) is applicable
- The aspect ratio should always be integer (4:3, 2:1, 5:4, ...), since most codecs are optimized for this (else performance loss at playback).

from : Prof. Dr. Roland Riempp