MPEG-4: Optimization of Picture Quality and Data Rate

900 MB Video On One CD-ROM

Depending on the type of CD-R used, a certain amount of data can be written. Most CD writers, however, only process media up to 700 MB. Selected writers can write up to 900 MB.

The number of minutes that fit on a single CD-ROM does not just depend on the data rate of the MPEG-4 video. The CD-Rs available on the market vary as regards storage capacity. Standard CD-Rs include the so-called CD-R74 media, which provide a data volume of around 650 MB. The CD-R80 CD-Rs with 700 MB are better suited to MPEG-4 due to their higher capacity. Anyone who has a special CD drive can even process CD-R90 or CD-R99 CD-Rs. These CDs can be read in very few drives and mostly have to be read by the CD writer that was used to create them.

Conclusion: Fast Motion Is Very Efficient, But Not A Universal Solution

Our comparisons between the two DivX-codecs based on the important criteria of resolution and data rate show that there are no universal settings for all application areas of DVD videos. Action films with a high movement content and fast scene transitions achieve good picture quality with the fast motion codec at a comparatively low data rate. The fast motion codec displays weaknesses in the case of still frames or sequences with low movement content. Here the codec works at a very low data rate, which has a rather bad impact on video quality.

For landscape shots with a fixed camera position and low movement content, the slow motion codec is clearly the one to be recommended. With fast scene changes, the slow motion codec works less efficiently and the picture quality is significantly worse than when using the fast motion codec with the same data rate. The slow motion codec is ideally suited to low resolutions up to a resolution of 352 x 288 pixels. At low resolutions this codec achieves better results in terms of picture quality than the fast motion codec at the same data rate.

To copy entire films from DVD to CD-ROM, it is recommended to use the fast motion codec with a maximum data rate of 6000 kBit/s. This poses no problem as the average data rate in the case of a full PAL resolution with 720 x 576 pixels is normally no more than 1200 to 1300 kBit/s. In this setting, 90 minutes of video do in fact fit on a CD-R80 CD-R. With longer DVD videos there are, in principle, three possibilities: either to split the film between two CD-ROMS or marginally reduce the resolution. Otherwise the maximum data rate can be restricted, with the downside that losses in picture quality have to be accepted.