Video compression

Professional filmmakers deal with uncompressed video, often at 768x576pixel resolution at 25 frames (50 fields) per second. Now imagine dealing with uncompressed video on your home PC.

A 10-second uncompressed video file takes up 340MB of storage space, so in order to capture a 20-minute video file, over 40GB is required. Obviously, a PC system will quickly be unable to deal with hours of uncompressed video.

The high data rates needed to work with uncompressed video on a PC is also an issue. For example, a TV image with a resolution of 720x576 pixels and a colour depth of 16-bits produces a data stream of 1.35MB per frame. As 25fps are a standard for TV footage, a data volume rate of 33.75MBps is required. Few systems outside professional studios can save this video data stream in a consistent way.

Thankfully, within the various video formats such as MPEG-2, MPEG-4, DivX and QuickTime, there is a combination of compression, resolution and frame rate settings that allow video to be delivered on the Web, a CD or DVD, or even a mobile phone.

Compression formats were devised to reduce file size dramatically, yet retain acceptable quality. These compressors work by throwing out data that are unimportant to the overall quality of the image.

Compression rates operate up to 100:1, reducing the 1+MB video frames down to as little as 100KB. Video compressed at 100:1 will not give you movie quality, but it may be ideal for viewing video over an Internet connection. You may be surprised to know that DV, often portrayed as pure video, has a compression rate of 5:1. This rate provides a nice compromise between high-quality video and a data rate well suited to the processing and storage capacities of today’s average PC system (see here for a screenshot).

Video formats such as MPEG-2, DV and MPEG-1 combine various levels of com­pression with different frame resolutions and frame rates to deliver video suitable to their audiences.

MPEG-2 is a great format for digital TV and video films delivered on DVD disc, with resolution and data rates scalable over a range to suit the intended audience. Generally, PAL MPEG-2 (DVD) is produced at 704x576pixels and a two-hour movie takes up around 4GB of hard drive space. However, recent releases such as the Superbit DVD series from Sony/Tristar take up much more space, with higher bit rates providing an even higher quality video experience than previously available. Many DVD authoring packages available today — such as Ulead DVD Workshop and Cyberlink PowerDirector — allow you to control the bit rate of your DVD video content in much the same way, so you can optimise the quality of your video to fit onto a DVD disc.

Video delivered over the Web uses a combination of higher compression as well as lower frame rates (5 to 10fps) and lower resolutions (down to 120x90pixels) to allow the viewing of video over even the slowest Internet connections, albeit at a lower quality.

First devised for VideoCD, MPEG-1 also happens to be ideal for broadband Internet distribution, although MPEG-4 seems to be gaining a lot of momentum. It offers impressive video quality for extremely low data rates, in the range between 10Kbps and 1Mbps. SVCD, a popular format for many PC video enthusiasts, is actually MPEG-2 video recorded at a lower bit rate and resolution. The video resolution is 480x576 PAL running at 25fps with a bit rate of 2.4MBps, and up to 40 minutes of SVCD video can be stored on a conventional CD-R/RW disc.

The playback options for SVCD are Windows-based PCs with a CD-ROM drive and an MPEG-2 player (such as PowerDVD), as well as some home DVD players.

All desktop video systems allow the editor to decide the output quality of the final project, so take the time to work out the best way to get your video file down to a size you want. If you are developing a video for the Internet, you may want to use a compression format with high compression and a low resolution such as RealVideo, QuickTime or Windows Media Video. On the other hand, if your final output is to an S-VHS or VHS tape, or other high-quality format such as DVD WRAM, you would use less compression and a higher resolution.

Working with video on your PC

The figures in the table below give a fair indication of which sort of system you need to work with the various video formats. The data rate indicates the demands made on your PC performance, while the storage indicates how big your hard drive needs to be.

If you want to work with DV-quality video, the data rate demands more from your system’s processor and hard drive: for example, over 13GB is needed to store an hour of material in this format.

FORMATS COMPARED
Video quality Selected compression Data rate (MBps) Storage (Minutes/GB)
VideoCD Multimedia 12:1 1.5 11
VHS 9:1 2 8
S-VHS 6:1 3 5.5
DV 5:1 3.6 4.5

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Denis Gallagher

PC World
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