| Video Compression |
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(BMAS Website 2003-2005) With raw video which has been digitised at 25 frames per second and 24 bitplanes of 768 by 576 pixels (the standard size for full frame PAL video on DVD) the resulting data can occupy a vast amount of space. 768 * 576 * 24 * 25 = 265,420,800 bits = 33,177,600 8bit bytes or around 31 MBytes of video data to be transferred per second So some form of compression is required to be able to transfer the video over almost all but the highest bandwidth data links. To achieve this for a more restricted bandwidth, some compromises will have to be made on things like:
If you have ever watched digital TV (either via satellite; cable or digital terrestrial), then you have seen a compressed video stream in action. Digital TV uses a variation of the MPEG-2 (MPEG = Motion Picture Experts Group) standard for video encoding. Feature films on DVD are also presented in another variation of the MPEG-2 standard. In a typical DVD movie of around 90 minutes duration, this can still occupy around 5 Gigbytes of data. The actual video data in a DVD is less than this as the MPEG-2 data can include several soundtracks and/or several subtitle tracks, but it still means around 1MByte of data per second. Whilst the high bandwidth data links can manage this level of data rate, the lower bandwidth data links (2 - 10Mbit) would still have problems so something more agressive is required. Video CDBefore the advent of MPEG-2 video, a VCD (Video CD) used MPEG-1 as a base. The quality of VCD video is similar to that of an average quality VHS (Video Cassette) recording, and should be viewable over a 2 - 10Mbit data link. With this format, it is possible to pack an hour of PAL format TV video into around 650MBytes (i.e: space for two episodes of Coronation Street on a 700MByte CD-R.. - God forbid!). The framesize is limited to 352 by 288 at 25 frames per second for PAL format VCD to remain within the MPEG-1 VCD standard. It is possible to use larger framesizes but with unpredictable results - for example: a video file of 512x384 can look fine on a computer screen, but a domestic DVD player squashes the vertical and the audio will probabably drop out of sync. There are three basic VCD MPEG-1 formats, two of which cater for NTSC (American TV format) and the other for PAL (Mainly European TV format).
MPEG-4More recently Apple have been pushing a new version of Quicktime incorporating the MPEG-4 audio/video codecs. MPEG-4 video itself has been around for quite some time and is often used as the video codec in the infamous DivX format. DivX is capable of a 10:1 compression ratio while still retaing most of the original quality, so a 5GByte DVD feature film will be compressed down to around 500MBytes which a much easier size to handle. DivX achieved a lot of its reputation along with the 'defunct' (?) Napster P2P (peer to peer) network as a large quantity of pirated video (including unreleased feature films) became available over the internet. DivX is a combination of two codecs - one for video and one for audio. The video codec is usually a variation of MPEG-4 whilst the audio codec is normally MPEG-1 layer III (MP3). (Napster in its original form has bee closed down, but the concept is still alive and well under several other P2P systems, we aren't going to reveal what is available as that could be construed as approving of P2P piracy.)
For really low bandwidth data links you could resort to the RealMedia or Windows Media format which allow video to be compressed and streamed over links as slow as a 14.4k modem although the quality is non-existant.
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