written by: Kumara Velu•edited by: Rhonda Callow•updated: 6/27/2011
Digital video is all about video compression. Video is compressed so that it would be suitable for delivery across various mediums. Video is successfully compressed when its file size is reduced and there's no noticeable quality loss.
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The Need for Video Compression
Why is compression essential for digital video? Digital video in its uncompressed form, takes up a huge amount of storage space. This kind of space is not always available and as such the video has to be compressed to cut down on storage space requirements.
Take for example a video from a digital camcorder transferred to your computer hard disk via a Firewire cable. You need 13GB of hard disk space for an hour of video footage. This is after the video has been compressed at the ratio of 5:1. What this means is if the video is not compressed, it would take up 65GB of disk space. Nowadays 65GB is nothing to shout about, but not in the days when hard disk space began at 10GB and ended at 40GB.
Later when this video is fitted into a DVD, it’s further compressed. Two hours of compressed video in your PC takes up about 26GB. But it could be reduced to a mere 4.75GB with compression to be turned into DVD video.
You may be asking how it is possible to reduce the size of a video file drastically and yet able to obtain quality not much different from the high-quality video captured into the computer or the video quality you get when you play back your video from the camcorder directly to your television.
Here’s the explanation.
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No Noticeable Quality Loss
Does compression work some kind of magic on video quality? No. It has all to do with how our eyes register colors. There are billions of colors around us, but our eyes can’t register all of them. Since we recognize only 1024 shades out of the billions, we don’t need to retain what the eyes can’t take in.
Similarly, there is no need to save a video data relating to a particular detail in every frame if the detail remains constant for a particular stretch of time. Take for instance a video clip of a person talking against the background of a painting in a room. There’s no need to include the data of the painting or other `constant’ details in every frame of the video. When video compression is done in this manner, you end up with a smaller file size without any noticeable quality loss.
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Video data stream is measured in kilobytes per second or kbps also known as the bitrate. In other words the bitrate tells you how much data a video stream transmits in a single second. The higher the bitrate, the higher the video quality.
For instance VCD has a fixed bitrate of 1150kbps and SVCD has a variable bitrate of up to 2600kbps. This shows that SVCD transmits more video data in a second and therefore its video quality is higher than that of VCD.
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Sometimes in the interest of saving storage space, you may want to compress a video at a lower bitrate. You must do this cautiously because removing too much video data may result in the appearance of artifacts in your video.
While this will not affect your viewing pleasure when played over your computer on a small screen, the same can’t be said if played back over your television screen.
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Video bitrate alone doesn’t determine video quality. Much depends on the codec you use. For example, video compressed with the Xvid Codec at 1300kbps shows much superior quality than VCD, SVCD and rivals the quality of DVD.
Similarly the H.264 codec offers superior quality video for a file size that’s impressively low.
As to which codec you should use depends on your requirements. Anyway, you should try the various codecs available in your video converter or video editing program and judge the results for yourself.