Video capture is the
process of recording an analog video signal and converting it to digital
for storage as a data file. Essentially, you capture a video signal by sampling
the signal several times a second and convert the resulting images to digital information, then
stringing the digital images together to play in sequence. You end up with a
video file which is set to play at a certain resolution (like 640x480)
and frame rate.
It's a similar process to that of creating MP3
music files by sampling an analog recording several times a second to create a
digital version of the music.
A simple process with one problem. If you've ever converted music
files from your CDs to MP3 format, you will have noticed that
the music takes up a LOT of space on your
hard disk prior to being compressed into MP3s.
This is nothing compared to video. Raw video
captured from VCR or TV takes up an ungodly amount of hard disk space, slightly
less than a gigabyte for every minute of recorded video. To get around this
limitation, various compression codecs were designed. Using one of these runs
the raw video signal through a compression process after it is converted to
data, but before it is written to the hard drive.
Different codecs have different compression ratios
and differing effects on the quality of video. They break down into two rough
groups: lossless codecs cause little or no degradation in the quality of the
original signal, while lossy codecs reduce the quality of the signal as they
compress it by introducing artifacts and reducing detail. Unsurprisingly, lossy
codecs compress video data far more efficiently than lossless codecs do, so
there are compromises involved.
Besides the obvious problem of disk space
that capturing 'raw' video poses to the average user, there is a second problem, computer
performance. If your computer's processor, memory, hard disk and video subsystems are not fast enough to keep
up with the constant stream of data from the video source, you wind up
with dropped frames, sections of visual data which could not be captured, and are struck from
the final file.
If you drop one or two frames over the course of a
couple of minutes of video, it's unlikely that the loss will be noticed. Drop
thirty or forty though, and the video capture file will be jerky and jumpy on
playback as a result of the missing visual information.
Video compression can help in this regard, or
make the problem even worse. While compressing video takes the load off your hard disks, both
in terms of storage space and how much data they have to move around
every second, it shifts the load to your processor and memory.
Since each frame of captured video must be analyzed
and compressed before it is written to the hard disk, a faster computer is
required for compressing video 'on the fly.' If you store the video in its 'raw'
form to your drive, then compress it afterwards, you can get around this
problem, but only if you have sufficient drive space to store the original huge