Expert: Jeff Greenberg - 9/27/2009

Question
i have a 7-zip compressing software and i have a foler full of videos that add up to little over 10GB. i saw that someone used .rar file with a video online and the video itself was around 200MB but when it was in .rar file it was somewhere around 50MB, and the video is in awesome quality, sound, screen and all that. how can it compress my videos into a way smaller MB size without messing up the quality?

and how are zip/rar files get corrupted? i cant loose these videos.
thanks

Answer
Krystine,

Let's do each question separately:

"Can I use compression software to make video much smaller."
Not really.  Basically, RAR, Zip, 7-zip and all the other common lossless compression software aren't optimized to handle video well.  Usually I see less than a 10% difference with that sort of compression.

I'd have to see the exact video for the 200>50, because I haven't ever seen anything compress that well.  Ever.  Maybe 200 to 180, but never to 1/4 it's original size.  

The mathematics of how the information is compressed and then restored isn't optimized for video, rather, it's optimized for text - say compressing a Microsoft word file.

How do they get corrupted?  Any file has been technically corrupt when a single bit of it has been changed.  

We need to talk about bits and bytes to have an idea how small (and fragile) a bit is.  A bit is either a zero or a one.  Computers store information in bits.  It takes eight bits to describe a letter of the alphabet.  If a single bit has been changed, the file is compromised.

For common files, say a MS word file, there are some extra checks done, to make sure the integrity of the file is sure - but it does happen.  

The only real strategy is to:
A) keep more than one copy
B) keep some 'redundancy' or extra information that can check the integrity of your files and repair them if necessary.

Here is a link about PAR files (Parity) that will explain it in more detail: http://en.wikipedia.org/wiki/Parchive

Best,

Jeff