Define Compressed/Uncompressed Video

[Joe_Hicks]

Will someone help clear up the difference between compressed and uncompressed video.
We all know that DV is compressed. So, what are the advantages to buying a capture card that captures uncompessed?

This question may sound simplistic, but until I have a complete understnading of the two and how they work together, I'm holding off buying a capture card.


Thanks,
Joe

 

[Flash]

There is no advantage if you're shooting DV. The best way to get that into a workstation is Firewire. Uncompressed is for people bringing in telecine stuff from film scans, or Betacam, or DigiBeta SDI, or even VHS, where the footage is being "digitized" (converted to digital) instead of just being "captured" from an already-digital source. There's 8-bit uncompressed and 10-bit, which you really can't see the difference with your eye, but the big advantage is when it comes to compositing or keying the footage, which is much cleaner around the edges and such.

Storage also becomes a major factor. Compare 3.6 MB/sec for DV to 20-30 MB/sec uncompressed, and it adds up fast! Most of the time, even when I'm working with an analog source like Betacam, I'll digitize it to like DV50 (3.3:1 compression) or something instead of uncompressed to save a ton of space, unless I need that extra resolution. There are some people who will capture their DV using "uncompressed" settings, and they claim to get SLIGHTLY less degradation of the signal over time as it goes through multiple passes of processing, but for most of us, it's insignificant. Some also say that capturing their DV through analog connections softens it up just a tad to improve keying, but there are better ways to acheive that, in my opinion.

 

[mmm]

IMO, video isn't truely uncompressed unless it is 4:4:4 (1:1:1) colour. "Uncompressed" 4:2:2 has only half the colour resolution and that can be seen as a type of compresssion.

Most video formats have some type of compression in order to make the data rates easier to manage. Some are known as lossless because literally no quality is lost. Others are so good that they are virtually lossless. A format like HDCAM SR or Digibeta in SD look basically as good as uncompressed. I have heard people say that dubbing to Digibeta can actually INCREASE quality because of its error correction. Not sure if that is true..?

Codecs like DV, HDV and even DVCPROHD are quite heavily compressed and it is reasonably easy to see the quality degrade, especially if you do a multiple compression passes.

The weaknesses of 8 bit compression (like DV, HDV, DVCPRO) are easier to see in particular circumstances. Heavy colour correction, particularly in gentle gradients can bring out terrible banding/stepping in the colour.