HDV Explained

mjroddy wrote on 12/31/2005, 11:38 PM

I've been to the VASST HDV training, so I have a loose idea as to what HDV's Mpeg2 long GOP format is and what a DI is. But my grip on it isn't strong enough for me to explain it to a friend who is just (today) learning about it.
Is there a free sourse I can suggest he visit to learn about the long GOP, "P" frames, "I" frames and all that?
I did a quick search on DVInfo.net, but didn't see anything immediate.
I'll look more thoroughly next year :-)

Comments

busterkeaton wrote on 1/1/2006, 2:31 AM

Canopus has a good HDV FAQ up:

HDV uses MPEG-2 Compression

MPEG-2 is exactly the same type of compression that DVDs use — so it's well tried and tested. The only difference is that the pixel count is scaled up to cope with HD resolutions. MPEG-2 is very good at using similarities between frames, so it divides video into bunches of frames called a Group Of Pictures or GOP. A GOP contains several different types of compressed frames. There's no need to go into too much detail here, but these are the basic types:

“I” frames are compressed frames that do not depend on any frames around them.
“P” and “B” frames are predicted from the content of adjacent frames. You can't decompress an isolated “P” or “B” frame because of their dependency on other frames.
There is a version of MPEG-2 used by broadcasters that doesn't use GOPs, it only has I frames. It doesn't compress the video as much as long GOP formats. DV compression is like this too, with good reason. When you're editing video, you need to have equal access to every single frame. Editors want precise control over their footage so they can make cuts in exactly the right place. If you were only able to make cuts every five or ten frames, it would be difficult, not to say impossible, to edit

Spot|DSE wrote on 1/1/2006, 2:42 AM

Mathew, look in your HDV book as well.
Sony GOP has a 15 frame sequence, consisting of 1 I-Frame and the rest are B and P frames.
JVC has a 6 frame sequence consisting of same.

GOP is one of the reasons an intermediary is more or less necessary.

mjroddy wrote on 1/1/2006, 11:30 AM

Thanks for the info. I explained that exactly to him (in theory only), but I had it translated wrong in my head. I thought the "P" frames were "perfect" or "picture" frames and "I" frames were "interpreted."
And, yes, I can easily open the book. But my friend lives about 100 miles from me, so instead of me trying to educate him, I though directing him to a web site would be best, where the pros could teach him.
But what you guys say makes perfect sense and I'll move forward from there.
Thanks very much!