recording HD live from camera to PC?

[clang]

as I mostly work on animation with lots of short shots, I've been in the habit of connecting my SD videocamera directly to my laptop and capturing video live to my NLE - i.e. no tapes are used. No problem, works fine, very convenient.

But when I try the same thing with a HDV camera (in HD mode), there's an appalling lag from anything happening on screen to the NLE seeing it, and any movement is horribly jerky. From my limited understanding of HD, the obvious possibilities are:
- Firewire just ain't fast enough to send a live HDV signal (which logically must contain a lot more data than SD)
- or the signal is sent fine, but is heavily compressed and my laptop isn't grunty enough to uncompress it in realtime

Any ideas? I'm using Vegas 8 and a Canon HV20, if that makes any difference

 

[mikkowilson]

The HDV signal is comprized of groups for frames (Known as GOP - Group Of Pictures) that are 0.5 secconds long. Any frame within the group requires the whole group to be calculated and displayed. As a result, the computer must recive the last frame of the group before it can even start to calculate the display of the first one - this adds a forced 0.5 sec delay into the chain.

Of course the camera needs the whole GOP captured before it can start to encode them, so there's ANOTHER 0.5 sec!

Add to that total of 1sec GOP delay the inherent lag from firewire tranmission (somewhere to the tune of 0.2sec noramlly (it varies) and the time for the camera and computer to actually process/encode/decode the image, and you start seeing quite a mighty delay in a live feed from the camera.


Transfers from tape are some what faster, as the camera has allready done it's half of the encoding.

- Mikko