Open or Closed GOPs?

Stuart Robinson wrote on 8/8/2006, 8:35 AM
Folks, I'm a long-time user of DVDLab Pro, but am getting reports from one important customer that the audio (Dolby Digital) and video are miles out of sync when discs are played back on his Sony home theatre system.

The MPEG-2 was created by Vegas 6.0d, as was a WAV file, and it in turn was converted to AC-3 by the TMPGenc encoder. Everything is in sync here and when the disc is played on my varied collection of stand-alone machines, but not on his Sony.

Anyway! I think DVDLab is probably the culprit, so am going to try DVDA.

The first question is whether to use open or closed GOPs in the MPEG-2 file that Vegas creates. DVDLab always warned of open GOPs but I see that is the default in the Vegas DVD Architect profiles.

What would folks suggest, open or closed GOPs? Thanks!

Comments

SimonW wrote on 8/8/2006, 9:36 AM
You've hit upon a good question there Stuart. I have been wondering about this for a while too. DVD Lab does indeed warn people that they should use closed GOP's.

But as you say Vegas defaults to open GOP's, and actually has a warning not to use closed ones!

So it is very confusing. Which one is best to use?

I have noticed that one of my colleagues has a DVD player that will not play anything that I make with DVD Architect properly. Yet it plays discs that were created with other programs. I wonder if this might have something to do with it.
Former user wrote on 8/8/2006, 9:56 AM
From what I understand, you would want Closed GOPs for the occasion where you might want to re-edit the DVD/MPEG video.

Dave T2
Coursedesign wrote on 8/8/2006, 9:57 AM
Closed GOPs are helpful if you need to edit the MPEG file later, or if you're using the Multi-Angle feature (anyone actually used this?).

They also help some DVD players when you are whizzing through content to find something visually.

Unfortunately closed GOPs increase the file size, so if you have already hit the disk limit, you'll end up with needing to reduce the quality to fit.

This is unrelated to the need for chapter points to be on I-frames.

Stuart Robinson wrote on 8/8/2006, 1:59 PM
Thanks guys. As the rendering for DVD should be the final step in Vegas, and I can't imagine the footage being re-edited from the disc, I'll leave the GOPs open and see how that turns out.

Fingers crossed!
DJPadre wrote on 8/8/2006, 9:42 PM
"or if you're using the Multi-Angle feature (anyone actually used this?)."

Yes.. i am.. quite a constant now... a little more than id like, but for stage shows and seminars this feature in DVDA has nailed afew clients for me..

HOWEVER... u CANNOT import prerendered MPGs when wanting to do this, u must import either a CF AVI or DV AVI and let DVD Architect encode the MPG natively...
What i do is render an DV AVI, and an AC3, then import the 2, then let DVDA take care of the compression while i sleep...
Steve Mann wrote on 8/8/2006, 10:57 PM
"HOWEVER... u CANNOT import prerendered MPGs when wanting to do this, u must import either a CF AVI or DV AVI and let DVD Architect encode the MPG natively..."

----------------------------

Works fine for me. I encode my MPEGs and AC3s in Vegas and have no problem making my multi-angle tracks in DVDA.

Steve M.
DJPadre wrote on 8/9/2006, 8:08 AM
Now THATS bizarromundo... rasonm i say that is because from what ive been told, and from my understanding of it, DVDA MUST render the 2 clips into the single stream, therefore requiring further rendering.. and if u import an MPG2.. that means your rerendering an already compressed file, which is somethign u DONT want to do...

Id be curious to know your settings, as i wil bring this up with the powers that be, becuase if DVDA DOESNT require u to rerender these MPG's it would be the first instance ive ever heard of it happening.. (or not happening.. if u knwo what i mean.. )
Kanst wrote on 8/10/2006, 1:38 AM
Quote from CCE manual:
In closed GOP, individual frames in a GOP do not refer the frames outside the GOP, and B pictures can be correctly decoded even in random access mode.

- Even if Close all GOPs is not selected, the GOP at the scene
change point is automatically closed.

- If Close all GOPs is selected, image quality will be slightly
degraded. Therefore, do not select this except when necessary.
Steve Mann wrote on 8/10/2006, 1:38 AM
Perhaps you are using the wrong terminology?

You don't render MPG files, you encode them. DVDA takes the assets (MPEG, AC3 and menu files) and makes the DVD files in the VIDEO_TS folder. DVDA doesn't render anything (though the program itself uses the terminology incorrectly).

I encode my MPEG and AC3 files in Vegas and load them into my DVDA project, then DVDA makes the VIDEO_TS file. In Vegas I use the Main Concept MPEG2 CODEC and the DVD Architect NTSC template. It never has to re-encode the MPEG files.

Steve M.