Mine isn't turned on either, so the default is "off"
Why would it be "off" by default if it should be turned on?.
What is a "valid" dvd/cd?
Does it have to do with playing the dvd in standalone dvd players?

Inquiring minds want to know!!

Anyone?

A valid DVD mpeg2 file should have this written for every GOP, but in practice - will read only the first GOP header .. but not always ..
Some dedicated mpeg2 device require this to be on. Software playback doesn't care at all.


A MPEG2 file contains a SHC (Sequence Header Code) that defines the horizontal and vertical resolution, aspect ratio, etc. Each video sequence is proceeded by a SDE (Sequence Display Extension) also with horizontal and vertical resolution for the current sequence. The following from the MPEG2 specifications:
...
6.3.6 Sequence display extension
This specification does not define the display process. The information in this extension does not affect the decoding process and may be ignored by decoders that conform to this specification.
...

Here is a direct quote from the manual for the standalone Mainconcept encoder, upon which the Vegas version is based:

"The Sequence Display Extension (SDE) is entirely optional in MPEG-2, SVCD and DVD. The information in this extension does not affect the decoding process and may be ignored by decoders."

I don't know anything beyond what I read, but based on this, I don't think this matters, and you can safely leave it in its default off state.

that information is entirely incorrect .. it is required by both svcd and dvd specifications to be compliant .. ..

but as i said -- it is not always necessary..

a lot of encoders at one time had to have their output patched as they wrote weird SDE's ... ATI and TMPGenc were culprits in this (we are talking tmpgenc 1.2 here) .. you can also repatch the SDE with restream and pulldownGUI , except for SVCDs ...