SVCD Scan Offsets

[Jeomite]

Does anyone know what 'SVCD Scan Offsets' are used for? I notice it's an used option when multiplexing SVCDs with bbMPEG. It causes the final filesize to be a bit larger? What does it do? I heard it was part of the 'SuperVideo Standard Specifications'. Do I actually need this?

Also, there's another related option "Sequence header aligning" which is the real reason why it takes up a few more MB to the final filesize.

Here's what it says about them in bbMPEG:
<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR size=1 color=black></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
Add SVCD scan offsets
Enable to embed SVCD scan offsets in User Data blocks after I frames. This option requires the Align Sequence Headers option to also be enabled.

Align sequence headers
If enabled, all video packets containing a sequence header are aligned to the start of the packet.
</BLOCKQUOTE></FONT></TD></TR><TR><TD><HR size=1 color=black></TD></TR></TABLE>

Still quite confusing, yet I would like to know what's the difference if I have these enabled or disabled...

<font size=-1>[ This Message was edited by: Jeomite on 2001-09-28 14:44:11 ]</font>

[Jeomite]

Is there any reason to enable SVCD Scan Offsets and Align Sequence Headers at all? I see no significant difference. I played SVCD created WITH and WITHOUT these options. Both discs played just fine.

Again, any reason to use these two options?

[lemmy999]

I made SVCDs with TMPGENc generated MPEG2 files (that supposedly have no scan offsets or non-standard ones at least) and it worked fine on both of my players. However, in TSCV I always select to "Update Scan Offsets". I think (but may be wrong) that Scan offsets are just like index points for the MPEG2 file. I think it is part of the standard but many players do not need them. I could be WAY off on that though.

[themamboman]

They are used when jumping to specific points in the mpeg-2 file. These are mirrored in the SCANDATA.DAT file in the EXT directory as far as I know.

If you don't have them right, you can't jump to a specific point in time correctly.

That's the biggest benefit of using bbmpeg. It does the scan offsets right for tscv/vcdimager to use.

I'm not sure, but I think they are byte offsets to get to each GOP.

[adam]

The svcd offsets are like little waypoints along your movie. When you jump to a point in the movie or fast forward having the offsets means your player looks at the previous offset and basically counts up from there to determine how to sync it. If you don't have the offsets it uses the data contained in the file header of the track to establish sync.

On some dvd players the svcd scan offsets arent needed, it will maintain sync without them. On other players the offsets will in fact interfere with how the dvd player maintains sync. But on most dvd players the svcd scan offsets are either necessary or do not hurt, so its best to just use them unless you experience problems.