Placement of Chapter ID's

sean@oregonsound.com wrote on 9/18/2003, 10:47 AM
I'm working on a training DVD with Chapter ID's at the front of each new section. In importing the markers I made in Vegas, which were placed right on the first frame of each new section, the first audio was consistently cut off when the DVD was cued to the chapter ID and played back on my consumer DVD player. Is there a standard ramp up time similar to CD players, or is it hit and miss? How much space do you generally allow between chapter ID's and first audio? Thanks,

Sean McCoy
Oregon Sound Recording

Comments

jetdv wrote on 9/19/2003, 10:53 AM
Chapter points have to be on an I-Frame meaning that the chapter point will be at the marker +/- 15 frames.
sean@oregonsound.com wrote on 9/19/2003, 11:22 AM
Thanks for the info, but what the heck is an I-Frame?

Here's what Sonic Foundry, er, I mean, Sony tech support had to say about it:

Thank you for the email. When dealing with consumer DVD
players, audio time is different per player. We would recommend that
you leave around a 2 second pre-roll when setting chapter points that
need to start abruptly. This will ensure that the player is able to
decode the audio stream in time to begin that song/video clip.
jetdv wrote on 9/19/2003, 11:39 AM
That makes sense too.

In MPEG2, not all "frames" are complete. Only the I-Frames are - the rest are updates to that I-Frame. Therefore, to start with a full picture, the chapter point must be on an I-Frame. You can specify the distance between I-Frames using the Custom tab on the Render As dialog. By default, it is every 15 frames (or twice per second). So, your chapter point will actually start at the Marker +/- 15 frames (or, more likely, +/- 8 frames if you go to the "nearest" I-Frame).
sean@oregonsound.com wrote on 9/19/2003, 1:34 PM
Thanks for the lesson in MPEG technology! Interesting stuff. So is there any way to identify an I Frame, or otherwise pinpoint the location of one in an encoded file? Logic would dictate that the placement of markers in Vegas before rendering should be on 15 frame increments to avoid any surprises. In addition to the decoding onset time of DVD players mentioned by tech support, it's possible I placed some markers in locations that were slipped after decoding by as much as 7 or 8 frames--enough to exacerbate the audio glitches.