[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: 3/4" dailies without windows and curtains

Jeff Kreines
"This is strange... most compression systems (JPEG and
MPEG) preserve the VBI.  It's especially easy to do this
since the VBI usually contains no picture info, just
relatively low-rate data.  If it really is the case that
even the new Avid PCI cards trash the VBI, that should be
a firmware fix, no?

Sorry Jeff, no agree.
We are talking about "lossy " compression techniques. These rely on a succession
of lossy and non lossy data rate reduction processes. The point is that the
instanaeous factor of reduction from one frame to another may vary greatly. The
greater it is reduced the greater are tricks used to render it less visible.
These tricks would distort VBI. 
In addition, VBI is information that is non-coherent with the actual picture
information and therfore potentially a nuisance to have to incorporate into the
picture coding process (motion prediction).This applies to both inter and intra
frame coding - MPEG and JPEG. Therefor, at least in professional codecs - VBI is
extracted and separately multiplexed into the picture datastream along with
audio and other data. This is put back (mostly) in the right order at the other

This is one of the reasons why actual editing of interframecoded material is not
as easy, albeit desirable. An editable "pro" MPEG format is underway though,
hope we will enjoy seamless timecode handling for a change.

Mike Reichel