Image Processing Reference
In-Depth Information
on a previous compression pass. If your macroblocks are the same size and position,
this artifact can be minimized. If you move the uncompressed image and recompress
according to a different macroblock layout, things may go bad very quickly. So, uncom-
pressing and then moving or scaling need to done with care.
Telestream Flip Factory transcodes video clips by decompressing only as far as nec-
essary before recompiling into a new format. When translating from one format to another
where both use a discrete cosine transform (DCT) algorithm, it is not necessary to fully
decode the image. The conversion may just require a repackaging of the macroblocks.
Use transcoders such as Flip Factory rather than cascading (or concatenat-
ing) encoders.
Avoiding Other Problems During Capture
Depending on the hardware you use, you may experience synchronization problems
when capturing footage from legacy analog formats. Video recorded on VHS, Betamax,
and Video 8mm formats may have an occasional timing glitch in the recorded video.
The capture card needs to maintain synchronization with the video and may have trouble
following these timing instabilities.
You could try inserting a time-base corrector between the playback machine and the
capture card. Some capture cards are designed to be upgradeable. Blackmagic Design
video cards are loaded with firmware operating code every time the computer is restarted.
This allows a new revision of that software to be deployed to the video card without need-
ing to change firmware ROMs by hand. This will improve the performance of your cap-
ture card as later revisions are published.
Making sure that the signals are as clean and robust as possible is always the best
solution. Be watchful for problems due to electrical noise, video, sync levels, and time-
base glitches.
Sometimes, routing the video through another piece of equipment and using a mon-
itor output or capturing to a DV camera will give a more stable input. While DV cameras
are generally not ideal analog-to-digital conversion devices, sacrificing a little detail is
probably better than losing complete frames due to an unstable time base.
Time and Relative Dimensions in Space
Getting the time-based aspects of our video sorted out is the next task. Dealing with that
right now is important because we want to get the frame rate correct and get rid of any
nasty interlacing artifacts if we can. This is probably the hardest part of the preprocessing
to deal with, so let's crack on with that in the next chapter.
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