CAPTURE
GUIDES -> Field Order Corrections
Although this guide has specifically been written
for correcting a known issue in certain versions of ATI MMC,
it will apply to any situation dealing with reverse
interlace. As you may have read in the UNDERSTANDING YOUR
SOURCE guide, interlacing is a process of displaying video
in a weaved pattern (although it will appear perfectly
normal played back on an interlaced device like a television
set). The fields, often called "A" and
"B", "top" and "bottom", or
"odd" and "even", can be in either
order. That is, one will go first, and the other follows.
The headers of the MPEG file stores the information about
which field goes first. If these header flags are
incorrect, the wrong field will play first, causing a
horrible stuttering effect in the video, especially
noticeable on side-to-side motion and panning shots.
The fix is simple: reverse the interlace information in
the file, and it should be correct. This can be done
with a simple MPEG properties adjustment in the freeware
software ReStream,
or with a full re-encode in an encoder program like TMPGEnc
Plus.
Restream
This is a very simple
program. Everything is contained in the main window of the
software, there are no menus or other fancy settings. What
you see is what you get.
Click on the folder at the top right side of the program to
open your source file. Once it loads, it will auto-select
the current interlace method. If the MPEG files is BOTTOM
FIELD, it will have no boxes checked. If the TOP FIELD box
is checked, the MPEG file has top field flags in place. If
the PROGRESSIVE box is checked, the file is not interlace.
Note: If the MPEG is supposed to be interlaced, and
has the combing affect ... AND is saved as a progressive
MPEG format, the video was encoded improperly and may need
to be re-encoded.
To alter the MPEG interlace information, merely alter the
settings in RESTREAM to pick the opposite of what was
loaded. In this example above, TOP FIELD was select when the
file was opened. The file plays back with a stutter on tv,
and therefore has incorrect interlace settings. In the
example, UNCHECKING the top field box will make the file
BOTTOM FIELD, which is the correct MPEG setting.
Name the new file in the DESTINATION window, and then click
the SAVE icon (floppy disk) to save the new file. It should
only take a few minutes at most. This is NOT a re-encode,
but merely a correction of the MPEG header data.
Advanced Usage Note: The GOP options (reset
timestamps, zero broken flags), the Sequence Extension
option, and the Stream Extras (remove user data, end codes)
may be useful for repairing damaged MPEG files. This is
outside of the scope of this guide, but a good feature to
note for the "just in case" scenario when dealing
with bad MPEG source. Altering the resolution, framerates,
aspect and bitrate is NOT suggested under any circumstances.
TMPGEnc
Plus Encoder
Although correcting an
MPEG without encoding is the best solution, it may not work
100% of the time. Some files are just stubborn. This is
often the case after an MPEG file has been re-encode too
many times, and the field information becomes jumbled beyond
repair.
Guide note: This section of this interlace correction
guide will only show the pertinent settings found in TMPGEnc
for correcting interlace. See the Convert
Using TMPGEnc Plus page for a full TMPGEnc Plus encoding
guide.
On the ADVANCED settings page, at the top, is the SOURCE
information regarding interlace. This tells TMPGEnc how to
interpret the source that it will be encoding an MPEG from.
Always be sure to correctly instruct the encoder how to read
the source file.
Do not rely on TMPGEnc to properly read the interlace
information when it opens the file. Very often it is
wrong and assumes the source is BOTTOM FIELD INTERLACED,
which is often incorrect because most capture cards grab
from the top field first. Only DV and a minority of devices
grab bottom field interlace.
After the SOURCE information is set up, be sure to set up
the DESTINATION information:
The ENCODE MODE must specify INTERLACE as the
encoding format. It will encode the new file in the same
interlace mode specified in the SOURCE SETTINGS. If the
source was top field, the new field will be top field. If it
was bottom field, the new file will also be bottom field.
When using this guide to correct MPEG interlace problems,
open the file up with the correct interlace settings, and
then the correct interlace encode mode. Match the video
bitrate and resolution of the source for the fastest encode
times (often approaching realtime or faster).
Page Last Updated:
August 13th 2004
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