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CAPTURING FROM VHS - PART 2 - INVERSE TELECINE | Diogo Figueiredo |
INTRODUCTION:
What do you think about increasing the quality of your DivX ;) movie-CDs while decreasing
the file size? Even more: making the encoding time shorter!! A miracle? No. It is possible
(sometimes) with a VirtualDub function that I forgot to mention in my first article (see
"Best resolution for capturing from a VHS source", by myself). To be honest, I
didn't forget to mention it. I just didn't know it existed. Well, living and learning!
That "almost miraculous" resource is called "inverse telecine", and
I'll be sharing this experience with you now.
LIMITATIONS:
This article is useful if you are working with a video source that uses the NTSC color
system (thus, uses 29.97 frames per second), and that was originally made for the cinema
(24 frames per second). Most of the movies you find in your "Blockbuster video
store" are included in this category. This is completely useless if you are capturing
a TV show, for instance.
I don't know how this applies to PAL movies, which work with 25 frames per second, because
all my equipment is NTSC. If I find anything about it, I'll post it as soon as possible.
LEGAL NOTICE:
It's never too much to say that making copies (it doesn't matter the media) of any
copyrighted material is illegal, unless you own the copyrights or have an authorization
from the copyrights owner. This is specially important in this article, since it deals
mostly with the capture of movies first intended to the big screen. Also, I'm not
responsible for any problems or damages caused by the information hereby expressed.
Proceed at your own risk. If you disagree with any of these, stop reading this guide now.
Otherwise, good luck.
PROCEDURE:
Let me go straight to the point: the NTSC video uses 29.97 frames per second. The films
intended for the big screen (movie theaters) use 24 frames per second. So, when you put a
cinema film into a NTSC cassette, some conversion must be made. How is that made? That's
simple: first, the film is slowed down by 0.1 percent, to be at 23.97 frames per second.
Then, a new frame is inserted in every block of four frames, resulting in 29.97 frames per
second. The problem begins in that "frame insertion". You can't just add a blank
frame or repeat a frame, which would make the movie choppy. Thus, the video fields are
used. At this point I suppose you know what video fields are. If you don't, this article
is not YET for you. Try to learn about video fields and come back later. Here's what is
actually done to every block of four frames:
-The first frame is kept untouched.
-The second converted frame is a mix of the first and the second original frames, both
separated by the fields, which means that the even field is the first original frame and
the odd field is the second original frame.
-The third converted frame is a mix (the same way as above) of the second and third
original frames.
-The fourth converted frame is the third original frame.
-The fifth converted frame is the forth original frame.
At the end of all that, what you get is a sequence of frames that goes like this: three
non-interlaced frames, then two interlaced frames, then three more non-interlaced frames
followed by other two interlaced frames and so on.
Every interlaced frame will be a mix of two actual frames, and in case you have a high
motion (a lot of difference between the two original frames that were mixed together),
they will look like this:
INTERLACED VIDEO FRAME
Compressing an interlaced video source is a waste of space. Thus, something has to be
done. Besides that, you would save 29.97 frames per second of a video that was made at 24
frames per second. That is 25% of completely useless data, because those extra 6 frames
per second were not in the original movie!
If you just deinterlace your video before compressing (as I recommended in my previous
article - sorry!), you are not getting rid of those extra frames. All you would do is
mixing both fields together. The result would look like this:
DEINTERLACED VIDEO FRAME
As you can see from the picture above, both images are still present. I just got rid of
the field lines. Besides, by doing this you lose half of your resolution, since the
deinterlace process mixes the horizontal lines two by two.
And then there's this function called "inverse telecine", which can be found
under the "video/frame rate" pull down menu in VirtualDub. What it does? When
set to "reconstruct from fields - adaptive", it "demixes" the fields
of the frames, so that they will be exactly how they originally were, eliminating one
frame in every block of five, giving you the final result of 23.97 frames per second. The
frames would then look like this:
INVERSED TELECINE VIDEO FRAME
Notice that no information of the original movie has been lost. It's all there as it
should be, and you don't loose any resolution at all! Isn't this enough for you??? What if
I say it's even faster than deinterlacing? (This sounds like one of those
"Ginsu-Knife-Don't-Delay-Call-Today" TV show!).
To put a 2 hours long movie in a single CD using the simple deinterlace process, I used to
use 600 kbits/second with low-motion DivX ;-). Now, after inverse telecine, I use 800
kbits/second and the movie fits the same space. That's 33% more quality! Notice that if
you are using "inverse telecine", you don't need to (and you shouldn't) use the
deinterlace filter. The "inverse telecine" does it all. Another important point
is that you movie must have been captured at 29.97 fps and with both fields complete. That
means 480 of horizontal lines.
SHORTENING IT ALL (THE PRACTICAL GUIDE):
-Capture at 512 x 480 @ 29.97 frames per second. VirtualDub does it, although I strongly
recommend "freeVCR", which is a freeware that can be downloaded from
"www.downloads.com" (perhaps you guys from divx-digest should try to put it on
your downloads section - if it's not already).
-Load you captured movie into VirtualDub and set the resize filter to 512 x 384 with the
"precise bicubic" option.
-Go to the "video" menu, then go to "frame rate" and choose
"reconstruct from fields - adaptive" in the "inverse telecine" box. DO
NOT change the frame rate. "inverse telecine" will do it automatically.
-For the compression, I recommend DivX :-) low motion @ 800 kbits/second if you want to
put a 2 hour movie in a single CD.
-Choose your preferred audio compression (recommended is mpeg layer3).
-Set audio to "full processing mode".
-Go to the "audio" menu, then go to "interleaving". Set the
"interleave audio every" to 30 frames.
-Save your work.
-Have fun.
Some further explanation about "inverse telecine" can be found in VirtualDub's
help file. Enjoy!