IVTC, UVTC, we all VTC for IVTC

[KoalaBear]

To borrow an ancient American aphorism, "People like to talk about Inverse Telecine (IVTC) for VCD production, but nobody wants to do anything about it." In other words, understanding the theory is one thing, but a practical illustration of how to apply that theory to your day-to-day benefit is quite another.

Due primarily to design (and an accident involving a spare half-height drive bay that won't accomodate a DVD-ROM) all of my VCDs are made from DVD or VHS captures and not DVD rips. People who transcode movies from videotape to VCD will probably gain the most from this mini-tutorial, but since the underlying theory is the same regardless of the source, you should be able to adapt the technique to DVD rips as well.

What you'll need:

- Video card capable of capturing minimally-compressed AVI at 352x480 with no dropped frames. I use an ATI AIW 32 and HUFYUV, you can use whatever you've got provided (a) the AVI codec is substantially lossless and (b) using it doesn't cause you to drop frames.

- Commercial VHS tape made from motion picture source. Most DVDs are macrovision-enabled, but some (Thirteen Days for example) are not. I own about 60 movies on VHS and I haven't encountered one with macrovision yet, so if you happen to have a macrovision-free DVD handy you can use it, otherwise stick with a movie on videotape.

- VirtualDub. You won't need any filters. Whatever version you have on hand will do.

- AviSynth. Again, we're not relying on anything fancy, use the version you've got.

- Panasonic MPEG encoder. I would include TMPGenc instructions as a courtesy, but I only use the program for its handy built-in (de)multiplexer and would have to do alot of unnecessary work to make sure the recommended settings produced truly comparable results. Maybe somebody who's familiar with both encoders can adapt these instructions for TMPEGenc users.

The procedure:

(1) Locate a relatively motion-intensive portion of the source film and capture a few minutes of it at 352x480, 29.97fps, 44KHz stereo. You should be able to get four to six minutes into a file less than 2 Gb in size. Note that you MUST capture at a vertical resolution of 480, because we'll need both fields in their entirety to undo the telecine.

(2) Create an AviSynth script named IVTC.AVS using these directives:

AVIFileSource("d:\capture.avi")
Trim(0,0)
DoubleWeave
Pulldown(0,3)
VerticalReduceBy2

(3) Load IVTC.AVS into VirtualDub and look at the resulting frames.

You might get lucky and hit the correct start frame right off the bat, but if not, you'll see weird frame patterns like ghost/no ghost and superimposed frames at scene boundaries. If you do, change the second line of IVTC.AVS to Trim(1,0), save, reload and repeat.

Statistically speaking, there should be two good start frames per second of video, so you shouldn't have to increment the trim more than two or three times before you find the right one. You'll know when you do because the frames will be perfect, zero interlacing artifacts, clean transitions between scenes, and consistently perfect pictures wherever you slide the preview pointer. This is why dropped frames must be avoided at all costs -- once you find the cadence, or rhythm, of the film, a dropped frame at some point will cause all subsequent frames to be processed incorrectly.

(4) Go to VIDEO | FRAME RATE and note that the speed of the clip has been reduced to 23.97 frames per second. Say "Hmmmmm." Save the AVI as a new clip named IVTC.AVI for the encoding phase of the procedure.

(5) Load the IVTC.AVI clip into Panasonic. Make sure the FPS figure in the "Material Files" section and the "Output File" section both read 23.97 fps. Select "VCD/NTSC Stream" as the encoding type. Go to "Advanced Settings" and change the GOP sequence to 12/12/3, press OK, then press START ENCODING.

(6) Load the encoded IVTC.MPG into Media Player and play it back full-screen. If you've done everything correctly you'll be amazed at the crispness, quality and reduction in visible artifacts that makes the hassle of performing IVTC worthwhile. Because what we've done is eliminated six pseudo-frames per second that were never part of the original film to begin with, we've effectively increased the bitrate by 20% which sharply improves the quality of the encoded MPEG.

If you like what you see on-screen, burn the MPEG to VCD and watch it on TV. It should be more or less transparent (i.e., artifact-free) and identical to the original source albeit at lower resolution. IVTC can lead to truly dramatic quality gains, but again, it is suitable for materials that originated on motion picture film that were transferred to NTSC video at some point and absolutely nothing else.

Give it a try, and share your results with us.


<font size=-1>[ This Message was edited by: KoalaBear on 2001-08-22 08:57:34 ]</font>

[VidGuy]

Personally, I capture at 720x480, then use TMPGEnc's automatic IVTC. Then create the project file (*.tpr), use VFAPI to create a VFAPI *.avi file and away I go - no pain. TMPGEnc does a pretty good job of automatic IVTC.