Strange MPEG-2 encoding behavior, help!

pdp wrote on 1/22/2003, 1:12 AM

I just finished my first short movie (whoo!). I did the first edit using Video Factory. I didn't really like the MPEG-2 encoding job that it did, and I figured I might want to try a higher bitrate or variable bit rate. I also wanted to use DirectX plugins, so I upgraded to Video Vegas, opened the old project (.vf), saved it as a new project (.veg), used some plug-ins, made a few minor tweaks to the cuts, etc.

Now, when I encode an MPEG-2 in VF, it looks just OK. When I encode an MPEG-2 in VV, it looks a lot better - BUT - when I previewed the DVD on my TV set, the opening sequence (left to right motion, tracking a guy walking down the street) has these .... jitters, or stuttering pulses, that look like there is a problem with the encoding. All the other scenes in the film are fine.

I first assumed I had encoded with the bitrate too high (8 Mbit fixed). So, I tried to render the one scene at several lower bitrates, lower DC coefficient (tried 10bit and 9bit), as an experiment, but they all show the same pulsing jitters. These do not happen when I encode with VF. But, the quality of the VF MPEG-2 seems lower, even though I have compared 6 Mbit and 8 Mbit fixed encodings using both VF and VV! Also, the VF software now lets me tweak all the parameters, like the VV software. But the results are still different, even though (I think) the parameters are all the same!

I have 2 MPEG-2 files posted online if anyone wants to look at them. Watch the tree as the character walks past it. When I view the files on a computer monitor, the VF file shows 'comb' effects consistently. The VV file shows 'pulsing comb' effects which I guess are causing the problem when I view it on a TV screen.

VF file: http://www.brucix.com/vvtest/VFtest.mpg (2-3 MB)
VV file: http://www.brucix.com/vvtest/VVtest.mpg (2-3 MB)

Can anyone help me figure out what's going on??? I'm trying to get a final set of MPEG-2s ready so I can burn about 20 copies of this movie.

Thanks
[pdp]

Comments

PeterMac wrote on 1/22/2003, 4:35 AM

This sounds very much as if you have the 'fields order' wrong, something that you can't see on a computer; you have to play the movie through a TV before it becomes evident. (The comb effects you refer to on the computer are the way movement is portrayed on a non-interlaced display - unless you use something like PowerDVD, WinDVD, etc.)

The default 'bottom field first' used by Vegas is usually correct, but not necessarily so. Actually, there's no 'correct' or 'incorrect' in this, which is why the confusion arises in the first place, but you must follow suit. So, if your original footage is 'top field first' and you've used 'bottom field first' in encoding, then you will see this jerky effect, especially on pans or objects moving across the field of view.

Try a sample of the original footage using an alternative field order. I think you'll find that does the trick.

-Pete

mikkie wrote on 1/22/2003, 8:48 AM

Looked at the files, stepping thru them a frame at a time. Something funky is going on because you see the interlaced tweeners in both directions from the tree branches - I'd expect one.

As Pete suggested, try reversing your field order - the dvd template automatically assumes lower first, where as vegas usually gets it right. at any rate, with the original footage loaded in Vegas, right click on it & select properties - the 2nd tab should tell you what Vegas thinks it is, & I'd leave it there for the moment.

The property dialog should have shown either progressive, upper, or lower -> change the project properties to match if needed -> then render a test portion of your video, making sure to change the dvd template settings under custom so that the field order is the same as the properties etc.

If it doesn't work, try changing the settings all the way through -- if you chose upper for field level, choose lower. If that still doesn't work, something may be off in the original footage -> go progressive all the way through *If* you can (some DVD software won't accept it), or at least through to the mpeg2 render.

FWIW, the average DVD player should send a 29.97i stream to the tv, regardless your cutting a DVD using progressive settings. If you take an off the shelf movie DVD and take it appart so-to-speak, you'll find the movie's at 24p. There's a lot of folks who go through an extra couple of steps de-interlacing and doing an inverse telecine in an effort to get as nice a picture as possible before going to mpg2 - running filters for either or both can *sometimes* smooth out problem footage.

mike

pdp wrote on 1/22/2003, 9:44 AM

Hey everyone, I figured it out (I think). I had slowed down the clip by 5%, to match up an audio turntable scratch with the frames where the actor scratches the back of his head - and to make him walk more in time with the music tempo. When I selected the clip properties, to check the interlace order (which is bottom first) there is another option there called 'resample' - I did a test using the resampling, and that did the trick! The clip looks great on the TV now, esp. at 8 Mbps fixed!

Thanks all
[pdp]