A coupla' general framerate questions.

[absinthecarolinas]

Hey all. Just a couple of quick framerate questions.

1) I'm getting ready to encode an AVI to DVD, and the AVI source shows up as 29.971 fps in VirtualDub. Seems odd, but do you think that being .001 too fast in framerate will cause a problem with either authoring (I usually use DVD Lab, or TMPGEnc Author) or playback? Or is VirtualDub just being over-exact, maybe?

2) My second question regards a different AVI and a setting under the Video tab of Cinema Craft Encoder, where you can check "zigzag" or "alternate" depending upon whether you have an interlaced vs. a progressive source. I have read about how to visually check a clip to see if it's interlaced, but this AVI is 23.976 fps. Can I safely assume that something with that framerate is most likely interlaced? I don't believe that this particular clip is available on DVD at all, so assuming it's a "rip" from a VHS tape, wouldn't it be interlaced? I've never ripped a VHS, so I dunno ... :0

(on a side note, I'm really only asking the second question in theory as I've already encoded it with the setting for interlaced source. It really looks terrific ... just wondering if I might have done better).

Thanks in advance to you knowledgeable folks ...

-abs

[BJ_M]

1. .001 is not to fast , since the exact speed is 29.9712 but different programs round it out different (29.971 , 29.971206 , 29.97 i have all seen)

you question 2 doesnt make sense -- do you mean the source was 29.97 instead of 23.976 ?

[absinthecarolinas]

you question 2 doesnt make sense -- do you mean the source was 29.97 instead of 23.976 ?

In question #2, I'm referring to a different AVI than the one in question #1. VirtualDub reports the framerate as 23.976.

I'm just wondering where such a framerate would come from (I mean I know in theory it's a result of inverse-telecining a 29.97 source, but in practical terms where did it come from -- a VHS tape??), and is it or is it not likely to be interlaced based on that knowledge alone.

Thanks,

-abs

[junkmalle]

A 23.976 fps AVI file is almost certainly progressive. You can leave it progressive with 3:2 pulldown when you convert to MPEG for burning to DVD.

I've inverse telecined from movies on VHS tape before. So it's possible.

[absinthecarolinas]

A 23.976 fps AVI file is almost certainly progressive. You can leave it progressive with 3:2 pulldown when you convert to MPEG for burning to DVD.

I've inverse telecined from movies on VHS tape before. So it's possible.

Ok, thanks. But I remain confused

Why would an AVI had a 23.976 framerate anyway? I mean, where would it come from? If a flick plays back on an NTSC TV, whether from DVD or from VHS or any other source, at 29.97, what reason would someone have to IVTC it? (or does this have something to do with capturing/video cards, etc? -- something I know very, very little about?)

As above, I did already convert this using the CCE setting for interlaced, and it looks great (a correction: the setting I mentioned above is under the "Quality" tab, not the "Video" tab). But maybe I'll try a few hours of encoding again using the "zigzag/progressive frames" setting to see if it looks any different.

Maybe someone could clear this up for me if I ask the question this way: When a clip is captured from a VHS source, I presume via a video capture card, is the resulting capture rendered as "progressive," or does it retain the characteristics with which it was played on the television (or, as is probably correct, does it depend on who's doing the capturing, what hardware/software they're using, and how they go about it)?

-abs

[junkmalle]

NTSC television signals consist of 60 fields per second -- that's all a normal television can display. When you capture from a VHS tape you get 60 fields per second. If one field contains all the odd scanlines of the image the next one will contain all the even scanlines. To recreate a full frame requires that you merge the two fields together. But while watching on a television you only see one field at a time.

If the original source was a video camera those 60 fields were taken at 60 different times (ie, every 1/60 of a second). You can't really convert that to a progressive picture.

If the VHS tape was made by telecining a 24 fps movie you can restore the original film frames by inverse telecining (normally done after capturing, not while capturing). It make sense to inverse telecine if you want to be able to watch the movie on a computer as well as a television. The frames will be free of interlace artifacts when viewed on a computer monitor. On a DVD you can leave the video as 24 progressive frames per second and flag the player to perform the 3:2 pulldown while playing.

[absinthecarolinas]

Okay. I think I see what you mean. In other words, video captured from NTSC TV is by definition captured at 29.97 fps. So ... if I have such an AVI and it's 23.976, then it's probably been IVTC'd.

So I guess the reverse of what I thought is true.

I have yet another AVI (this is the third one I've referred to) which is also a clip I feel sure was captured from VHS (and which is not available on DVD as far as I know). This particular AVI is 29.97 fps. So is it more likely this one is interlaced as opposed to progressive?

I think I'm on the right track here ....

-abs

[junkmalle]

Actually, a 29.97 fps AVI file has most likely been deinterlaced (by dropping the second field and interpolating from lines of the first field, or bluring the two fields together) because interlaced video looks terrible on a computer monitor. Normally one would only leave the video interlaced if the AVI was a transition format -- eg, when making a DVD from a DV camcorder.

You can tell for sure by using VirtualDub (or VirtualDubMPEG for MPEG files) and looking at individual frames. The comb-like interlace artifacts are very obvious. Anywhere where anything moves you will get frames where it's obvious each field came from a different picture.

[absinthecarolinas]

Actually, a 29.97 fps AVI file has most likely been deinterlaced (by dropping the second field and interpolating from lines of the first field, or bluring the two fields together) because interlaced video looks terrible on a computer monitor.

Well, crudbuckets. I just started a 3-pass encode with the setting ticked for interlaced. Ah well, the last one I did that way came out looking great anyhow.

Live and learn

ThAnKs!

-abs