Interlaced/Progressive question

[pfh]

Sorry if this question has been hashed to death and I just haven't come across enough debate on the issue but I hope nobody's rolling their eyes and groaning..........
I've read some discussions about interlaced and progressive formats and understand the reasoning behind it but here's a question that still leaves me a bit confused:
If film is a progressive source and vhs is interlaced then isn't the original progressive footage on a vhs "degraded" by a no-no conversion? I mean, if we really shouldn't be changing an interlaced source when we capture/transfer to dvd then how does the film industry get away with it by going the opposite way?
Or put another way, are vhs tapes actually progressive footage that gets converted by the vcr and/or tv on playback?

I ask this question for a couple reasons:
1. To understand video transfer process more thouroughly and
2. I have a hardware capture device that has a chip capable of interlaced and progressive caps. However, the engineers from Plextor say it captures/converts to progressive only- with the current software used to operate it. In working with a couple reps from Plextor testing beta drivers, I'm getting pretty decent quality but it's still progressive. Looks good though when played back on tv set. Just wish I could see what happens it I could leave the source (vhs) as interlaced. Unless of course vhs is actually progressive that's changed during playback as per my opening questions.

[groyal]

If film is a progressive source and vhs is interlaced then isn't the original progressive footage on a vhs "degraded" by a no-no conversion?

Film is 24fps progressive, TV is 30fps interlaced: if there wasn't some way to convert between the two, you could never watch movies on television let alone store them on videotape. But the telecine process which achieves this conversion doesn't degrade the picture, it degrades the motion. This is a more subtle defect that escapes our attention most of the time.

I mean, if we really shouldn't be changing an interlaced source when we capture/transfer to dvd then how does the film industry get away with it by going the opposite way?

Deinterlacing a picture isn't a simple "do" or "don't" proposition, it's a decision that involves where the source originated from, what kind of digital-domain processing is required, and how you intend to watch the finished product.

For purposes of editing and image enhancement it's somewhere between desirable and necessary to deinterlace in order to do some kinds of work even if you reintlerlace the picture later. If the source was progressive (e.g. film) it's to your advantage to "undo" the telecine process because MPEG-2 has the ability to perform that operation on-the-fly. If the DVD is intended solely as a replacement for the VCR and you're not even going to bother to remove the commercials, deinterlacing is purely a waste of time.

Note that creating a pseudo-progressive source by applying a filter that removes interlace artifacts is NOT deinterlacing (even if such filter is called a "deinterlacer") and is outright ruinous to the picture because it cannot be undone. It has its place in the scheme of things, but again, the decision to use one depends on the specific case and can't be reduced to a standard practice.

[pfh]

Thanks for the reply groyal:
My head is swimming trying to comprehend all the info and just had some thoughts floating in there.
After more thought I realize now that the telicine process doesn't ruin any of the progressive nature of film. You've helped clarify things and now I realize (once again) that digitizing video is very situational. Not are we only constrained by the industry standarized output devices we also add to this equation source quality, editing needs, codecs, formats, etc. etc.. Then lets add poltics, NTCS, PAL and SeCAM to the mix!! Jeesh.!!!! I certainly have a lot more respect for those of you that do this stuff for a living! My editing needs are very minimal so IMO I'm putting too much concern into the progressive/interlace thing. As long as my output looks good then I'm satisfied and have educated myself in the process.
Hats off to ya. ..........................Paul

[FulciLives]

It really is best to leave interlaced video as interlaced UNLESS you intend to watch it ONLY on a PC MONITOR.

It is possible to take interlaced NTSC and telecine back to film rate for making a DVD but this only works with NTSC that came from film and even then the process can be tricky unless you know what you are doing.

The best way to do an IVTC (inverse telecine that is 29.970fps interlaced NTSC to progressive 23.976fps NTSC) is to use DECOMB for AviSynth.

However there is nothing wrong with leaving video interlaced for the purposes of making a DVD.

- John "FulciLives" Coleman

[Cornucopia]

Just a point of clarification for the poster...

Any time you "digitize" any video footage with a standard, consumer capture card (notice I'm ignoring HD, lorez webgrade, and scientific models) at full rez, it is capturing as interlaced, whether the source was originally that way or not.

Your choice is to use the info about the source and your needs about the destination to determine what (if anything) you need to do with the signal.
There are some general rules of thumb, but also quite a number of exceptions.

Scott

[pfh]

I hear you John. Using Avisynth for inverse telicine is what I found to try for my situation, however, as you noted this can be tricky as well. This maybe why Plextor is "leaving well enough alone" in a device they sell, eventhough the WIS chip supports interlaced and progressive modes. My mpeg captures are progressive for sure but it comes out looking pretty good on my tv ( no scan lines or anything). I just can't help but think it maybe even better if I could take full advantage of the chips capability.

On a side note- Vdub has added support for the Plextor M402 unit which I'm using but I haven't yet had the time for trying capping with vdub.

Here's my quoted email to Plextor support:

It's progressive, not interlaced.


Plextor America Support Team - Jack



--------------------------------------------------------------------------------
From: Paul Hecky [mailtohecky@lowesisland.com]
Sent: Friday, February 18, 2005 10:26 AM
To: Tech Support
Subject: M402U


I’ve searched thru the website and couldn’t find specific answer. My question is whether or not the ConvertX M402U converts the input of anolog interlaced footage to progressive mpeg2 or does it leave it as interlaced?

Thanks. Paul

[dvd3500]

What about when converting between NTSC and PAL? Should you deinterlace beforehand?

[John Whittle]

There are some pitfalls in trying to convert film source interlaced video to progressive.
1. Film is 24 fps (nominal) video is 30 fps (nominal-color is slower).
To get this converted, a 3-2 arrangement is used where FIELDS are repeated and a frame of video will consist of two different frames of film (a field of each).

2. Reverse telecine spots flags and can build back to 24 fps film IF (and this is a big IF) nothing has been done to violate NTSC laws along the way, such as off NTSC color frame edits, speed changes, bad reel joins. NTSC color frames consist of four fields (to get the subcarrier back in phase) and a cut at the wrong point throws the hoizontal timing off plus the interlaced field flags.

So unless you're responsible for the transfer and editing of the source material you are trusting or risking problems.

It is usually best to build back to progressive before a PAL conversion (which is a 4 percent speed increase and matches film frames and fields) and also when chopping or making 16:9 out of 3:4 from a film source.

There is another devil and that was a practice of "speed" changes which was widely done at one time. This would make a program fit a time slot or was just used to speed up or slow down a piece of footage. The problem is it throws out or adds in fields.

I was trying to make a 16:9 version from a letterboxed tape I recorded in 2001 from PBS and after two days of "converting" I all did was create a 16:9 version with "stuttering" from the speed conversions. I should have known better since playing a DVD on a computer it tried to put up a progressive display and I noticed some stuttering then bit it didn't click until later.

The 3:4 version looks great on an interlaced display and sounds great.

You may also run into sound synch problems when you strart building back since you can also run into dropped frames.

If everyone obeyed the NTSC and PAL laws, we wouldn't have these problems and now I fully appreciate why the engineers hated it when we'd do a speed change in telecine.

John