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Question re: interlacing

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Jeff Strachan
Question re: interlacing
on Oct 28, 2010 at 4:06:40 pm

I am one of those old codgers that sometimes get put down on these forums. But I'm a nice guy!

I was recently asked a question about interlacing that I simply don't know the answer to.
In analogue each field is a partial image separated in time by 1/60th(approx) of a second.
In progressive digital each frame is a complete image of a single instant in time and displayed 1/30th of a second apart.
Now interlaced digital; are the two fields partial images of two different instants in time that are separated by 1/60th of a second, or are they partial images of the same instant in time but displayed separated by 1/60 sec?


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Dave LaRonde
Re: Question re: interlacing
on Oct 28, 2010 at 4:42:19 pm

[Jeff Strachan] "Now interlaced digital; are the two fields partial images of two different instants in time that are separated by 1/60th of a second, or are they partial images of the same instant in time but displayed separated by 1/60 sec?"

You're gonna love this answer: it can be BOTH!

At 1080 vertical resolution, almost all cameras can shoot interlaced or progressive-scan. Progressive scan gets recorded in the way you describe above: partial images of the same instant in time. They give it a fancy name: progressive segmented frame.
This system of progressive segmented frames is also used when shooting at 23.976 frames/sec (NTSC film frame rate), adding 3:2 pulldown to maintain a 29.97 frame rate.

Extremely pricey cameras can shoot actual, complete frames at 1080. You don't encounter them very often... at least not at my level of the food chain.

At 720 vertical resolution, it's a slightly different story: the most common method of recording is at 59.94 FRAMES per second. However, they can also shoot at 29.97 complete frames and 23.976 complete frames. And dare I say it? In-between frame rates, too.

And here you were looking forward to a nice, clean and simple system of digital HD video. Ha!

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Gary Hazen
Re: Question re: interlacing
on Oct 28, 2010 at 6:29:46 pm

That's a bit confusing Dave. Interlaced and PsF are not the same thing.

[Jeff Strachan] "...two fields partial images of two different instants in time that are separated by 1/60th of a second..."

This is interlaced.

[Jeff Strachan] "...partial images of the same instant in time but displayed separated by 1/60 sec"

This is PsF.

You could argue that PsF is interlaced, but you wouldn't win that argument with an engineer.


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Dave LaRonde
Re: Question re: interlacing
on Oct 28, 2010 at 7:03:58 pm

[Gary Hazen] "You could argue that PsF is interlaced, but you wouldn't win that argument with an engineer."

Yeah, yeah, yeah, and an engineer's gonna bust my chops for calling it YUV Color, too.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Jeff Strachan
Re: Question re: interlacing
on Oct 28, 2010 at 8:07:37 pm

"In between frame rates"
Ugh! glad I am not the one writing NLE programs that have to work with this %$#%$

yes, I had expected an array of possible frame/field arrangments but not something quite that messy.

So the answer to my question is, "both, and more"

Jeff Strachan
CJBN TV
Kenora, Ontario


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Dave LaRonde
Re: Question re: interlacing
on Oct 28, 2010 at 9:02:33 pm

I should probably add those in-between frame rates are found almost exclusively on cameras carrying price tags similar to brand-new Corvettes. Thus, there aren't many made, and these frame rates see limited application.

The frame rate situation isn't as bad as I apparently made it sound. Didn't mean to mislead.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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