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#1 Ollie Bartlett

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Posted 30 May 2008 - 08:36 AM

Hi there,

Im having real problems getting my head around some format issues and would love it if i could get some clear answers. ive got an HDX-900 in front of me, and within the menu system it says, for example, that i can record 1080-50i, but as either an 50 field interlaced or 25 frame progressive signal.

I've understand the difference between interlaced and progressive scanning, and the different look it will give you, but how can the two co-exist within the same signal? ie 1080-50i 25p. Ive always known a signal to either be interlaced or progressive, never both. Certainly, or as far as i know, this is not the case with say a sony 750p (or at least, i hope not otherwise ive missed something everyday at work for the last 3 months)

Or am i being stupid and not realising that within the setting '1080-50i', you can record either a 50i or 25p signal, and that 1080-50i is simply the name for the system mode, and then camera mode sets up the actual recorded signal?

Moreover, can anybody understand what ive just written, or is it all nonsense... :blink: ?

Cheers people,

Ollie
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#2 Andrew McCarrick

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Posted 30 May 2008 - 10:23 AM

With the DVCPro HD codec, when recording to tape, in a interlaced signal it will take a progressive source and record every frame twice. The camera will always scan in progressive, but if you are in an interlaced frame rate it will take every other line from those progressive frames.
So with 25 progressive frames (with alternating fields) you get 1 (odd),1 (even), 2, 2, 3, 3, 4, 4, ect. into a 50i signal.
And with 50 interlaced frames, you get 50 progressive frames.... 1 (odd), 2 (even), 3 (odd), 4 (even), ect.

HDCAM uses a similar method but only records progessive frames in the 2:2 pull down.... so you shoot 24p, it records 48i (from the 24 progessive frames); you shoot in 25p, it records 50i. It's just the way the codecs were set-up.

Edited by Andrew McCarrick, 30 May 2008 - 10:25 AM.

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#3 Ollie Bartlett

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Posted 30 May 2008 - 11:33 AM

Ok so let me see if ive got this...

25p is 25 progressively scanned frames but yet the signal is still recorded to tape as 50 interlaced fields anyway? And even though the 750's can alter between 50i and 25p, the recording will always be interlaced? As in a 1080-50i recording that was scanned at 25p?

Just in case this is right, would anybody be able to explain what happens when the system is set to 720-50p but the camera is set to 25p? That seems, and again going on the long shot that i understand any of this, that it would scan progressively, but for each time it scans it would record two full frames to tape, 25 times a second.

wow im confused.

Ollie
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#4 Andrew McCarrick

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Posted 30 May 2008 - 12:07 PM

Ok so let me see if ive got this...

25p is 25 progressively scanned frames but yet the signal is still recorded to tape as 50 interlaced fields anyway? And even though the 750's can alter between 50i and 25p, the recording will always be interlaced? As in a 1080-50i recording that was scanned at 25p?

Just in case this is right, would anybody be able to explain what happens when the system is set to 720-50p but the camera is set to 25p? That seems, and again going on the long shot that i understand any of this, that it would scan progressively, but for each time it scans it would record two full frames to tape, 25 times a second.

wow im confused.

Ollie


Yep; You got it right. In 720p 25p records at 50p with a 2:2 pull down... so it has every two frames being redudant frames. Then in post you would remove those redundant frames... each NLE has it own way of doing that.

When you move away from tape based workflows and into data/harddrive cameras.... things tend to be easier, as you can record natively in the framerate... so if you choose 25p, you only get 25 progressive frames instead of 50 interlaced.

Edited by Andrew McCarrick, 30 May 2008 - 12:11 PM.

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#5 Ollie Bartlett

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Posted 30 May 2008 - 01:03 PM

Wow... sweet. Cheers Andrew.

In one foul swoop you've cleared up my format issues and also in a roundabout way answered my questions as to what pulldown actually is.

2 for the price of 1... bargain!!! Should we ever meet i owe you a beer.
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#6 Andrew McCarrick

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Posted 30 May 2008 - 02:03 PM

2 for the price of 1... bargain!!! Should we ever meet i owe you a beer.

lol... thanks for the offer but I don't drink (I'm 19, so it's not legal here in the u.s. anyway).
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#7 Ollie Bartlett

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Posted 30 May 2008 - 02:49 PM

Same as... gave up drinking and smoking exactly 82 days ago and had nothing since (you can tell i probably needed to by the fact its still counted in days).

Anyway, hopefully moving to Hoboken sometime early next year so if and we can both have cokes.
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