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Progressive-scan CCD


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#1 Krstic Zoran

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Posted 01 September 2008 - 12:17 PM

I found many explanations about progressive-scan CCDs but I'm not sure what is truth.
I don't undarstand readout mechanism of progressive-scan CCDs (in HDW-F900,Varicam,etc.) when we work at 24 PsF or either progressive-scan mode (24 p,50 p...).How long is exposure time at 24 PsF,25 p or 50p?Does it 1/24s or 1/48s,1/25s or 1/50s respectively?Is the shutter used in 24 PsF,etc?

When we shot at 24 PsF,all 1080 lines are read from CCD at same time!?
HDW-F900 record at 24 PsF.How it record 24 frames on the tape?Is the frames divided on odd and even fields or 1st field contents 1st half of 1080 lines?

How Varicam record 24 p on the tape?Does it record real frames on tha tape?

Does 50p mean that we have real 50 frames read from CCD at second?

Sorry,my post is too much confused but I hope that you will understand what is on my mind.

Thank you in advance.
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#2 Phil Rhodes

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Posted 01 September 2008 - 12:48 PM

Hi,

First of all, please change your display name to your real first and last names as required by the forum rules.

How long is exposure time at 24 PsF,25 p or 50p?Does it 1/24s or 1/48s,1/25s or 1/50s respectively?


You can alter it on most cameras, but usually, unless someone says otherwise, it will be twice the frame rate - at 24p, shutter would be 1/48, at 50p, 1/100. It's the same as a film camera, where the rotating shutter exposes the frame for one half of the time the frame is in the gate.

PsF doesn't make any difference, it's just a technical change in the way the picture is stored.

When we shot at 24 PsF,all 1080 lines are read from CCD at same time!?


More accurately, all 1080 lines stop receiving light at the same time. On a CCD, they are then read out sequentially, one after the other.

HDW-F900 record at 24 PsF.How it record 24 frames on the tape?Is the frames divided on odd and even fields or 1st field contents 1st half of 1080 lines?


How it's recorded on the tape doesn't really matter; what you care about is how it comes out of an HDCAM VTR. PsF means that the odd lines are sent, then the even lines.

How Varicam record 24 p on the tape?Does it record real frames on tha tape?


Varicam, as with all DVCPRO-100 tape based devices, always records 60 frames per second onto tape. It duplicates frames from lower frame rate sources to make up 60, and the VTR will play back all 60 frames. Often, special techniques are required when editing to recover the 24 original frames.

Does 50p mean that we have real 50 frames read from CCD at second?


Yes.

Hope this helps.
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#3 David Mullen ASC

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Posted 01 September 2008 - 01:29 PM

PsF is a recording mode -- it takes true P frames captured by the sensor and splits them into two fields for recording, and reconstructs them as whole P frames for playback on a progressive-scan monitor or when working with the frames in post. This allows a 24P camera to use 48 Hz for recording and display as 48i on some pro HD interlaced-scan set monitors.

A 24P camera with no shutter engaged would expose the sensor for 1/24th of a second (a 25P camera would expose for 1/25th of a second). But typically some electronic shutter speed is selected to shorten this time, usually to 1/48th of a second to match the motion blur of shooting 24 fps film with a 180 degree spinning mirror shutter (or 1/50th for a 25 fps camera). On the other hand, a lot of people who shoot 60i on an NTSC or HD camera often just use 1/60th for the shutter speed, i.e. no shutter.

The Varicam shoots at selected progressive-scan 720-line frame rates but records everything to 720/60P, so if you shoot 24P, it adds 36 redundant, repeated frames in a sequence/pattern to add up to 60. These redundant frames are flagged so they can be deleted for true 24P post or upconversion from 60P/720 to 24P/1080.
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