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Guest Pete Wright

Varicam vs. PAL Digital Beta

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chrisM

 

I heard that a DVW 700 does exactly as you say and simply makes a frame out of two interlaced halfpictures. At least that is how I undestand it non-technically. The DVW 790, so I was told, does real frame capture. I shot a commercial a couple of years ago where the director wanted frame setting on the DVW 700. Though there was an interesting effect (strobing, motion blur) it was far from what I know from a film camera.

I would like to know if the DVW 790 would deliver a true frame recording. Thanks for your help.

 

Markus

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simply makes a frame out of two interlaced halfpictures.

If it really just puts two fields together from an interlaced source, that doesn't work so well. If you have a vertical edge moving horizontally, it's in two different places in the two fields, so you get what is called "mouse teeth".

 

 

 

-- J.S.

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If it really just puts two fields together from an interlaced source, that doesn't work so well.

 

That's simple weave deinterlacing. The "frame" mode with a PAL DigiBeta is different.

 

This how I understand it works.

 

In "field" (normal) mode the CCD reads out every photosite every field. Charges from adjacent fields are combined alternately, i.e. for the first field line one is actually 1+2 and line 3 is 3+4 and for the second field line 2 is actually 2+3 and line 4 is 4+5.

 

The limits of electronic shutter in "field" mode is 1/50th of a second.

 

With the camera in "frame" mode, (also EVS and extended clear scan), lines of photosites are read out consistent with field times, i.e. line one is read out first and line 2 is read out 1/50th later. The photosites therefore have 1/25 of a second between "readouts" to accumulate charge. This is why shutter times can be slower than 1/50th in these modes. Also, vertical resolution is higher because photosite charges are not being combined.

 

A mechanical shutter can create a true progressive capture characteristic. I'll try and illustrate why:

 

The top two lines depict the times these photosite lines are accumulating charge. The breaks in the lines represent their "readout" time. They read out alternately every 1/50th of a second, but have 1/25th of a second to collect charge.

 

The bottom broken line depicts the time the mechanical shutter is open, XXXX = closed.

 

Time

Line 1 __________ __________ __________ __________ etc

Line 2 _____ __________ __________ __________ etc

Shutter XXXX _____ XXXX _____ XXXX _____ XXXX (open 1/50th closed 1/50th)

 

The shutter only allows the lines to accumulate charge for 1/2 of the normal time. When it closes one field is read out immediately while the other simply "waits" for the next field readout. Usually this second line would continue accumulating charge, but it can't because the shutter keeps it in the dark. The image you get is, in effect, 576 25 PsF, because the photosite charges where all captured within the same 1/50th exposure time.

 

I hope this makes sense :unsure:

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That's right. It is the Lumier guy at the dvinfo HDV thread that has the inside info on the new HDV cameras, that was his name.

 

Digi Beta (PAL) is compressed 2x, Varicam 7x, the recorded resolution is about the same. (Varicam cuts down 25% horizontal resolution when recording.) I've seen footage from both. Digi Beta picture is a lot better. And you can't even get Varicam in Europe.

 

 

You can, I shot a shortfilm on Varicam. With the aid of www.alfacam.com. The biggest HD facility in Europe.

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Hi,

 

As far as I know there's less than four varicams available in London, and one of them is privately owned. I found the London varicam people to be universally standoffish and useless.

 

Phil

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