ASC vid manual... let's figure some stuff out

[skippyfetus]

As I approach the center of the American Cinematograher Video Manual, a few confusing things haunt me and I was wondering if anyone could shed some light on them.

Topic #1 -

Is DV 480i SDTV? Why do I ask? Because 480i resoltion is defined as 704x480. DV is 720 pixels wide and it uses them all. We know that it uses them all because you can see all 720 in our NLE programs.

Here are some resolutions from the American Cinematography video manual. It says that DV's resolution is 500 lines... I'm assuming it means TV lines, since there's only 480 vertical lines used in DV. It's kinda strange though because the XL1S claims 530 lines.

Topic #2 -

Quote from the manual:

The dynamic range of a high end CCD may exceed 70dbs - the ability to reproduce a contrast range of 600:1, which is a range from absolute black to two and a half stops beyond the whitest white displayable on a video monitor. It should be self evident that compression of the highlight information is necessary in order to reproduce the signal.

It sounds like he's saying that a high end CCD can capture 9.2 stops of contrast, but in order for it to be displayed as a video signal, it must be compressed to about 7 stops... is this correct... that there can be no more than 7 stops of contrast in video? This would be consistant with what John Jackman says in his book "Lighting for Digital Video and Television". He says

some film manufacturers today are claiming 500:1 on their latest, best film stock. But remember that this is an idea on the camera original; you'll never get a distribution print with that range. Video cameras haven't reached that far yet. A good prosumer camera like the Sony VX200 or the popular Canon XL1s might expose well through six stops, or 64:1. Pro camcorders like the Sony DSR-500 can consistently hit seven stops, or 128:1.

Barry Green and I were debating this point because in this: http://www.theasc.com/magazine/product.htm article by Jon Fauer, it is claimed that the DVX has 8 stops of contrast. I think this is wrong... not only is it one stop more than is even mentioned in the lighting book, but it's also only two stops less than human vision. The quote above from the AC video manual suggests that a video monitor can't even display more than 7 stops of contrast.

Here's another question that arises with this discussion... do regular photographers and film users measure contrast ratio the same as video people? Because in regualar photography we count the "usable" contrast ratio... it sounds like they count pure white and pure black in video. Here's a quote by Barry Green:

Michael tested it and found almost 8 stops of latitude. 16mm showed about 11 stops, and 35mm film showed 13 stops. The pictures are posted on his site at http://24p.com/ASC_Pictures.htm

and here's my response:

We've got a conflict of facts. One of two things are possible:

a) Michael doesn't know how to measure contrast range

or

b) Michael is using different standards than are used in still photography and every book I've read on the subject... which is still doing it wrong.

The pictures he has posted on his website are meaningless (for this discussion) unless someone was there to measure the actual values of the subject.

Human vision is said to be about 10 stops and no film on earth has topped that yet. B&W negative film is 7 stops, color negative film is 6 stops, color slide film is 5 stops, digital SLR's about about 5 stops. There isn't a film stock out there that claims to produce a ratio of more than 500:1, which is 9 stops.

Topic #3 -

I found this a little odd:

From the AC Manual:

[Because of diffraction] HDTV lenses should be operated at wide apertures and never beyond 5.6

The manual is very vague and seems to suggest that HD's resolution is the reason for the diffraction issue. If that's the case then why isn't it an issue in still photography past f5.6?

Topic #4 -

From the AC Manual:

HDTV cameras require lenses with much higher performance characteristics. The wide screen aspect ratio lowers the CCD's sensitivity by ten percent because the area of active pixels is smaller.

Hmm... I know that larger pixels are more sensitive to light because each pixel has a larger aperture (so to speak). I don't understand how not using part of the CCD would affect the pixels that you are using... do you? Also, because of the way the XL2 does this, it won't have the same affect will it (actually the opposite)?

 

[Barry_Green]

Re: ASC vid manual... let's figure some stuff out

I would not presume to correct the American Society of Cinematographers, so I'll throw what I can in to clear it up, but for real answers we need someone more knowledgeable to write in...

Is DV 480i SDTV?

Yes, DV is SDTV.  Recorded pixels and "active" pixels may not coincide.  The 720x480 pixel array is not what gets broadcast, remember.  What gets broadcast is an analog representation of that.  Furthermore some pixels are not "active" pixels.

Here are some resolutions from the American Cinematography video manual. It says that DV's resolution is 500 lines... I'm assuming it means TV lines, since there's only 480 vertical lines used in DV. It's kinda strange though because the XL1S claims 530 lines.

That chart seems to round things off.  They list the Panasonic HD camera as having "700 lines", when the resolution is 720.  And they list DVCPROHD as having 500 lines, which seems incorrect.

I would suspect that they just rounded numbers off, but I don't know why they printed what they did.

It sounds like he's saying that a high end CCD can capture 9.2 stops of contrast

That's not me talking, that was me paraphrasing Michael Phillips' test.  For the answers, you'll need to go to the source, fellow DVXUser.com member MichaelP.

The manual is very vague and seems to suggest that HD's resolution is the reason for the diffraction issue. If that's the case then why isn't it an issue in still photography past f5.6?

Diffraction is not my keenest subject, but from my understanding it relates to the size of the aperture dictating the spread of the beams of light, and how the size of that light corresponds to the size of the pixels on the CCD.  With 2 million pixels crammed onto a 2/3" CCD, HD's pixels are obviously much smaller than a DSR570's, which fits 500,000 pixels in the same physical area.  So diffraction would occur at a wider f-stop on the high-def sensor than it would on the standard-def sensor.

 

[skippyfetus]

Re: ASC vid manual... let's figure some stuff out

720x480 pixel array is not what gets broadcast, remember

You know I've finally got it! If you take 8 pixels from each side of the 720x480 image captured by DV, you get the 704 that the video manual states as the width of 480i video. Now, if you multiply 704 by .9091 you get 640, which makes a perfect 4:3 aspect ratio.

That chart seems to round things off. They list the Panasonic HD camera as having "700 lines", when the resolution is 720. And they list DVCPROHD as having 500 lines, which seems incorrect.

Ok, if they're rounding things off, they could be talking about scan lines or tv lines, which is it? Vertical resolution?

That's not me talking, that was me paraphrasing Michael Phillips' test. For the answers, you'll need to go to the source, fellow DVXUser.com member MichaelP

MichaelP, you've got some splainin' to do!

 

[skippyfetus]

Re: ASC vid manual... let's figure some stuff out

Man, I thought we had some real video geeks around here, but out of all of us we can't answer a few simple questions. I guess that's the difference between photo and video. In photography, you can find the answers to any question rather easily, but it seems like the technical stuff in video is kept on the down low. Why is that?

Oh well, like always I'll try my luck over at DVinfo

 

[taubkin]

Re: ASC vid manual... let's figure some stuff out

Yes, they mean TV lines. It's not a roundoff. TV lines is a different measurement than pixel count. I guess you know what TV lines is because I'm almost sure it was you who wrote about it, or maybe Slapdragon. TV lines is the maximum DIAGONAL lines the image could display (like a grid, black/white/black, etc...). I guess Canon's got more lines because of it's aspect ratio.

 

[skippyfetus]

Re: ASC vid manual... let's figure some stuff out

No buddy, TV lines is the horizontal resolution of the largest circle that will fit on a TV screen. It's not diagonal and the width doesn't matter. It's not a count of the pixels, but how many black and white lines can be sharply rendered horizontally accross the screen. The maximum is 540 because you cannot have more TV lines than pixels in that area. Very confusing, I know.

 

[taubkin]

Re: ASC vid manual... let's figure some stuff out

Got it. I just got confused over a bad memory I guess.

It is confusing, but I tought you had cleared it up a few weeks back...

 

[Barry_Green]

Re: ASC vid manual... let's figure some stuff out

[quote author=Tony_H link=board=news;num=1091492906;start=0#5 date=08/03/04 at 16:54:32]but how many black and white lines can be sharply rendered horizontally accross the screen. The maximum is 540 because you cannot have more TV lines than pixels in that area.[/quote]
Clarification (which you already said, with the circle analogy, but...) it's how many b & w lines that can be rendered horizontally, across an area that's equal to the height of the image. So it's not the # of lines horizontal across the entire screen, but the number that can fit in a square area the same width as the height of the screen. So for DV, that means you can only measure 3/4 of the screen (because 3/4 of 4/3 is 1:1) and so 3/4 of 720 pixels is 540, so the maximum possible resolution in "TV Lines" for a DV system is 540.