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Grading with Computer Monitor Instead of Broadcast Monitor


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#1 Peter Moretti

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Posted 06 September 2008 - 06:57 AM

I've read that a lot people recommend grading with a broacast monitor. But it seems to me that a properly calibrated high quality computer monitor might be at least as good (if not better).

I believe by using a computer monitor--instead of a broadcast monitor with HD-SDI--one cycle of RGB to YUY/4:2:2 to RGB conversion is eliminated.

Here's a diagram of the path of the video stream assuming Avid. (I know Avid is hardly recommended for serious grading and CC mostly because of its long render times and somewhat crippled CC tools, but I think the comarison holds no matter what software is used.):

Computer Monitor

DNxHD/YUV/4:2:2 >> Computer Video Card/RGB >> DVI Out/RGB/Uncompressed >> Computer Monitor/RGB

Notice only one YUV to RGB conversion.


Broadcast Monitor w/ HD-SDI

DNxHD/YUV/4:2:2 >> Computer Video Card/RGB >> HD-SDI Out/YUV/4:2:2 >> Broadcast Monitor/RGB

Notice a two extra conversions; one RGB to YUV (to meet HD-SDI's specs) and one YUV back to RGB (by the broadcast monitor to convert the computer's RGB color space to YUV/4:2:2 to comply with HD-SDI's specs
display the image).


Does this makes sense? I'm still learning about all this, so I could very well be missing something.

Thanks MUCH.

Edited by Peter Moretti, 06 September 2008 - 06:58 AM.

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#2 Phil Rhodes

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Posted 06 September 2008 - 07:20 AM

You can do this; I know of at least one HDCAM-originated feature which was graded on a normal computer monitor and with knowledge of what you're getting into it can be done.

Mike Most will, if he notices this, probably come by and tell you it's absolutely unthinkable and will cause the sky to fall in, but it won't, I promise.

The penalty of course is accuracy. Computer monitors are ideally supposed to be sRGB devices, which happily have exactly the same RGB primaries as rec.709 monitors, if that's what you wanted to get back to. The gamma is somewhat different and you would want to do something about that. In the example I mentioned above, there was a brightness offset which was happily caught during a preview screening, and was easily tweaked out.

Ideally you would want to get it side by side and eyeball match the display to one of known quality; it might be worth renting a proper broadcast one for a day to match it.

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#3 Michael Most

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Posted 06 September 2008 - 09:36 AM

Mike Most will, if he notices this, probably come by and tell you it's absolutely unthinkable and will cause the sky to fall in, but it won't, I promise.


I don't think the sky will fall in, and I don't think it's unthinkable. It's just a really bad idea.

The problem isn't specific color accuracy. The problem is blacks, gamma, and overall contrast range. Computer monitors don't have true blacks, don't have the same gamma as television monitors, and have different resultant contrast. You can use profiling, but it only gets you so far. Of course, if you're not working on a professional project that must pass QC for the specific format of the deliverables, then knock yourself out and use whatever you have.
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#4 Stuart Brereton

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Posted 06 September 2008 - 10:24 AM

My Dell monitor has been calibrated using a Datacolor Spyder probe, and now has profiles for PAL, NTSC and Rec 709 along with the normal computer profiles. It's as close as it's ever going to get to proper broadcast settings. That said, I recently saw one of my projects on a Sony Grade 1 21" CRT, and the difference is immediately obvious. Profiling will get you part of the way, but there is no substitute for having a proper Broadcast monitor.
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#5 Blase Theodore

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Posted 06 September 2008 - 12:16 PM

I know someone who seems to be doing this successfully, whereas I did not succeed. I cannot tell you definitively either way, but here was my experience..

My setup consisted of using a sony CRT fed through a DVI port connected to my X1900 graphics card. The resulting signal was tuned by an ISF engineer to compensate for any gamma/color shift from the signal path and then re-profiled with an EyeOne Pro. In theory, everything was ideal, direct RGB with no conversions, and the LUT ensured perfect 709.

However when I compared it to broadcast monitors, there was a clear difference. I believe there's a scaling issue in pc levels to video levels which causes colors to clip at 235 instead of 255. (Additionally, there's a gamma shift being introduced by the graphics card itself.) The result was that I had crippled the available gamut of the display, but couldn't see it unless I went outside of "broadcast safe" space. (Hence all my bars/test patterns looked good). So I would grade things, and then they would go over-saturated and I'd never know.

For me the issue is not the CRT, its the data path to the CRT. And a graphics card pipeline, no matter how good the card, didn't work for me.
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#6 wolfgang haak

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Posted 06 September 2008 - 12:40 PM

Hi,

My background is more still imagery, with the workflow target towards four colour print process, so I may be stepping on a few feet here, but...

(read my blurb here...)
http://www.cinematog...showtopic=33250

The assumption that computer monitors don't have black is based on the knowledge that nearly all monitors are shipped out with too high brightness/background light level (200-350cd/m2) . This encourages burn out of the colourants, and increases sales when your £2000 proofing monitor starts showing colour fringng after a couple of years... (sarcasm off)

Anyway a properly (hardware) calibrated and software profiled device will have proper black. (assuming that 0 cd/m2 is black, tolerance 0.1 cd/m2)
Modern proofing monitors don't implement a gamma correction but rather use L* for the tonal response. (quato monitors do)
The gamut of the monitors can not only be described by their primaries, the ranges look like strange beasts. See my current profile attached. (white is FOGRA isocoated, White my current display)
Posted Image

The L* TRC can be adjusted to your liking values between 1.6 and 2.4 are common.
The advantage of using a monitor like this is that that the deviation between hardware calibration and ICC profile towards an output target is not done by the graphic card (at a loss of display gamut) but rather in the monitor. The monitor is connected via USB cable to the computer, where the calibration software feeds the monitor the ICC profile. The graphic card remains unaware of this and continues to send a full 255, 255, 255 range of signals. The conversion towards the target profile is then executed by a chip inside the monitor. (in 12 bit/channel)

On a system like this I would assume that you can safly carry out any grading you like, as (from experience) you can expect your colour rendering to fall within ΔE0.5 (ΔE1 being the limit where the "Standard Observer" will start to differentiate colours, and ΔE4 being the accepted deviation in print processes)

Hope this helps,

Wolfgang
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#7 Phil Rhodes

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Posted 06 September 2008 - 03:39 PM

As Mike quite correctly points out, the problem will be that you will have an inbuilt tendency to crush blacks too much if you grade on an LCD. This can be worked around to an extent by a) keeping this in mind and B) referring to a waveform display of some kind.

Most DLP projectors, especially 3-chip types (don't ask me why the contrast is better on 3-chip models but it is) will be closer than an LCD and may be quite affordable.

And Mike, the HDCAM feature to which I refer was professionally graded in that the people involved in the grade were paid for their efforts and make the majority of their incomes out of such work. I'm sure they'd appreciate your opinion...

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#8 Peter Moretti

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Posted 08 September 2008 - 04:04 AM

So if I understand correctly, the gamma correction is performed by the computer/video card/(possibly monitor) but not by the NLE.

So when I choose the Rec 709 color space in Avid, Avid uses the color range values and luminence weigthing for 709 but not the proper gamma correction?
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#9 Paul Bruening

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Posted 08 September 2008 - 12:52 PM

Hello Phil and Wolfgang,

Fellas, what's your opinion on using this monitor (ViewSonic P220f) for grading:

http://www.viewsonic...es/p220f/#specs

I've got the P220f with separate R, G, B, H/H+V, V BNC inputs. I also have the graphics version, P220fb.

If the P220F is actually usable, what are some good cards to drive it and what should I do to calibrate it for both video and film space?

Any help is appreciated.
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#10 Phil Rhodes

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Posted 08 September 2008 - 01:13 PM

Well it'sa CRT so it has at least the potential to have decent blacks.

At the end of the day everything - including professional monitors - invariably need calibrating using something like Truelight and then eyeball tweaking to achieve literal "it will look exactly like this" results. The difference is whether it's actually possible to achieve those results on a given piece of kit. I suspect something like that CRT would be within a workable range; I have seen Truelight barf on consumer TFTs as black isn't black enough.

Of course most of the broadcast monitors used for this sort of work are not calibrated in this manner which makes it a bit of a crapshoot anyway.

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#11 Hal Smith

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Posted 08 September 2008 - 09:18 PM

The post pros who do spend their working life in dark rooms are loving the 50" high end Pioneer Plasmas. I know of at least one who has found they're suitable replacements for the discontinued and fast disappearing Sony BVM series (I got lucky and scored a good 19" BVM off ebay for my KEM telecine for $250 + shipping). The Pioneer pro version TH-50PF10UK is about $2200 at places like B&H.

I've seen one running some Pioneer demo BluRay content at an Ultimate Electronics store and can honestly say it has absolutely the best picture I've ever seen. It's the first new generation display that is tempting me to retire my 55" Mitsubishi Platinum CRT HD projection set.
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#12 Peter Moretti

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Posted 08 September 2008 - 09:43 PM

Wolgang,

The problem I believe isn't so much that modern high-end LCD's like an Eizo CG or a Quato can't display the proper color, black and grey scale levels, it's unclear how to get them to do that.

We say proper calibration, but what does that mean? I send RGB signals through the profiling software and its read by the probe. But aren't those RGB colors sRGB colorspace? That is different from Rec 709, which is used for HD.

http://www.13thmonke...ammacorrection/

So when you calibrate a monitor, how do you send Rec 709 color values to it instead of sRGB? I have Datavision's Spyder 2, and there is no Rec 709 option, AFAICT.


Phil,

Actually, this question goes for broadcast monitors as well. How do you properly calibrate a broadcast monitor if the calibration software is sending sRGB values to it?


Thanks very much for all the help guys.
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#13 Peter Moretti

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Posted 08 September 2008 - 11:27 PM

Well, actually, my Spyder 2Pro does have a setting for Rec 709 calibration. How accurate it is, IDK. I also don't know how accurate Avid's composer window is. Does it add anything to the color that the ColorVision software does not account for?

But it looks like it is possible to use a calibration probe for Rec 709. Again, how accurate the calibration will be is still a VERY open ?.
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#14 wolfgang haak

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Posted 09 September 2008 - 04:01 AM

Peter,

Actually, this question goes for broadcast monitors as well. How do you properly calibrate a broadcast monitor if the calibration software is sending sRGB values to it?


Good question. I put my faith in the quato's as their modified Betac McGreth DT94 Colorimeter is externally certified byUGRA.
Their calibration software has presents for HDTV (HDTV (ITU-R BT.1361 / BT.709) or DCI (Gamma 2.6, 6.300K) compliant calibration).
Actually I've just spotted a dedicated Broadcast Monitor on their site: IP260 Braodcast Monitor for those interested.

To be pedantic you're right Peter I haven't verified any one companies claims about their product myself. But needless to say, no one really does.

regards,
Wolfgang
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#15 Keith Mottram

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Posted 09 September 2008 - 04:10 AM

If you are going down this route then the monitor to get is the HP dreamworks monitor, the colour accuracy is stunning and not to far from the likes of a cinetal (I actually prefer it as am not fond of the noise cinetal generates in the blacks). it is cheap as chips for such a good monitor. if you are going diy the trick is to hold as much info as possible in your final grade as you will need to tweak for film output and this cannot be done in your bedroom.

Keith
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#16 wolfgang haak

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Posted 09 September 2008 - 04:31 AM

Peter,

a quick one re: what colour space. PC monitor calibration is based on ICC profiles V4. The Probe /software covert targets via a profile connection space (PCS) which is CIE L*A* B* (1976).
The calibration works in two parts: 1) send and measure greyscale and RGB components signals to the graphic card, and use the measurements to hardware adjust the TFTs colorants to the desired primaries/whitepoint of your calibration target.
2)measure and create a ICC profile for the output device for ICC aware applications to use. Here is the trick: The ICC Profile is actually empty (well it's linear and does not show any deviation). This causes ICC aware applications to correctly send the full range of RGB signals to the graphic card. The Calibration software "uploads" the ICC Matrix to the screen via USB, where the color nessesary transformation according to the ICC profile takes place.

This is better then software calibration, where ICC profile causes the application to send less then full 8bit data to the screen in order to achieve whitepoint/target primaries. As screen with bright red phosphors will cause the ICC profile to set the maximum RGB values for that screen to i.e. 225, 232, 253, resulting in possible banding on attempting to replicate large gamut colorspaces, as the remaing bits have to "strech" across 8 bit of colour data.

Anyway, jsut a bit of geeky info. :rolleyes:
Wolfgang
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#17 Daniel Porto

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Posted 09 September 2008 - 10:02 PM

If I just connect my computer to a normal television and then do my color correction would this be ok in terms of gamma?

Do a projector and a normal television have the same gamma?

If I am releasing a copy of my short film to both the internet and television, does that mean I need to do 2 different color grades because of the change in gamma?




Sorry guys I am new to this but thanks for all your help!
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#18 Peter Moretti

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Posted 10 September 2008 - 02:12 AM

If I just connect my computer to a normal television and then do my color correction would this be ok in terms of gamma?

Do a projector and a normal television have the same gamma?

If I am releasing a copy of my short film to both the internet and television, does that mean I need to do 2 different color grades because of the change in gamma?




Sorry guys I am new to this but thanks for all your help!

Daniel,

If there is one thing I've learned, it's that it all totally depends on many factors. Even though HD TV's should decode with the Rec 709 gamma, there is no way to tell for sure w/o using a monitor probe. That's b/c what looks good and sells more TV's versus what is accurate can be at odds with each other.

The internet uses sRGB, so grading for that should be much easier, b/c it's using the same color space as your computer. You should still calibrate your monitor and try to set the black point as low as you can get away with.

There are just too many links in the chain to get a definitive answer. I bet the best answer is to go with something like an eCinema monitor which was made with broadcast in mind. (The Quato broadcast monitor is not available in the US.)

But short of that, we are all just trying to save a few grand here and there until the picture becomes clearer, LOL.
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#19 Peter Moretti

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Posted 10 September 2008 - 02:57 AM

Wolfgang, et al,

How important do you think is having HD native resolution? Eizo's CG222W has just about everything the CG241W has, except it uses a 1680 X 1050 resolution. Should that be a deal killer?

I'm tempted to drop $1.4K on one of these and use it until things sort themselves out more. Realistically, I'm not going to be doing my final grade for quite a few months. It'll be a big step up from what I'm using now.
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#20 wolfgang haak

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Posted 10 September 2008 - 05:31 AM

Peter,

you've probably noticed that forcing a TFT to run at anything then native res makes your eyes hurt. Expect a jaggered image, and tired eyes, and a visit to you optometrician of trust.

But on a serious note, bear this in mind, while calibration software uses "solid patches" on the screen for measuring, it will work. The interpolated pixel sizes of a screen running out of native res however will mean that fine detail will get lost and color perception can change!

By all means do get a screen that fits in your budget, (I would) but I would make sure it has a res higher than HD, and watch your footage "letterboxed". There is a case to be had to strive for technical perfection with a system setup, but IMHO in every project there comes a point where artistic skill of those involved outweighs the merits of academic number-crunching.

You can grade your film till the cows come home, you know the majority of people who watch it will do so on a random combination of a) on a burnt out screen, B) wearing coloured specs, c) with glare and reflen on the screen d) before morning coffee with hangover in bad mood. Oh stop it Wolfgang you grumpy old man! not all clients are from hell...

Wolfgang
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