Color curves instead of Broadcast colors

RichMacDonald wrote on 2/1/2004, 9:31 AM
As useful as the Broadcast colors filter is for finalizing legal video, it does have the unfortunate effect of crushing the lights and darks to a single max/min value. That is, any color outside the legal range is simply "truncated" to the end point; there is no gradual limiting. Roughly corresponding to the limiter vs. compressor in the audio world.

In doing some experimentation, I find that I can produce a better looking result if I tweek the video with the Color Curves filter instead. IOW, with the color curves I can simply adjust the end points to eliminate the extreme lights and darks. This (seems to?) prevent the illegal colors, and it allows detail to remain in the lights and darks that would have been removed with the Broadcast filter.

1) Caveat: I'm no expert in the various color spectrum issues, but from what I've read I think the legal broadcast color issue is more complicated than simply preventing the extreme "end-points". Naively, from the viewpoint of the color spectrum, the end-point truncation is like applying a triangular filter (or cutting off the end of the triangle), but the legal broadcast color spectrum is more like a truncated triangle with curved sides. Am I right? If so, does the Broadcast filter handle this? If so, then I should use a Color Curve then add a Broadcast filter downstream as extra security.

2) One can think of a some useful Color Curve presets. (a) Keep the curve linear and just adjust the endpoints - move the dark (lower-left) point up, and move the light (upper-right) point down. But this would also reduce the contrast of the entire picture. (b) Keep the same dark and light points as (a), but use an S-curve instead. This will maintain the contrast for most of the colors and only "compress" the contrast at the end-points. (c) Keep the same dark and light points as (a), and "step-change/ramp" the curve, adding 2 additional points on the 45 degree diagonal horizontally aligned with the end points. This is identical to the Broadcast colors filter, with the caveat in (1).

I'm mostly interested in the preset (b), since it gives the most pleasing results in general, although its usually only the starting point since I can never resist an extra tweak or two. Regardless, I need to figure out how to accurately set the end-points. Unlike Photoshop, the Vegas Color Curves filter doesn't show the digital values of the colors as I move my mouse around. I could eyeball the vectorscope/waveform while I adjust the curve, but I'm too much of a dope to really understand broadcast video, so I'll probably screw up :-) (I don't even understand all the presets on the Boadcast colors filter.)

It occurs to me that this Color Curves filter(s) preset would be useful to all Vegas users, either added to the standard Vegas package or available for download from some 3rd party site. Has anyone done this...who knows what they're doing:-? If not, could anyone recommend a link to educates me how to do it properly and is willing to proof my results?

Comments

taliesin wrote on 2/1/2004, 9:42 AM
I also find color curves useful handled that way. But did you ever try the smoothness function of the Broadcast Color filter? This makes gradual limiting.

Marco
craftech wrote on 2/1/2004, 9:53 AM
None of the features such as presets and/or scopes are a substitute for decisions based upon what you see on a well calibrated external monitor.
Too often the best settings visually are not within "legal" limits. A Sony tech recently observed the same thing in a post here.
John
RichMacDonald wrote on 2/1/2004, 10:01 AM
>did you ever try the smoothness function

Doh. No, LOL. Given that the presets all use smoothness=0, I never bothered to read the instructions :-)
RichMacDonald wrote on 2/1/2004, 10:07 AM
>None of the features such as presets and/or scopes are a substitute for decisions based upon what you see on a well calibrated external monitor.
>Too often the best settings visually are not within "legal" limits.

Agreed. I know how to make a good looking picture (no calibrated external monitor yet, though), but making it legal using the standard "legal-for-dummies-button" negates much of my hard work. Now I want to keep it legal but expand my options for nudging up against that boundary.
MUTTLEY wrote on 2/1/2004, 11:00 AM
To sound nieve, what are the possible consequences of not useing " legal " colors ? I have yet to use this option but do all my tweaking, as others, by eye on an external monitor. Thusfar everything I've done that has been on TV has looked just fine, but I have wondered.

Would this be similar to the old school philosopy that all your web images should be 256 color ? That pretty much what I imagined it was.

- Ray

www.undergroundplanet.com

RichMacDonald wrote on 2/1/2004, 11:35 AM
>what are the possible consequences of not useing " legal " colors

Non-technical explanation ahead. There are links to explain it but not handy for me, right now, I'm afraid.

Ever been watching late night TV, a commercial comes on with some white text or a big white background, and your sound suddenly starts buzzing? Perhaps not with new TVs, but definitely with the old ones.

Think of your TV as an analog "spring" trying to follow the oscillations of the input. As a "spring", it doesn't turn on a dime so its always a bit behind, especially overshooting the peaks of the oscillations. So when your input goes from 0 to 100, your TV might actually go from -10 to 110. Anything between 0 and 100 is "bad", e.g., could overflow into the audio channel, causing that buzzing, among other things.

The solution was to clamp the legal input values. So you may only receive input between 10 and 90, and your old TV now only overshoots between 2 and 98. Inaccurate, but legal.

Web images in 256 color was because it was the lowest common denominator. For someone using a 256 color video card, your 257 color page would look like cr*p. And actually it was less than 256 (230 something if memory serves) because there were a number of reserved/required web colors.
GaryKleiner wrote on 2/1/2004, 1:03 PM
>None of the features such as presets and/or scopes are a substitute for decisions based upon what you see on a well calibrated external monitor.
Too often the best settings visually are not within "legal" limits. <

craftech,

Your second sentance is contrary to your first.

When you say "the best settings visually are not [legal], the next sentance should be :"Therefore you MUST use a waveform monitor."

I don't care if you like how it looks. If it is going out beyond your personal use, video must be set to be legal. I usualy push the pedestal to close to zero, but I use my waveform monitor so I make an informed decision.

I have used waveform monitors for many years and I "know" when what I see on my monitor is pushing or past legal limits. Most people on this forum do not have the benefit of my experience, and hence my words of caution.

Gary
craftech wrote on 2/1/2004, 6:47 PM
"I don't care if you like how it looks. If it is going out beyond your personal use, video must be set to be legal. I usualy push the pedestal to close to zero, but I use my waveform monitor so I make an informed decision.
I have used waveform monitors for many years and I "know" when what I see on my monitor is pushing or past legal limits. Most people on this forum do not have the benefit of my experience, and hence my words of caution."
Gary
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A recent quote from SonyEPM:

"So, do you need to use the broadcast filter for DVDs?"
"I'd say NO. "Make it look good" and forget about clamping for DVD playback. l recently ran some really good looking DVDs* through my waveform monitor, and the Vegas-equivalent composite levels were coming in as high as 115 (see Vegas scopes, composite view). I asked some of the transfer guys at the Sony digital authoring center about this very issue, and as soon as I hear back,I'll post that info here- should be interesting to hear how they do it."

* DVDs were: Stuart Little, Bad Boys II, Daddy Day Care.
SonyEPM


Guess he too doesn't have the benefit of your experience Gary.

John
Jay Gladwell wrote on 2/2/2004, 9:41 AM
John, far be it from me to speak for another person, but I think Gary was referring to video that was created for broadcast. If that was indeed his intention, then both of you were right!

J--
Former user wrote on 2/2/2004, 9:49 AM
Legal levels are really more of an issue when the program is to be used in Broadcast. A TV will accept pretty much any saturation or brightness with no ill effects, but when that signal is broadcast, it will tear, buzz in the audio if it plays at all. If you are only making DVD's for player/TV use, it is not as big an issue.

Dave T2
SonyEPM wrote on 2/2/2004, 9:50 AM
Just to be clear: If you are doing video for broadcast, yes you have to care about legal video levels. I have never said otherwise. If you have out-of-spec levels, and the station doesn't reject your master, they could "compensate" for one hot scene by reducing the overall video level and by doing that make the rest of you production look bad (too dark for instance). They might also just air the thing and some users will have buzzing TVs. Transmitters could freak out even- I've seen it happen (LONG ago however).

In terms of DVDs and only DVDs: You don't have to be nearly as anal about proper levels as you do with content for broadcast. I'd still scope the show prior to rendering MPEG-2, but if you are under 115 (composite) on our scopes you should be just fine.
GaryKleiner wrote on 2/2/2004, 10:39 AM
Sony EPM,

If a signal that is too hot will make TVs buzz when broadcast, why does it not have the same effect when the source is DVD?

Same question about black levels that can be so low that they interfere with sync.

Gary
SonyEPM wrote on 2/2/2004, 11:06 AM
Color levels that are hot to the point of audio buzz (overmodulated video signals leaking into the sound subcarrier) or blacks that interfere with sync (extreme superblacks) are going to be a far more rare occurence when the video is coming off of a DVD than they are in "straight into the TV from analog source" scenarios.

I'm sure there are some TVs that will freak out when playing DVDs with extremely hot levels, hence my advice to not exceed 115 composite. Digital blacks should not be a problem on DVD. You can't put analog style -20 superblack on a DVD for instance. ( if you can please tell me how, and with what).

I'm sure there are "exceptional" monitors that exhibit all sorts of bad behavior with even marginally illegal video...if you have one, tell us about it.

Chienworks wrote on 2/2/2004, 11:51 AM
The buzzing also has a lot to do with RF moducation too. I can produce a DVD that buzzes and the screen flips out when i play it through a modulator into my old TV that only has RF inputs. The same DVD played into a video monitor with A/V inputs plays fine.
craftech wrote on 2/2/2004, 1:01 PM
In terms of DVDs and only DVDs: You don't have to be nearly as anal about proper levels as you do with content for broadcast. I'd still scope the show prior to rendering MPEG-2, but if you are under 115 (composite) on our scopes you should be just fine.
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Sorry, my post was't intended to address video intended for broadcast. I know that Rich MacDonald mentioned broadcast in the original post and I should have made the distinction when I started my first response above.

In my experience it doesn't seem to be as big an issue with VHS tape production either. I drop the black level down to below legal limits sometimes and it really helps eliminate the video noise in dark stage lit video intended for VHS. It also enables me to saturate the colors more without bleed, particularly with reds. I test my videos on between 4 and 7 different TV's using a test tape with several corrections adjustments on it before finalizing my decisions and rendering, and never have any complaints from customers who are often used to buying washed out looking VHS videos adjusted to legal limits. I have never done work for broadcast as I am an event videographer specializing in stage productions. Sorry for the confusion.

John
farss wrote on 2/2/2004, 1:48 PM
Here's an interesting anecdote. One of the local TV stations discoverd a way to make all their pictures look 'better' than everyone else's. Drop the blanking level from 0.300 to 0.290 Volts.
They got away with it for a while, I guess something about having their licence revoked got their attention.
scotty_dvc80 wrote on 2/2/2004, 3:23 PM
hey Rich you never did follow up with any contact info : )
Chienworks wrote on 2/2/2004, 4:03 PM
Here's an amazing broadcast example: FOX 33 just broadcast a "Winter Storm Watch" warning for my area. It looked like the image was pure white text on black background, but what came out on the screen was more static than picture, looking like about 5 other stations were bleeding over it. The audio sounded like a 5Mbps MP3 file. And ... the real kick was that it knocked my cable modem offline for the duration of the warning. Now that was nasty.

I wish i had been thinking a little faster. I would have captured a few seconds of it to post.
RichMacDonald wrote on 2/3/2004, 7:43 AM
hey Rich you never did follow up with any contact info : )

After taliesin made me realize I'm a dope, I dropped the request. Simply add some smoothness to the Broadcast filter and the crushing problem can be controlled. Seems like a safer option compared to using color curves.

Its rich@clevercaboose.com, fwiw :-)