Cool DVX Noise Reduction Technique

cardmaverick

cardmaverick

Well-known member
This is an excerpt from a post I made at the Reel Stream forum regarding how push your footages noise floor even lower. It's great for Andromeda, and I can imagine this will also benefit those recording to DV as this technique affects the sensors.

Enjoy!

White Balance and Color Temperature: The Last Noise Frontier... In Camera

Ok, I'm back at it again! For the past few weeks I've been carefully studying the affects of color temperature, white balance, and channel gain.

Lets first talk about the white balance and channel gain.

In the DVX100, whenever you white balance, you adjust the cameras blue and red channel gain in order to achieve balance. The problem with this is that you create channels that clip at different times, thus causing the ever familiar magenta highlights. You also increase the noise present in each channel. If you use the cameras preset white balance settings or balance manually, green always clips first, AND, it also CANNOT be adjusted EVER, no matter what. The ideal setup for the DVX100 noise wise, is to somehow balance in a way that makes all of the sensors equal in gain, thus making them clip at the same level, not necessarily at the same instant in time (that will vary depending on the color temperature your shooting under), just at the same level of exposure. In other words, if the green channel is fully exposed at a level of 100 IRE, then the other channels should also be fully exposed when they hit 100 IRE, not below it, and not above it. For most current Andromeda users, their white balance settings make their red and blue channels clip way too high up, which causes the odd colored highlights, usually magenta in color.

Color Temperature

Color temperature is basically a way of measuring the color of light, in degrees Kelvin.To be simple, indoor light is around 3200K and is very orange, outdoor light on a sunny day is about 5600K, and is very blue. Color temperature can be as high as 7000-8000K on a very cloudy day, especially in the shade. Because of the varying amounts of blue and red in light, these two colors need to be adjusted in an image to achieve a correct balance of color.

How to balance the Red, Green, and Blue Channels Gain

Ok, I'll say this up front, this is very costly in terms of lighting, some might argue impractical, but theoretically, if you have the lighting for it, this is the absolute best set up for the system noise wise.

What you'll need are about 5 magenta filters. Like this one:

http://www.ritzcamera.com/product/241663780msk.htm?bct=t13031003%3Bcidigital-cameras-and-accessories%3Bcifilters%3Bcifilters-round

You also need a white card, and some way of producing light that is about 12,200 Kelvin. I did this by taking a 3200K photo flood (500 watts), and placing 4 sheets of 1/2 CTB gel in front of it.

Strap on the filters and white balance on the white card under the very, very blue light. If you don't have that many magenta filters, just put a deep shade of magenta under the light and white balance on it. I tried this using a color sample called "Dragon Fruit" from Sherwin-William Paints and it also worked.

Make your balance. To check your success, point your camera at the the white card, free of any filters, and blow out all the channels, you can take the gel off the light if you need more light. All the channels should be clipping at the same time. If any channel clips before green, you've gone too far! Pull back on the magenta.

To shoot with this you'll need a good dose of light. You will also need varying densities of blue filter as well to correct for all types of common lights, even daylight will need to be corrected for with a blue filter, tungsten needs even more correction.

If you analyze footage shot threw this filter setup, you'll quickly notice that noise is almost totally equal in all three channels. The decrease in blue noise is very drastic.

Another interesting point is that when shooting with proper correction, you will NEVER have off color highlights. Theoretically, this setup increases dynamic range, but I haven't tested that yet.

As a side note, you're camera, free of filters will produce green looking video. This is a setup very similar to the Viper Filmstream.

More Practical Setups

Ok, so shooting threw like 6 or 8 filters isn't exactly friendly for those with a tight lighting budget. But don't worry, you can still benefit from this technique. You just won't reduce the noise quite as much, but I still find the results are worth it, especially in the red channel when shooting under 3200K light.

For 3200K Light:

Balance threw two magenta filters and shoot threw them.

For 5600K Light:

Balance threw two - three magenta filters and shoot threw them, how many depends on the time of day, cloud coverage, etc..

BE SURE TO CHECK YOUR CLIPPING! Take the filters off and check to see where everything clips, nothing in cinematography is perfect, the color temperature of your lights could be a little bit off, etc..

The best realistic setup is to shoot under the highest color temperature you can (12,200K seems to be the sweet spot) using two or three magenta filters. HMI's would be a good bet, if you can gel them with full CTB and be fine exposure wise, even better.

Another road is just shoot with your camera setup for perfect channel gain balance and then correct in post. I haven't done enough work with this yet to verify if its worth it. Given the amount of boosting you need to do digitally to the red and blue channels, I would say its probably not ideal, as the red and blue channels are usually underexposed without filtration because you lowered their gain so much. The filter basically evens all the channels out so that you can get a strong, equal exposure in all of them, thus none of the channels will need any post boosting. Boosting with light on the sensor is far better than boosting digitally, in my opinion.
 
This is another excerpt from the post, hopefully this will clear up any questions people might have about the technique being employed:

Balance threw them, take them off to check your clipping, put them back on, and shoot threw them. You take it off in order to make sure you're not making the blue or red channels clip before green. It won't work right if you don't shoot threw them. Either way you'll have lower gain in the red and blue channels, but without the filter, you will have the red and blue underexposed. The magenta basically blocks the green light, forcing it down to the level of the red and blue channels. This is needed because there is apparently more green light in real world images. Even though everything can be equal gain wise, the fact that there is more green in the scene means it will clip faster. So basically what your really doing is:

Lowering gain in the red and blue channels, then pulling the green channel back via the magenta filters to make the channels all even exposure wise, then you can get a strong exposure in all three channels. Without it, red and blue will be underexposed, while green is more fully exposed.

You can't really have an end all be all balance unless you are committed to putting additional CTB or CTO filters on top of your magenta filters. If you balance threw magenta under 3200K, you'll still need CTO to shoot under higher color temperatures. If you balance under 5600K you'll still need CTB filters to shoot under tungsten light.

Optical filtering is a major part of the process, without the filters, this technique isn't really gonna help you out, given that you'll be underexposing and then boosting underexposed data in post.

Make sense?

When I get the time, I'll upload some comparisons to show how much noise you can reduce in each channel.
 
I shot a crude, but effective test today to see if the magenta filter trick would help my dynamic range... it does!

I found that I gain more usable HIGHLIGHT dynamic range after balancing threw, and then shooting with a magenta filter on. I can even see the highlight range increase on the DV footage too.

I'm very excited to see how much range I can pull off using this trick when the 12 bit firmware upgrade comes :)
 
Interesting post Chris - I'd love to see some examples of this from your DVX (not just from the Andromeda, but also off of DV)

If you've got the time - upload some clips.
:thumbsup:
 
I'll see what I can do, I've been very busy this month shooting a pilot. I had to trash the original tests because I needed the space for the pilot real quick, but its not hard to reproduce, and I've kept my "uber balance" stored in the camera just for the heck of it. Its really neat to work with this trick.

Also, I've realized that since green tends to blow out first, I've taken to ISO rating based off of the green channel, but I'm also experimenting with a "triple rating". In other words, creating a 3200K balance threw the magenta, and then rating each channel's sensitivity. My current approach, for economical reasons, doesn't allow me to shoot with perfect balance, just heavily reduced red and blue.... so thats why I'm curious about channel ISO rating, and I've also noticed that some scenes do not blow the green channel out first due to color content, so thats another reason to mess with a triple rating too. I just shot a stand up for the pilot this past week, and when looking at the channels, red was approaching full saturation first.... it goes against the norm, but every now and then you have those types of situations.
 
Chris, very interesting. I am still trying to get my head around it, without having viewed any before and after images/samples. I hope I have no misconceptions seeded by my interpretation of the information provided.

I see we are going to need a lot more light for an indoor shoot. What I am trying to achieve is lower noise in shadows darker areas of images, about 40 IRE and less. Granted, the HVX 1/3" CCD and DVCProHD (even using 1080pA24) encoding may be contributing substantially to those noise/crawling problem areas. As an adapter user, the magenta filter(s) are going to make me think of using HMI, which is only a wish at the moment, unless I hire in that extra gear, which I will likely do for a prospective Feature I am working toward, but not currently for testing.

Do you think your process will help for shots like night scenes or dark and shadowy setups?

I am interested to learn if this process can be done as a white balance on a say a magenta balance card, resulting in a green cast image like the Viper, and then in post, adjust the white balance (like a LUT). I may misunderstood some of the above, but would this type of magenta white card balancing result in the red and blue channels gain being reduce, thus not peaking typically before the green?

Until Hydra is released, I want to find a way to reduce noise in low light footage, without crushing black, leaving nothing to work with in post. Otherwise I need a lot of light and then bring it down in post.

Thanks for the insight.
 
It is certainly a bit confusing. It took me a good couple of weeks to finally figure out what was actually going on. As for dark scenes, filter or no filter, it's a losing battle to be honest. You'd be better off just going without the filter and getting a stronger exposure I believe.

It works its best using optical filters, thats how you create a more even exposure between the channels, plus the obvious reduction of gain in the red and blue channels.

I haven't tested it enough going to tape yet. In fact, I think the DSP does more than we know, but either way, if I'm getting cleaner footage right off the chips, then I know the DSP is as well, so that can't possibly be a bad thing.

Even though I have a large amount of range, I never use it. The bottom end of Andromeda is rather dirty, better than compressed dirty, but still, very grainy, so I limit myself to about 7 stops or less to keep the image high quality without the need to reduce grain in post.

Hope that helps!
 
Pumping a scene with more light, proportionally, seems like the best way to overcome the noise in the shadow. Once Hydra is available, pumping light and recording up to 14-bit full RGB 4:4:4 will do just nicely, although 10-bit RGB 4:4:4 via record LUT will likely do just nicely for a Feature length production (data management/quality rational).

Looking forward to some A/B comparison posts on this magenta magic, in due course.
 
The key to avoid magenta-highlights is to white-balance both the environment lighting and camera exposure. The more white-light and white-balanced exposure, the less is the magenta-highlight.

Let me go through the whole process as I understand !

Assumptions:
1. I can't afford to buy or rent magenta filters (even a single one).
2. I can't go for CTB or CTO gels (as I don't have pro light gear).
3. I have bare DVX100B with a tripod.
4. I have lots of fluorescent lights to start with (Yes! no 500 watt tungsten).
5. I usually shoots indoor.

See, I'm that poor :D

Considering above mentioned statements my process would be:

1- Fluorescent lights are neutral lights. So, there is no need to place CTB or CTO gels to balance them (they may produce greenish effect so good quality fluorescent lights are needed)

2- I will place magenta/orange-transparent sheet according to the environmental color temperature in front of my camera to see things more white-balanced, do manual WB via the sheet will equalize the levels in all channels. Hopefully!

Any suggestions ...
 
That is essentially what I was thinking with my first post above.

I have a roll of Lee 1/4 minus green (No.249) that I used years ago to balance key fluorescent lights on a greensceen. I was thinking to either stack multiple squares (1/4 to 1/2 to 3/4, etc) and place in front of the HVX lens, then WB on a white board, or alternatively, layer the filter gel on the white board and WB. Thereafter, remove the magenta gel. End result should be a green hue in the camera image thereafter.

Reading a Viper PDF that was posted by a member in another of Chris' threads, reveals that the Viper uses a combo of filters also. The resulting Viper image has a green hue, which leads me to interpret that the filters are used only to WB. To overcome the green hue in the recorded image, they apply a LUT, through a colour matrix, describes the image as a potential correction in post or final output. When setting up the camera to stay within 100 IRE, they set the Viper monitor source to the green WB setup (no LUT) and make sure the Zebra is under say 100% or 98%, and not blow out the highlight. Expose to the recorded image, not the LUT.

That is how my brain is interpreting the info I have read, although I may be mistaken about how the Viper functions. For me, I want to shoot in low light situations, however this magenta technique, as Chris mentioned above, will probably not be of benefit to me. I need more light. HVX DVCProHD codec in low light does macroblock in the low IRE range. Hydra may overcome the macroblocking/compression artifacts issue, however, Hydra coupled with more light is probably the best outcome.

Aziz, light!
 
Christopher Barry said:
That is essentially what I was thinking with my first post above.

I have a roll of Lee 1/4 minus green (No.249) that I used years ago to balance key fluorescent lights on a greensceen. I was thinking to either stack multiple squares (1/4 to 1/2 to 3/4, etc) and place in front of the HVX lens, then WB on a white board, or alternatively, layer the filter gel on the white board and WB. Thereafter, remove the magenta gel. End result should be a green hue in the camera image thereafter.

Reading a Viper PDF that was posted by a member in another of Chris' threads, reveals that the Viper uses a combo of filters also. The resulting Viper image has a green hue, which leads me to interpret that the filters are used only to WB. To overcome the green hue in the recorded image, they apply a LUT, through a colour matrix, describes the image as a potential correction in post or final output. When setting up the camera to stay within 100 IRE, they set the Viper monitor source to the green WB setup (no LUT) and make sure the Zebra is under say 100% or 98%, and not blow out the highlight. Expose to the recorded image, not the LUT.

That is how my brain is interpreting the info I have read, although I may be mistaken about how the Viper functions. For me, I want to shoot in low light situations, however this magenta technique, as Chris mentioned above, will probably not be of benefit to me. I need more light. HVX DVCProHD codec in low light does macroblock in the low IRE range. Hydra may overcome the macroblocking/compression artifacts issue, however, Hydra coupled with more light is probably the best outcome.

Aziz, light!
Click to expand...

Hi Guys,

You MUST shoot threw the filters. Even with the Viper. The Viper Filmstream without filters looks green. You use the filters to correct for it. Your DVX does not come balanced this way to start, so you need to trick it first, thus the reason for balancing threw the filters. If you take them off, you'll see a very green image.

Make Sense? I'm thinking this merits a video tutorial........

Last but not least, you are messing with the sensors, then correcting for the balance via optical filters.
 
Makes sense Chris. Perhaps the Viper functions differently with it's internal WB reference, as opposed to the DVX.

WBing only to magenta, whilst we will see a green image result in camera, as you suggested and I agree, I was thinking that in post, we could then WB correct the green hue footage, like a LUT. The green hue image resulting from the magenta WB only makes me think that the Red and Blue (mixed makes Magenta) channels peaking in gain first, as per you tested, will peak later, or more closely together with the green channel. Not debating you, just thinking out loud. You have done the tests and this is very interesting to explore, that which you have done in the real world. :beer:

A video tutorial, A/B comparison and some RGB channel comparisons would be handy, coupled with seeing how the final image improvement compares. That is a big job in itself, so you would be going all out to show us this.

I might setup the HVX on the weekend WB under 3200k lights only and do a shot of the Macbeth ColourChecker card, together with a grayscale chip chart to observe colour channel differences and noise in the greyscale steps, and then apply my suggested Magenta gel WB technique discussed above, just for the sake of it, and see if any differences in the RGB channels can be observed using an RGB Parade scope, after CC white balance correction.

Any tips or suggestions or don't bother, Chris?

Edit: I might also shoot through the stacked magenta gel (I don't have the magenta filters), and test that as well.
 
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chris,
i want to ask you if after the WB with magenta filters and when i ceck a white card to rate the ISO based on highlight i have to do this without any filter??
whitout filter when i blow out the channels ,the green blow out first.

is this correct??:dankk2:
is 100 ire the point where i have to take blow out?? or less??

about the Iso of the camera based on highlight ,after i have found the f stop on my camera where the channel blow out i have to use this value(fstop) to calibrate my light meter??
 
Perhaps the answer to the light meter calibration part of your question can be found in this FreshDV article and video. What camera are you using? I think HVX, without an adapter, is rated at about 320 ISO, if I remember correctly.
 
Christopher Barry said:
Perhaps the answer to the light meter calibration part of your question can be found in this FreshDV article and video. What camera are you using? I think HVX, without an adapter, is rated at about 320 ISO, if I remember correctly.
Click to expand...

The FreshDV tutorial bases the rating off of 18% gray.

I don't really agree with that approach, as that shade of gray can be remapped drastically, your brightest highlight however, is a bit harder to change, and still have a normal looking image.
 
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so cardmaverick, by white balancing through the magenta (a lot of it) I will turn my image quite green, but that is okay because of the r g b channels' different level of clipping and noise levels, and green is most the noise-less of the channels. then in post i just color correct the image to bring the right color back in. is this correct, in essence? sorry if i'm being redundant.

thanks so much.
 
janusz said:
so cardmaverick, by white balancing through the magenta (a lot of it) I will turn my image quite green, but that is okay because of the r g b channels' different level of clipping and noise levels, and green is most the noise-less of the channels. then in post i just color correct the image to bring the right color back in. is this correct, in essence? sorry if i'm being redundant.

thanks so much.
Click to expand...

Do not shoot with the video looking green, simply continue shooting with the filter on. Optical filtering is the key here, it will not work without the filters remaining on the lens.
 
Well - this may sound stupid - but here is a logistical question for you -
How am I supposed to shoot with that many filters stacked on the camera?
Won't they vignette?
You linked earlier to a screw on filter - but there is no way I can screw 3 or 4 filters on the HVX without vignetting.
Even with a matte box, I'll probably have 3 filter stages at most - and I'd like to have space for a pola or something else when shooting outdoors.
 
Luis Caffesse said:
Well - this may sound stupid - but here is a logistical question for you -
How am I supposed to shoot with that many filters stacked on the camera?
Won't they vignette?
You linked earlier to a screw on filter - but there is no way I can screw 3 or 4 filters on the HVX without vignetting.
Even with a matte box, I'll probably have 3 filter stages at most - and I'd like to have space for a pola or something else when shooting outdoors.
Click to expand...

There are two approaches:

The first one I outlined was the "perfect" setup, but thats not very feasible...

The second one only uses two filters and is very feasible without any vignetting issues. If you want to put more on for daylight, I'd suggest some step up rings and using 85mm filters.

Also, you don't have to buy 30 ccM filters.... Thats what I use because thats what I have. You can search around and by single filters in thicker densities at places like B&H. I gave the ritz link to make it easier on people, but there are certainly other options out there, including the square drop in filters for matte boxes as you mentioned.

Hope that helps!
 
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