BMPC 4K low light for HD recording *hack*

Hi there

My applications commonly need low light performance.
If you want to understand why, please check our website : http://kimchiandchips.com

We have a BlackMagic Production 4K Camera. We bought it for a number of purposes (including scanning/calibrating).

I've noticed that its low light performance doesn't match a DSLR (neither does a RED for that matter). But for our purposes it's really important to record with a global shutter and we'd like to use our BMPC in low light situations.

(low light + global shutter + 4k is basically impossible I know)

Generally our delivery format is 1080p, and i'm wondering if there's a way to squeeze better low light performance out of the camera by adding together the pixels of each 2x2 pixel block when down-sampling to HD.

In theory, down-sampling should therefore give us 2 stops of extra low light performance. I'm curious if selecting the HD mode in camera (downsample before recording) has the same result. And if anybody has any experience with this?

In theory, if we record in 4k, and then downsample to HD afterwards, then the downsampled video will have less 1/4 as much noise per pixel. Meaning that if we add 4x gain in post (e.g. DaVinci Resolve), the post-amplified image should be the same as just adding together the pixel values in the camera

Pixels A, B, C, D in a 2x2 block:
Downsample then amplify x4 : (A + B + C + D) / 4 * 4
Simply add values : A + B + C + D
(these two are equivalent)

Personally, I think it would be great just to have higher ASA options (1600, 3200) in HD mode. This could be added with a firmware update and would kick up a lot of positive interest for the camera. But it may turn out that the same amount of noise at HD is unacceptable, whilst at 4k, it's too small to notice.

(PS : I don't have the camera with me now, it may turn out that there's already high ASA options for HD recording already. Which would be great!)

Elliot

Elliot thats a very interesting idea! As of now the BMPC4K has max. ISO 800 in all recording modes. What "might" work is also the same approach that Magic Lantern did with it's Dual ISO to gain massive dynamic range at the cost of half the resolution.

http://www.magiclantern.fm/forum/?topic=7139.0

Downsampling always improves S/N, so that is a good starting point.
If you want to improve the low light performance of the sensor you would have to modify the cooling system of the camera.

Interesting point Robert about heat being the source of noise on the sensor.
I've known this from physics experiments, but forgot to apply that same logic here.
It's easy to overheat by blocking the bottom vents so will be aware of that


It seems the ideal downsample for low light performance would be:
1. Shoot in 4K raw
2. Amplify gain
3. Downsample
this ensures that you don't lose the data (e.g. in rounding errors) before you downsample.

So considering the other alternative of giving the camera some extra cooling...

Since the body of the camera is metal, it should be relatively easy to add some serious cooling without any internal hacking.

I'm thinking an assembly like:
* 2 * 40mm peltier heat exchanger
* Heatsink
* 2 * 1/4" screws hold the assembly flush with the top plate
* Thermocouple heat sensor
* Microcontroller (e.g. AVR)
* 64x32 LCD display
* Standard battery input (e.g. 12V)

Check this post for gains in low noise when cooling is applied:
http://www.jtwastronomy.com/products/ultimate.html

This author suggests -15C to -20C as optimum operating range for his CMOS sensor. There'd have to be some risk taken to find the optimum operating temperature (i'm thinking +5C might be a good starting point). Also need to consider condensation on the sensor at low temperatures

You could do this in post of course, having a 4K timeline and putting a few layers 1 pixel out, and composite them together. Then put that timeline into a 1080 timeline

Elliot someone tried already to cool it down using an external peltier and cooler but it didn't worked well. Because the internal electronics is adjusting the temperature to stay constant that to the set value.

Read the full thread (not only the first post):
http://forum.blackmagicdesign.com/viewtopic.php?f=2&t=24688

Look up the term 'binning' to do with astrophotography, it is used all the time to get better SNR/Low light performance and works a treat, however it is usually implemented in the camera firmware.

You could potentially capture in 4K and use software to bin afterwards, which should still bring the noise down a bit, alternatively if you had very smart software, you could increase the chroma resolution substantially, by sampling colour from the 4K bayer grid and overlaying that over the 1080 luma grid, similarly to the way the C100 works.