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View Full Version : How many levels of Dimming?



ebrady
03-21-2010, 10:34 AM
I am a newbie at DIY Christmas, but an experienced engineer. I have brainstormed a controller design and am working on the schematic (for the fun of it). However I need to make a decision regarding it's performance.

So here is the simple question, how many levels of dimming are truly needed before the human eye can no longer tell the difference between adjacent steps? I have tried 11 levels (0 - 100%) and while I can discern a very small difference between levels, I can not envision a need to have any more then that for most displays..

What are your experiences?

Ed

P. Short
03-21-2010, 10:41 AM
Do you get smooth fades with that number of levels? That is the main reason for wanting a larger number of dimming levels, especially at lower brightness levels.

ebrady
03-21-2010, 10:51 AM
Do you get smooth fades with that number of levels? That is the main reason for wanting a larger number of dimming levels, especially at lower brightness levels.

Well, I am a newbie... I have not yet build a large scale display... Playing around on the workbench, I have done a few fades manually with strings of 50 and 100 lights. The fades are smooth, however I wonder if this is a truly representative result for the number of levels needed?

ebrady
03-21-2010, 11:14 AM
I have done some additional research, and I can see that existing systems are all over the map on this... X10 uses 64 levels, some DMX systems use 256 levels, some 65536 levels, some custom systems use 4096. Either way it is becoming evident to me that 11 levels is more then likely going to be inadequate. I have a hard time believing that 4096 plus isn't complete overkill, however my experience is lacking in this area...

budude
03-21-2010, 11:34 AM
I think nearly everything here uses 256 levels and a few with 192. Assuming you use some type of sequencing software like Vixen, the fade is going to be chopped into intervals (like 50mS). Because of this, each interval will be set to XX% of 100 and the equivelent dimming level (e.g. 50% = 128 (w/256) or 2048 (w/4096)) - but the end result (light level) is going to look exactly the same. The point here is as long as you can cover a typical fade up/down with a unique level of dimming, it will look smooth. A 256 level dimmer could cover a full 12.8 second fade up/down (at 50mS) and that would normally be more than enough for most folks. It's a bit more complicated than this because some lights/LEDs don't dim linearly but that doesn't really change the point made.

P. Short
03-21-2010, 07:38 PM
Also, the human response to light is not linear, it's logarithmic as far as I know.

ebrady
03-21-2010, 07:52 PM
Also, the human response to light is not linear, it's logarithmic as far as I know.

That make sense, I guess brightness levels produced by a certain bulb would not necessarily have to be linear either. Hmmm, sounds like that could be an interesting new plugin to Vixen. It could allow the user to add a table or curve to allow it to compensate the PWM levels to match the characteristics of the connected bulb types. Doing this, the intensity value listed in Vixen would more accurately represent true intensity...:idea:

budude
03-21-2010, 08:06 PM
That make sense, I guess brightness levels produced by a certain bulb would not necessarily have to be linear either. Hmmm, sounds like that could be an interesting new plugin to Vixen. It could allow the user to add a table or curve to allow it to compensate the PWM levels to match the characteristics of the connected bulb types. Doing this, the intensity value listed in Vixen would more accurately represent true intensity...:idea:

Vixen 2.5 had a dimming curves feature so you could tailor the output to anything you wanted. It's not in Vixen 2.1 however and 2.5 has been sidelined for now...

aussiephil
03-21-2010, 09:20 PM
I have done some additional research, and I can see that existing systems are all over the map on this... X10 uses 64 levels, some DMX systems use 256 levels, some 65536 levels, some custom systems use 4096. Either way it is becoming evident to me that 11 levels is more then likely going to be inadequate. I have a hard time believing that 4096 plus isn't complete overkill, however my experience is lacking in this area...

The number of available levels gives you granular control for single colours or greater colour mixing control for RGB.
X10 aimed at home automation, the 64 levels are in themselves likely overkill as people set a desired intensity and leave it there, you don't normally fade your room lights up and down constantly.

DMX at 256 levels and for commercial products hardware based dimmer curves being the norm allow for quite linear fades.
Digital PWM at 256 levels can lead to a visible step in levels at low levels and the implementation of 8bit (256) to 12bit (4096) granularity and hardware mapped curves can smooth this out significantly.
This shows the need for 4096 levels and how they can be used.

16bit (65536) resolution and above becomes about colour accuracy in RGB systems.

In all cases above the smoothness of a fade from 0% to 100% will be dependant on both the bit depth (levels) and the number of time intervals.
25ms timing has 40 intervals per second
that is translated to
40 Level steps for a 1 second fade
20 level steps for a 1/2 second fade
10 level steps for a 1/4 second fade
or
6.4 seconds to use all 256 levels.

Personally i have a lot of fades in my show last year that were 4-10 seconds long and even more that had now more than 6 steps from 0-100.

I would aim to implement a minimum of 256 levels though even 10 levels could be enough if every fade is a fast fade.

cheers

Entropy
03-22-2010, 01:56 PM
Also, the human response to light is not linear, it's logarithmic as far as I know.

Yup. This is why most display systems (PC monitors, TVs, etc) use a gamma curve. 256 bits/color used by PCs is NOT linear.

It's also why my I2C RGB project had an internal LUT to expand 256 levels of input to 12 bits/color - I applied a gamma curve to the inputs to make better use of the 256 levels. (Anything more than 256 and you have interoperability problems with many controllers - keep it to 256 levels and you can use DMX-512.)

11 levels seems to be WAY too little. It may seem enough when you are slowly stepping in between levels, but in addition to being logarithmic, I believe the human eye tends to be MUCH more sensitive to changes in brightness than constant levels, which is one reason for needing plenty of levels for smooth fades.