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    JHarpur
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Smartfade, raw DMX, zipper noise, and low-level LEDs

I'm running into an issue with a Smartfade controller and some LED fixtures in a new but very small theatre.

Here's the situation - The designer of one of the shows called for some very low levels (in just one colour, mind you) across several banks of identical LED fixtures - top washes, front washes, etcetera - Cue levels ranged from a high of 12 percent to lows of one or two, but always the same value for the varous areas being lit. When he called for 5-second fade to black, naturally with all the fixtures dimming simultaneously, the 'zipper noise' from these fixtures was horribly, horribly apparent. The fixtures are an inexpensive 3-channel RGB "PAR" LED-style fixture, and are by design permanently set at 8-bit fade resolution for each colour. As far as I can see this is one of the worst case scenarios for any LED-based rig.

The supplier involved is coming in to dispute the appropriateness of the gear he's supplied the space. As it is, we're replacing his console (I won't name it, because of ETC's request we not slam other manufacturers' products in the forums, but rest assured it deserves its own roasting) with an Element, and we have been using the loaned Smartfade for show control until it arrives.

My own feeling is that the problem is not with the console, and that the problems we're seeing in the shows is endemic to the LED fixtures' performance characteristics as a whole. What I'd like is confirmation that the console being used is not the cause of this problem, and that the Smartfade console is delivering a DMX signal at an acceptable resolution, ie. 255 steps in a single channel from 0 to Full. That would certainly jibe with what I've seen on those 5-second fades from 3 or 4 percent - as the jumps from one level of illumination to another as it fades simultaneously across all fixtures are more than 3 or 4 steps. I'd also guess that other fixtures, which have an intensity channel, would perform in a markedly better fashion - In this scenario, one could set the colour values at the low levels, have the intensity channel at full in the cue, and fade the intensity for a much smoother fade.

I'd appreciate any suggestions, hints, confirmations, or corrections available.

Parents

  • Noise? That has nothing to do with DMX! Most likely there are some small relays shutting power off to the driver. If so the manufacturer or testing might be able to tell you which level is their trigger point. If you know the absolute bottome you can see just how bright that is, and have all fades go to that level and then straight off. so all the clicks happen at once.

    DMX does have a transmission speed. It's possible that the board and fixtures disagree somehow, or are just slow enough to cause jumps during such an extreme fade. Again testing might find a workable setting. The supplier is probably your best bet to get some test equipment in and really look for solutions.

    I can confirm both "steppy" fading and low end cut off is an issue with many LEDs, both theatrical and architectural. Also some brands have good and poor dimming fixtures. Just as some 8-bit fixtures have internal smoothing and some don't. Generally you get what you pay for.

    I can not confirm that you will get better than 16 bit dimming with an intensity channel. Generally an intensity channel is a software feature and shouldn't give you better than the best mode the fixture has. But I think I want to do some testing on my own inventory to find out how it performs! Also ETC is bringing a big Selador road show to town this week. Fun with new toys!

  • in reply to RickR

    Hi,

    This is a common complaint and it is to do with the fadiing abilities of the LED lights, not the console.  You will find the same happens with any console if you run a slow fade between two settings at low levels just by driving values to the color channels of the lights.

    There is only one solution and that is to use lights with a seperate intensity channel.  That channel will allow you to run an intensity fade at full 8 bit resoolution even if the color channels are at lower levels.  Internally the better brands of LED lights actually have 12-16 bit resolution.

    However, if you are trying to fade between different colours at very low brightness then the intensity channel will not help and you will still see the steps.  Only lights with 16 bit color channels for each colour will be able to improve this.

    Alas, if you seek a really perfectly smooth fades at low levels I have to tell you that even 16 bits is not really quite enough.  Even with 16 bits at the very low end you can probably see the first 10 or so steps, if you look closely.

    I know this sounds a bit mad, but maybe the quick solution would be to fit some neutral density gel in front og the lights and run them brighter.  Of course, that is only if you only need them at the specially low level and not brighter at some other point in the show.

    You have come across one of the major limitations of low-cost LED lights.  Making step-free dimming is remarkably complex and needs serously fast processing because, unlike a tungsten light, there is no thermal inertia to smooth out the dimming.

    If you get the opportunity to try some Seladors I think you will find they offer greatly improved dimming, and now you know why there they posess a separate intensity channel

  • in reply to AdamBennette

    Thanks, those are precisely what I'd considered the salient issues - Nice to know I'm not losing my mind. (Well...)

    As for using ND, I think if we're actually stuck with these fixtures (as we haven't signed off on their installation yet) I'd rather stuff some nice thick diffusion in front of them to cover a larger area, as that is also an issue - And that will also knock the levels down.  Once we get a console capable of running more than 16 fixture types (Yay, Element!) we can stop grouping them as they are now, and delay or boost fade times by fixture to help hide the steps.

    [Mmm, Seladors. Nice.]

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  • in reply to AdamBennette

    Thanks, those are precisely what I'd considered the salient issues - Nice to know I'm not losing my mind. (Well...)

    As for using ND, I think if we're actually stuck with these fixtures (as we haven't signed off on their installation yet) I'd rather stuff some nice thick diffusion in front of them to cover a larger area, as that is also an issue - And that will also knock the levels down.  Once we get a console capable of running more than 16 fixture types (Yay, Element!) we can stop grouping them as they are now, and delay or boost fade times by fixture to help hide the steps.

    [Mmm, Seladors. Nice.]

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