Monday, April 2, 2012

Light Communications Idea

Once again, the entertainment industry inspired me to this one. In stage illumination, the most recent development is the use of multi-color LED spots and washers. Those devices contain either three or four differently colored groups of ultra-bright LEDs. Controlling of the spots or washlight is usually done a serial protocol called DMX, by so called DMX-controllers or DMX control software.

In stage lighting language one controls different settings of "fixtures" (the lighting devices) and stores this control settings in "scenes". The scenes than can be called either manually or automatically as a sequence called "chase". The frequency in which the scenes of a chase are being called usually can be set by a sliding fader.

So, what's the trick about all this and where is the link to amateur radio?
Very simple, in long range light communication or cloud scatter experiment, usually QRSS is used. Now the link should be obvious... the fixture(s) are, very obviously, the light source(s), while the DMX-controller serves a beacon keyer.

A simple series of unmodulated dots (A1A) can be programmed with the following 2 scenes:
  1. red on all fixtures to 100%
  2. red on all fixtures to 0%
The next step would be to program of a chase of scene 1 and scene 2.

Unmodulated signals may be hard to discriminate. However, with the strobe function, the entertainment industry offers a solution to this problem. The strobe will create sidebands in the known fashion.
So, for a modulated signal (A2A) the following scenes can be used:
  1. red on all fixtures to 100% with a fast strobe
  2. red on all fixtures to 0%
Again, the chase would simply repeat scenes 1 and 2.

In order to know what I am writing about, I actually bought some material at a local pro-audio store:
Reasons for the decision on just those devices:
The washlight can be controlled by either 3 or 7 DMX channels. 3 channel resemble the control of the red, green and blue LED groups. 7 channels include said RGB-controls and some more stuff, which can be found on the respective webpage (#4=hue, #5=strobe, #6=color cycles, #7=luminance).
The lighting controller employs 8 faders to control 16 channels before switching to another "fixture" (i.e. bank) is required. This 8 channels fader control comes handy to control 7 channels of the SlimPar 38 or (and that's another trick) 3 channels of 2 SlimPar washers. In the latter case, two devices are controlled by a single fixture channel.
Just for the interested: the trick is the address of the washer or spot. The address of the first device (officially called fixture, but this can be confusing here, hence, let's call the individual washers or spots "devices" for now) will be "1".  If the device is using 3 channels, the address of the second device could be "4". In this case, provided the above mentioned controller is used, the first 3 faders would control the first device's R, G and B groups and faders 4, 5 and 6 would control the second device's R, G and B groups. The advantage, the two devices are now dealt with as a single fixture.
Advice: With a 16 channel controller (as the one I am using) one could potentially control 5 3-channel devices, however, the assignment of the fader will be rather confusing. Therefore, I recommend controlling 4 3-channel devices only. For sake of convenience, I would assign the second device to address (channel) 5, the third to address 9 and the fourth to channel 13.

Back to QRSS. Even the cheapest of DMX-controllers with the cheapest of LED-spots would make a real nice light beacon setup. OK, I went for something more sophisticated... since I see a secondary use in my light beacon setup... just in case I want to through a party, I now have a club-worthy lighting setup.

Concluding, there may be "red only" devices. However, stage worthy multi-color devices would even allow for multiplexing, depending on the receiver filters. The ones I use through out 1500lx @ 1m each, all LEDs engaged (at a power consumption of about 20W). Since I bought 4 (for good measures) that would be 6000lx @ 1m in white or about 2000lx using just one color.
Now I need to work out some receiver concept.