aburkefl
 Second Officer
Flight distance : 78612 ft
United States
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At the risk of devolving into a graduate-level discourse on radio communications.....
For several years, the RC (remote control) community - mostly planes until relatively recently - became immersed in FPV - First person View. This has been accomplished mostly by the following:
2.4 GHz spread spectrum for control of aircraft - the spread spectrum technique permits many, many users to operate all at the same time in the same area without serious interference. Interference is not impossible, just an incredible reduction in its likelihood.
5.8 GHz analog video to transport the video signal from the craft back to the controller. Unlike the 2.4 GHz spread spectrum (see above) these analog signals are limited - you can't have a great deal of them before they each start "stepping on" each other. You cannot arbitrarily increase/decrease frequency for greater bandwidth - you quickly end up in regions licensed for other operations. You are also somewhat limited in the broadcast bandwidth of the accompanying video signal. Systems like FatShark (just to pick an example) do NOT provide 720 HD signals - it is significantly lower in resolution. Keep in mind, its original purpose was to give the pilot a view of what's pretty much directly ahead of his/her craft. Recordings of these signals make for a pretty crappy movie.
Some FPV systems (and some not-so-FPV systems) utilize WiFi. Without enormous amounts of power, the typical WiFi (there are a number of variations here I'm not going into) works indoors up to about 150 feet. Outdoors, we can essentially double that to a - insert raucous cheer here! - whopping 300 feet. And to further add insult to injury, let's don't get too deep into latency issues. If your WiFi network is right next to you, the latency (delay of transmitted signal) is still amazingly obvious. The further apart the two devices, the more serious the latency. A quick aside here.
The Blade 350 QX3 uses a WiFi system for its camera. You can use a smart phone or tablet to start/stop video or take photos and frame the picture. That's about it. Even from just a couple of feet, the latency is enormous. The gimbal/camera is providing the WiFi and it has a delayed signal from only a foot away! That's unfortunately not uncommon with WiFi video. Back to our theme.
LightBridge (apparently a DJI creation - every reference I find to it shows "DJI LightBridge" - stays pretty much in the same 2.4 GHz area as the controlling signal - NOT up in the 5.8 GHz range like "standard" FPV signals. Additionally, it's digital, not analog. And that's primarily what gives it that gorgeous (relatively speaking) 720 HD picture. Also, unlike our outdoor WiFi (remember that 300 feet rule of thumb?) DJI Lightbridge is "spec'd" at 1.7 km - many users have reported far greater distances, but the DJI published numbers are officially (if I remember correctly) at 2 km - 1.2 miles.
Apparently under semi-ideal conditions - good line-of-sight - no radio frequency interference - proper orientation of antenna to Phantom - no overheating of portable device, etc. - This provides enormous benefits over just about any WiFi configuration.
It's my understanding that the 2.4 GHz LightBridge signal is NOT utilizing spread spectrum. Frankly, I'm not sure what happens to video if spread spectrum is attempted. Consequently, there is not an unlimited amount of room for very many people to operate at the same time in the same place. Can't for the life of me figure out what's going to happen if 15 people are all in the same place with their Phantom 3!
In an FPV "meet" the organizers typically assign frequencies/channels a block at a time - a limited number of people can fly at a given time. This was essentially how frequencies were controlled in the "good old days" - before spread spectrum.
One last note - spread spectrum. An interesting and formerly trivial - but not any more - note about spread spectrum. One of the original patent holders was Hedy Lamar! At any rate, the technique consists of "binding" a given transmitter (your controller) with a given receiver (the receiver in your plane/quad/heli/car). That receiver will now recognize ONLY the signals coming from your transmitter. The technique involves "frequency-hopping" but it takes place so quickly it appears to be spread all over the spectrum - hence spread spectrum. It initially held great promise for stealth transmissions (like by the armed forces in wartime) because the individual frequencies involved are not used by much power and for very short periods of time - I think in milliseconds, so the "jump" from frequency to frequency is very frequent. If you tune across the dial with even a sophisticated radio it doesn't sound like there's any "intelligence" there.
Now it's your turn to entertain me for a while!
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