This is a laymen explanation of Lightbridge versus Wifi, the two primary technologies used to wirelessly stream video from a drone with a gimbal camera to a ground receiver for display, for monitoring purposes. The technology discussed here is not related to the ability of a drone camera to record video to the internal flash card.
Lightbridge is what sets DJI apart from all other drone manufacturers. Lightbridge is a DJI proprietary communication link technology used to broadcast 720P live "near real time" video from a drone to the ground, over a mile, often 2+ miles. Lightbridge uses the same 2.4Ghz frequency band that WiFi uses, however Lightbridge is much different than the standard WiFi communications other drone manufactures use (think Yuneec, Blade, 3DR). Lightbridge is a one way stream of data with no two way handshaking. It's literally a broadcast of data, like TV broadcasters use from the top of hills. This is very different than WiFi that requires constant two way "hand shake" communications with every packet of data. With WiFi, each data packet has to confirm a packet was received, and all 512 bytes in the packet was received intact. If one byte is lost, the whole 512 byte packet is sent again! This is where latency occurs. Resending data for one byte loss is a waste of time in the wireless video world of drone flying. Although wifi is very fast from a data point of view, the two way handshaking of the TCP/IP protocol WiFi hinders the goal of "real time" video, which everyone needs to fly drones FPV. There is NO TIME for resending packets when real time video is the goal.
Real time video display with no latency is what everyone needs to fly FPV, that's obvious. Even with Lightbridge real time video still isn't not possible. However, Lightbridge is clearly the best value implementation so far, minimizing video latency while maximizing flight distance. This is because Lightbridge uses a one-way broadcast of data. Latencies with Lightbridge are more consistent at distance as a result, versus standard WiFi. You will typically get 100 to 200ms video latency with Lightbridge, and this is pretty consistent at greater distances. Wifi latencies are higher, generally 25% to 100% higher depending on distance and the flight environment. The impressive distance capabilities of Lightbridge vs. WiFi have a lot to do with the simplicity of ONE WAY communication broadcasting to stream video.
When you reach the distance limits with Lightbridge the screen will typically show streaks and you'll get a warning of connection weakness. Most of the time reconnecting is generally fairly quick, because there's NO requirement for two-way handshaking to re-establish connection, like Wifi. LIghtbride is much faster at re-establishing a connection when it reaches it's distance limit. Lightbridge gives you a visual warning in the controller that signal is weak or lost, this instantly triggers the pilot to stop and hover. To reconnect you can typically re-establish a connection by turning the craft around slowly to phase shift the antennas of the craft, or increasing altitude if LOS is marginal. Again, this usually very quick, less than 10 seconds with Lightbridge. In comparison, with most Wifi implementations, when you reach the distance limitation the screen will lock up, no warning, and typically the screen will have a frozen image, which can fool the pilot and make him think all is OK for 3 to 6 seconds. Once he realizes he isn't moving, reconnecting can take 30 to 60 seconds because of of the handshaking required to re-establish a viable communication path. These WiFi re-connections typically require either a higher altitude when LOS is marginal, or hit the RTH to get closer.
Another weakness of WiFi is wooded flight locations. Briefly going behind a tree with leaves can disrupt WiFi signals and affect video a lot easier than Lightbridge. Wifi is more sensitive to lockups and disconnects with tree/leaf interferance. Trees contain moisture, a signal blocker, and will trigger WiFi will lose a few bytes, triggering re-sending of large packets, impacting the latency and video consistency. Lightbridge reacts a little different with slight data is lost from trees. This looks more like video streaks or static, but you'll always see an image, with a low latency of 100 to 200ms.
Due to the simplicity of Lightbridge broadcast one way broadcast data stream, the distance capabilities are two to three times that of Wifi implementations of non-DJI drones. Lightbridge may be more expensive (there is no one chip solution like Wifi) to implement than WiFi, but the distance and latency benefits are huge, well worth the extra few dollars in circuitry.
Wifi was never created to broadcast video from air to ground, hence it has inherent problems that hinder the distance and latency of drone to controller video streams. Wifi is cheap to implement, solely because the PC market has created high demand, and chip makers can put wifi into a small piece of silicon for pennies. However those chips have a fixed handshake protocols that were not designed for drone usage for video.
OTHER VIDEO COMMUNICATION SCHEMES
For the drones you see racing at 50+MPH (no gimbal/camera), they use analog video transmission broadcasting, which is near zero latency, much lower latency than Lightbridge. The other advantage of analog transmission is it doesn't freeze an image, lockup or lose total connection all of a sudden as the craft approaches the distance limit. It's a one way broadcast of an analog signal, much like old TV signals before digital TV broadcasting was invented. When the signal gets weak you will start getting snow in the screen, telling the pilot to turn around and GCTH (get closer to home). This real time analog video link technology is generally provided by a company called Immersion. The downside to analog video communications is it will not go 1+ mile distances like Lightbridge while being within the legal FCC power limits, and it's consumes more power, because it's analog.
One of the latest technologies to keep an eye on is from Connex which offers full 1080 HD quality video streams with no latency. Amazing stuff, but expensive today.
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