Der große HD FPV Feed Thread

Jim Tapp schrieb:

It seems like you removed your post (ugh!) about "it's odd that the signal strength bars doesn't correlate to my picture quality"?! Well, I wrote a long response explaining why that is and I'm sure other folks
will appreciate the explanation, so even though your post is gone I'm still going to reply since I took the time to write this LOL. Thank goodness I didn't lose the text despite you removing your post.
It's not odd, it's expected ;-) That's because unlike analog video, there's no direct correlation between signal strength and image quality with a digital system like this. I'll try and explain this as best I can in laymans terms and hopefully this will make sense:

Don't think of the system like an analog video system where everything is 1:1 between signal and picture quality. Instead, think of it more like you're streaming YouTube to your phone over 4G. Does your cellular reception affect your YouTube quality much if at all (until it gets real low or disconnects)? Nope, and for the same reasons as follows.
You are literally sending digital data / packets between the air unit, to the goggles, and then back again as an acknowledgement. Think of the Air Unit and Goggles, more like as a Laptop running off WiFi. That's a lot more like what's going on here - your own private little adhoc WiFi network.

Normal bandwidth is about 25 Mbps on this system. You need this bandwidth to have the best possible image, because, that allows the highest rates / speed for sending data. Think of bandwidth, as a measurement of how much water is FLOWING through a pipe. The pipe in this case, is your connection between the air unit and goggles. BTW, The system compresses and encodes the HD image in order to "fit" that bandwidth. When the image is too complicated, like flying slow around trees / leaves / water - where there's lots of detail all moving independently which is hard to compress; that's when you can see focus mode or pixelation kick in even when bandwidth and signal strength is high. This is because the compression has increased in order to get the size of the image / frame / packet underneath that current bandwidth limit.
Signal quality then, is more like the SIZE of the pipe - it tells you your potential for bandwidth, but not the bandwidth itself. If you have full strength signal, you should have no problem getting full bandwidth - the pipe (signal) is large, so as much water as you need (bandwidth) can flow through it. But as signal strength drops, the size of the pipe does too. And if you're far away (a very long pipe), this may eventually start to limit the flow of water in the pipe. Less water = lower bandwidth. With good antennas, you can still get full bandwidth out for miles, with good line of sight, because the signal strength stays high (the pipe is big). But if you fly behind obstructions, or you fly very far away on omni's, etc, you limit the size of that pipe. Signal strength drops, and eventually, so will bandwidth.

When bandwidth starts to drop, so will picture quality. The system has to compress the picture more in order to maintain the 120fps framerate, when there is less bandwidth. It can't send as much information. Increased compression = more pixelation / blockiness. As bandwidth continues to decrease, you can send less and less data, so the image becomes worse and worse. Eventually if you obstruct the signal enough and bandwidth falls low enough, the system can't resend the packets in time when they're missed, and there isn't enough bandwidth to send an image at all. This is when you start to drop frames / the picture freezes for a sec. This is when bandwidth is extremely low. If you continue to push it, eventually, the goggles can't talk back to the air unit at all. The conversation is over between them, and you get a DJI logo and lose all video. The good news is, you can anticipate this, and avoid it, if you watch bandwidth!

As such, looking at the signal bars for video is not very useful in flight, and it doesn't tell you much. Rather, bandwidth is the true indicator of performance and range etc. When you see the signal bars go red, start to pay attention to bandwidth - but don't read too much into red / low signal alone (unless it happens VERY close by, in which case, you may have a loose air unit antenna!). When you see bandwidth drop below 20, you are losing some data / performance. When it hits 15 Mbps, you should pay attention to it more carefully. When it hits 10 Mbps, slow down! When it hits 5 Mbps, STOP. Do a full 180 turn, to make sure you can fly back if you need to! Pushing it past 5 Mbps is sketchy. There's so little bandwidth by this point that just popping behind a tree could interrupt the signal very quickly, and you could lose the transmission entirely.
Also, because the system is bidirectional, the goggles can tell the air unit when they miss a frame (say you pop behind a tree) - and the air unit can resend it. The signal travels at the speed of light, so with good bandwidth, this happens real quick. Like 5ms. When it happens, you get a little bit of extra latency as a result. This is why the system has variable latency even when signal is good. You can see latency increase when you jump behind obstructions, but the picture can remain clear - it's resending the data you missed. To you, the image is never interrupted, the picture stays smooth and fluid at 120fps but the system is working behind the scenes to keep it that way. The cost is an extra 5-10ms of latency as a result. No free lunch.