Electronics > Projects, Designs, and Technical Stuff
Digital FPV video for drone racing
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Marco:
They don't care if something else is transmitting on the band, they'll just try to shout over it ... since your transmitter is close to your receiver that's a shouting match you usually win.
TheDane:

--- Quote from: hexahedron on January 19, 2019, 04:34:06 pm ---There are 480 scan lines (not including overscan) in a 480p video signal.
(1/480)*16 = 0.0333...
As it will take a little less than 16 scan lines to transfer all the data (for those 16 scan lines), and as all of the data is needed to re-construct those 16 scan lines, it will have a minimum of a little less than 32 scan lines of latency.
(1/480)*32 = 0.0666...
If we take into account that there are 60 frames per second:
((1/480)*32)/60 = 0.001111... seconds of latency
that is about 1.111... Milliseconds of latency!!!

--- End quote ---

Running some numbers:
50 [km/hr] = 50.000/3600 [m/s] = 13.89 [m/s] - breakneck drone speed?
30 [Hz] = 33.33 [mS] - progressive picture frame update rate
60 [Hz] = 16.67 [mS]
100 [Hz] = 10.00 [mS]
240 [Hz] = 4.17 [mS]

Distance the drone is moved pr. picture frame update at 50km/hr, if it can fly that fast:
13.89 [m/s]/30 [Hz] = 46.3 cm
13.89 [m/s]/60 [Hz] = 23.15 cm
13.89 [m/s]/100 [Hz] = 13.89 cm
13.89 [m/s]/240 [Hz] = 5.79 cm

Doing a rolling picture update could very well induce smeering/blurring - confusing your brain
 Doing a rolling picture update could very well induce smeering/blurring - confusing your brain
  Doing a rolling picture update could very well induce smeering/blurring - confusing your brain
   Doing a rolling picture update could very well induce smeering/blurring - confusing your brain
    Doing a rolling picture update could very well induce smeering/blurring - confusing your brain
Which is why motion-pictures@24 [Hz] are/were such a big thing - it is moving a frame, pausing and illuminating, moving a frame - so no motion sickness is induced
Most people will not notice the flickering on a big screen@24[Hz] - but it is not gaming. Scenes tend to be relatively slow moving and the director is directing your attention.

You are talking about 60 [Hz] image update rate = 16.67 [mS] image update rate, and it seems you worry about 1.xxx [mS] latency and partial scan updates.
An open WiFi network should have ping rates in the 1.xxx [mS] range, and I presume you don't need encryption - your competitors won't be looking on your screen trying to cheat like it was a game of CS:GO  8)
Another benefit is having the option of using a range extender on your network - ensuring packets are always received. No more drop-outs, though a bit of network setup is required to sort out duplicate frames. This is all about drone flying, not video jamming each other - right?

Another big requirement is your screen/viewing equipment. Does it support 60 [Hz] framerates, and even higher - without blurring the image itself?

A homemade system sounds great, expensive and quite heavy!
WiFi dongles and SBC's are easy to obtain and replace (and have spares of) in case of crashes, and it won't burn a hole in your wallet. Most are even CE/FCC certified  ;)

Looking forward to hearing more about your interesting project.
radar_macgyver:

--- Quote from: Marco on January 20, 2019, 11:18:15 am ---Nice thing about the cheap receiver modules is that you can do a diversity receiver by simply having lots of modules and switching the IF output.

--- End quote ---
Or even by switching the video. There are some commercial products out there which do this already, and they work decently well to avoid dropouts due to multipath. They don't do anything to avoid frequency overlap, though, which was one of the problems the OP was attempting to solve.


--- Quote from: Marco on January 20, 2019, 11:18:15 am ---... I'd just pay TI the eye watering price for the LMX8410L and design around that ...
--- End quote ---
OT, but have you done any tests on that part yet? I was eyeing it for a new design and some of the specs (like phase noise) almost look too good to be true, while others (19 dB noise figure) left me scratching my head. The mixers' spurious responses (other than 2x2 and 3x3) are not characterized in the data sheet either.
Marco:

--- Quote from: ogden on January 20, 2019, 01:58:54 pm ---Even if miracle happens and one can find analog video transceiver with phase performance good enough to run better than BPSK or 2FSK/4FSK modulation

--- End quote ---
It's good enough to see intensity grading of a video image, which is more than 4 levels, so you can do a hell of a lot better than 4FSK ... you'd probably drive it with a 8 bit DAC and since it's a FM modulator that is equivalent to 256-DPSK, you'd probably not use all those bits but 4FSK is needlessly pessimistic.

I was wrong about the cheap receiver syncing to the signal, it doesn't bring it to baseband DUH.


--- Quote from: Kilrah on January 20, 2019, 01:22:23 pm ---I was thinking of streaming stuff in a way similar to progressive JPEG, aka you stream the same frame multiple times with increasing "resolutions".

--- End quote ---
In and of itself that doesn't really help any, unless you encode the lower resolutions with more FEC.
dmills:
Thing is a 24 FPS cinema projector has a 48Hz flicker rate due to the two bladed shutter (Actually, some had a three bladed shutter for a 72Hz flicker rate!)...

TECO might be worth a look, it is a wavelet transform codec aimed at UHD baseband (Because 10Gb/s is not enough for 4k60 4:2:2, annoyingly you need 12Gb, which is boring given ethernet standards), it is low latency (as little as a single video line) and capable of up to about 10:1 data reduction.

That would get your ~100Mb/s 8 bit 4:2:0 down to something that could be coded in QAM8 or 16 with a reasonable amount of FEC.

I still think your receivers are the low hanging fruit, and I would absolutely start with some serious spatial and polarisation diversity probably with a pilot tone on the audio channel from the transmitter that my processing can use to assess quality. A few 20M or so cat 6 cables to carry the baseband back from the receivers and to carry power to the receivers, some processing magic (Fast ADCs, fpga with some sdram, video DAC or HDMI output) to dynamically equalise the path delays, and an alpha blend to combine the video signals based on signal quality indications, job done.

Incidentally, if combining multiple noisy signals, pick the darkest pixel, not the brightest, dark pixels due to noise are MUCH less of a problem then bright ones, which is why PAL transmitters had sync be full amplitude and white be minimum amplitude.

Regards, Dan.
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