Electronics > Projects, Designs, and Technical Stuff

Digital FPV video for drone racing

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richard.cs:
Because you can't use inter-frame compression (due to latency) you loose much of the advantage of digital video, being the removal of redundant information (and ideally replacing it with error checking of the more important information). The existing analogue systems essentially transmit lots of redundant information, and the Mark One Human Brain does a lot of postprocessing to tease the image out of noise and interference and it's actually very good at it. From an information theory perspective there isn't a clear advantage of a digital system in this application.

Going digital does not in itself solve the two-transmitters-shouting-over-each-other problem. You are still working with the same slice of spectrum, and anything which degrades your SNR will degrade your received signal. You can have active spectrum management with some kind of clear channel assessment or "permission to transmit" token, but that works just as well as a low-bandwidth digital control channel on top of analogue video as it does on a digital stream.

If the real problem is suddenly cutting to the wrong stream then that's essentially a capture effect, some types of digital stream might avoid that, but the degraded SNR might also just kill it entirely which is hardly any better. It seems likely that cutting to the wrong stream is the combination of two things, the FM capture effect, and which set of video sync signals the display manages to lock to. Switching from FM to AM (as all UHF-band analogue video always was) and then focusing the rest of your effort on making the sync as robust as possible (transmit it separately on a different narrow-band channel? Use a PLL to regenerate the sync so it can free run for a few frames if the sync is lost? Both?) You'd then have the sum of your wanted signal in complete frame sync, and your pits signal, almost certainly rolling, and I'm pretty confident that a human brain will separate them. Edit: if you have full control over the video source you could add a pseudo-random dither to the syncs which pretty much guarantees that your spurious signal will be smeared into fuzz, that should help the brain do the separation.

TheDane:

--- Quote from: Marco on January 22, 2019, 01:05:16 pm ---With 166 nanoseconds between each symbol you can swallow a few picoseconds.

--- End quote ---

5.8GHz FM transmitter chip - RTC6705 datasheet page 7 (sky.geocities.jp/oumeastro/RTC6705-DST-001.pdf)

Synthesizer counter default setting( 5.8Ghz band:5865MHz)
For 5.8Ghz band, FRF = 2*(N*64+A)*(Fosc/R)
Example: default FRF=5865MHz, Fosc=8MHz, R=400
5865=(2*N*64+A)*8Mhz/400=2*(N*64+A)*20KHz
N=2291(=8F3xH), A=1(=01xH)

Output freq is 5865 [MHz], Crystal Oscillator frequency is 8 [MHz] - an input/output multiplier ratio of 733.125
So a few (1,2,10) picoseconds suddenly becomes nanoseconds. 10 [pS] * 733.125 = 7.331 [nS]

Edit - added following: This is 'baseband' center frequency - it shifts up/down when subjected to motion. Any receiver must be able to follow in phase, if it is using phase-modulation-demodulation as an information carrier (As Digital video transmission most commonly does)

hexahedron:
Hey all, I have decided that I'm going to put this project on hold on my end. You guys have brought up so nsny good points that i had not considered that make this project damn near infeasible. Feel free to continue discussion, as I feel it is very constructive.

IDEngineer:

--- Quote from: Kilrah on January 22, 2019, 02:06:29 pm ---Again for repair reasons you can't fully disallow any kind of power source, if you did not let the pilots test for repairs other than VTX-related during other races you'd then have to lose way too much time between races with testing periods.
--- End quote ---
That's why I said earlier that this must be solved with REAL Engineering, not social engineering.

I too thought of the Faraday cage idea, so far I see two possibilities:

* Simply {grin} wrap the pit area in appropriately sized wire mesh. 5.x GHz would require some rather small pitch openings (haven't done the calcs). Perhaps just a grounded screen in the direction of the flight line and course itself. This *might* be feasible at a(n) (inter)national scale event, but lots of local events run by local clubs feed into those and I suspect most local clubs could not afford the screening material. As an example, a typical regional qualifier can involve a few dozen pilots and a total attendance of perhaps 100-200 people - enough to experience this problem but not so many that there's enough cash flowing to pay for expensive infrastructure.

* Have small dedicated test cages into which aircraft are placed during powered non-flight tests. This might actually be feasible, but it would have to be a glovebox like affair where hands and probes could have access while powered up and still providing shielding. When we're on the road we carry a decent set of test equipment - portable scope, meters, VTX spectrum analyzer, etc. - as we repair and test things during events that can last 1-4 days. Those probes and the hands that hold them or adjust things in real time would need access that doesn't defeat the purpose of the Faraday cage.

TheDane:
hexahedron - if you're willing to wait, and invest time and money, 5G network might be interesting for you:
https://en.wikipedia.org/wiki/5G#Advantages


It could be a (very) long time before chipsets become publically avaliable however, and projects open sourced like the GSM Baseband software implementation OsmocomBB (http://bb.osmocom.org/trac/)


Good luck, and happy droning  :P

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