Author Topic: Digital FPV video for drone racing  (Read 4762 times)

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Offline dmills

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Re: Digital FPV video for drone racing
« Reply #75 on: January 21, 2019, 09:09:47 pm »
Any reason not to be thinking spread spectrum?
Link could still be basically FM, with spreading by a frequency hop during horizontal retrace.

Advantage is that unless someone has set the same spreading code as you a jammer will take out at most a few randomly scattered video lines instead of the whole thing.

The issue with FM is that it captures to the strongest signal in the receivers pass band, which makes numb nuts in the pits a problem.

The RC flying community back in the day used to handle this by only handing the crystal pairs to the pilots when they were going to fly, and taking them back afterwards (And there was hell to pay if you were found to be in possession of extra rocks). 

73 Dan.
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #76 on: January 22, 2019, 12:10:30 am »
In terms of other people turning a transmitter on in the pits, this is a problem that has been faced by RC since it began. Before 2.4 GHz is was routine for big RC car races to have a transmitter impound. You got your transmitter back just before your heat.
Big difference this time is that the VEHICLE is the transmitter (video coming back to the operator). "Impounding" vehicles is a non-starter... folks have to work on/repair their aircraft between races. Props break, wires get sliced, PCB's and/or their components get dinged... it's a lot like an airborne demolition derby. Impound aircraft and the sport will cease to exist overnight.

And no, we cannot "just impound the video transmitter PCB". To keep size and cross sectional area to a minimum, the PCB "stack" is a tight assembly fully customized by each pilot. You can't just add or remove PCB's as a matter of convenience... even repairs can involve almost complete diassembly and reassembly depending upon how the aircraft is laid out. This problem needs to be solved with technology, not social engineering.

EDIT: Here's an example. At the World Championships in Shenzhen, DJI (the "stable camera platform" drone company) provided HD cameras with integrated video transmitters so the organizers could livestream high quality video from each aircraft's perspective during each race. Those little cameras were expensive, and they only had so many, so DJI staff were standing at the flight line to install them on each aircraft as they were carried out to start a heat and then they recovered them off each aircraft when they were carried back after each heat. They had a variety of ways to attach them - zipties, double sided foam tape, you name it - but the chaos was ridiculous. The pilots got into serious arguments when the DJI people picked where they wanted to mount them... many pilots didn't appreciate the DJI folks "manhandling" their aircraft as they treated the cameras like precious gems... the DJI people got angry each time a camera was damaged from an impact (as if the pilots were doing it on purpose?!?)... etc. This just illustrates the craziness of trying to remove/impound/add some piece of electronics prior to each heat, and in this case it didn't even have to tie into the other electronics on the platform.

There are at least a dozen popular video transmitters in current use in the FPV world. Each optimizes for some different set of features. Sometimes size is important. Other times features win out even if the board gets larger. Some people insist on coax connectors on the board, while others demand to have the coax soldered straight to the PCB. Some Tx's are standalone, while others tie into the flight controller to get overlay/HUD info such as battery voltage, present/peak current draw, elapsed time, etc. so the pilot knows how much is left "in the tank" and whether he has to nurse things along to survive the full heat length. There's no one size fits all, and if DJI's experience is any indication the swapping of hardware in real time isn't an acceptable solution.

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the issue I have with FPV is that we could just simulate the entire thing like a video game.
Simulators (aka trainers) do exist. However, I notice that real fixed and rotary wing RC modeling continues to be popular despite incredibly real simulators being available for many years now on a variety of platforms. There's something about "reality". Plus, a huge draw of this sport for a lot of the pilots is the merging of their love of flying with their love of electronics, assembly, craftsmanship, etc. And frankly from this Dad's perspective, I'm a lot more supportive of FPV than I would be if my son were living in the basement playing video games like so many other teens these days. The skills he's building in hardware, software, mechanical, 3D printing, social interaction with sponsors and other pilots, etc. are transferable to a wide range of career options and life lessons. He's considering majoring in Aeronautical Engineering when he starts college next year, which I doubt would have happened if he'd been playing video games.

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btw, if you've raced at major drone racing events then odds are very good you've used hardware I've built!
OK, you can't just leave that hanging out there! {grin} Fess up, what are you talking about? (And noting the flag you have in your ID, my son took first place in the Western Regional Canada Championships in Kelowna BC last year and 4th place (top non-Canada pilot) in Canadian Nationals in Ottawa ON a few months later, so if anything you've built is local to Canada the odds are actually quite high!)
« Last Edit: January 22, 2019, 12:38:36 am by IDEngineer »
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #77 on: January 22, 2019, 12:15:10 am »
Any reason not to be thinking spread spectrum? Link could still be basically FM, with spreading by a frequency hop during horizontal retrace.
No reason at all, and that's actually something we've talked about. Again, losing a line here and there is something the brain can accommodate.

EDIT: However, this only works if all pilots in the area switch to the new system. Otherwise the new system would be susceptible to a "old school" pure analog transmitter being powered up. I suppose if we used the entire available ISM band for the spread the effect of a single analog channel powering up could be minimized, but realistically any SS system would have to be designed to accommodate multiple pilots simultaneously... and we're back to channelized allocation again, where the subsequently narrower channel width makes that channel's spectrum overlap to a greater extent if the right/wrong analog channel is turned on. Figure support for eight simultaneously active pilots, so now the band is channelized to 1/8th the ISM band, the odds of overlap with analog get pretty high.

This is why I was talking about something similar to the tokenized system used for modern RC uplinks. It changes the presumption from "avoid interference" to "plan on interference and accommodate it".

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The RC flying community back in the day used to handle this by only handing the crystal pairs to the pilots when they were going to fly, and taking them back afterwards
That's a better solution than "impounding aircraft", but since virtually everything is synthesized these days there's no physical objects to control.

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(And there was hell to pay if you were found to be in possession of extra rocks).
I've never heard crystals called "rocks" before - very cool and singularly appropriate!
« Last Edit: January 22, 2019, 01:04:55 am by IDEngineer »
 

Offline ogden

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Re: Digital FPV video for drone racing
« Reply #78 on: January 22, 2019, 03:29:49 am »
This is why I was talking about something similar to the tokenized system used for modern RC uplinks. It changes the presumption from "avoid interference" to "plan on interference and accommodate it".

Right. In case of widely accepted digital FPV, every transmitter shall have unique "call sign" broadcast (like SSID) so you know who is to blame when *t hits the fan. There could be even some channel management system (like DHCP for IP) which assigns channels on request to avoid (un)intentional collisions.

Frame rate isn't necessarily related to latency. 30FPS, or even 24FPS, is a sufficient frame rate as long as the end-to-end latency is low enough.

Pure BS. Let your son run 30fps progressive instead of 30fps interlaced (60Hz half-frames) and ask his opinion ;) Alternatively you can ask virtually any FPS/race gamer what is better for games: 30 FPS  or 120 FPS. Kinda obvious answer, right?
« Last Edit: January 22, 2019, 03:35:12 am by ogden »
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #79 on: January 22, 2019, 06:06:19 am »
Pure BS. Let your son run 30fps progressive instead of 30fps interlaced (60Hz half-frames) and ask his opinion ;) Alternatively you can ask virtually any FPS/race gamer what is better for games: 30 FPS  or 120 FPS. Kinda obvious answer, right?
We're talking about two different things. You're talking about refresh rate. I'm talking about signal path latency. The latter is not related to the bandwidth of the data (read: refresh rate) in the signal path. A low refresh rate video signal can have lower signal path latency than a high refresh rate video signal, and vice versa. The discussions in this thread have been about signal path latency, e.g. delays imposed by compression and/or conversion to and back from digital.
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #80 on: January 22, 2019, 09:28:35 am »

This problem is almost always some pilot believing "It won't matter for just a quick second". They drop their transmitter power to "pit mode" (generally a couple of mW, whereas most racing is at 25mW these days), remove their transmitting antenna (never mind the SWR mismatch), etc. and "plug in for just a quick test".

EMC (Electromagnetic compatibility) is nothing new, and in fact something every designer/manufacturer must comply to if selling equipment, at least in Europe.

When testing equipment, say naval emergency transmitters, it is paramount not to disturb the open air frequencies - unless you want a visit from the coast guard and their huge chopper. So, smart ingenious people (or poorer  ;)) came up with a shielded box (Faraday cage) - and filtered inputs/outputs.

https://www.google.com/search?q=emc+shield+box&source=lnms&tbm=isch&sa=X

I know, organizers must have it and require its usage - but from the rules I have read it states that antennas must not be soldered on, so what is stopping the scene from utilizing these boxes, with screw on antenna cables + attenuation so the receiver input is not fried.
A great way to test your equipment, without disturbing others.
 

Offline StillTrying

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Re: Digital FPV video for drone racing
« Reply #81 on: January 22, 2019, 09:29:54 am »
Any reason not to be thinking spread spectrum?
...
Advantage is that unless someone has set the same spreading code as you a jammer will take out at most a few randomly scattered video lines instead of the whole thing.

Won't any other VTX have the same effect, especially when there's ~8 of them in use.
CML+  That took much longer than I thought it would.
 

Online Marco

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Re: Digital FPV video for drone racing
« Reply #82 on: January 22, 2019, 10:29:06 am »
I think you're in for a heck of challenge to successfully make a digital system that can replace the analog ones.

I'd just like to interject for a moment. What you're referring to as analog is in fact digital PCM + FM. Analogue is a corner case of digital modulation, the signal is generated by a DAC ... they just use a really weird encoding for legacy reasons.

Once you cut the legacy NTSC out of the chain you can trivially change the resolution/refresh rate. An OV5460 can do VGA at 90fps and QVGA at 120 for instance. Even if you encode it as PCM, you have much more control over it. You're not reliant on some blackbox circuitry to synch to the NTSC signal for instance. If you have signal interference there is no reason to get confused, keep counting and the next pixel you get will be in exactly the right place. Who needs sync signals every line when you have crystals? You can throw a reference pulse in there every couple of ms and calculate the path response and correct for it, etc.
« Last Edit: January 22, 2019, 11:08:06 am by Marco »
 

Offline ogden

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Re: Digital FPV video for drone racing
« Reply #83 on: January 22, 2019, 12:06:42 pm »
Pure BS. Let your son run 30fps progressive instead of 30fps interlaced (60Hz half-frames) and ask his opinion ;) Alternatively you can ask virtually any FPS/race gamer what is better for games: 30 FPS  or 120 FPS. Kinda obvious answer, right?
We're talking about two different things. You're talking about refresh rate. I'm talking about signal path latency.

When you know latency of existing analog FPV radios, you know that there's nothing to improve. That's my point which I already provided BTW:

If we want to beat analog TV transceiver latencyy using digital tech - we can't unless we increase frame rate. So, it shall be 120Hz or higher. Also whole frame buffering to compress/decompress is way too expensive waste of precious time. Analog TV transmits every scan line w/o buffering

Inter-frame delay time also shall be considered as latency - you want it or not.
« Last Edit: January 22, 2019, 12:17:16 pm by ogden »
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #84 on: January 22, 2019, 12:15:03 pm »
Who needs sync signals every line when you have crystals? You can throw a reference pulse in there every couple of ms and calculate the path response and correct for it, etc.

Crystals are NOT perfect, and will change frequency when subjected to g-changes (acceleration, deceleration, rotation). Ah relativity  :box:

An extensive subject, already looked into by TI, and not to be forgotten - it is not only reflection/multi-path/dobbler effect which affects system performance in this case.
When designing a clock oscillator into a system, the impact of vibration is often overlooked. However, as demonstrated above, oscillators with similar specifications can  xhibit vastly different behaviors when subject to vibration or shock, possibly resulting in the addition of tens of picoseconds of jitter and violation of design requirements. Very important when using mixers and/or multipliers - http://www.ti.com/lit/an/snaa296/snaa296.pdf

Acceleration Sensitivity Characteristics of Quartz Crystal Oscillators - http://www.greenraytoday.com/beta1/wp-content/uploads/2017/02/AccSens.pdf
 

Online Marco

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Re: Digital FPV video for drone racing
« Reply #85 on: January 22, 2019, 01:05:16 pm »
With 166 nanoseconds between each symbol you can swallow a few picoseconds.
 

Online Marco

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Re: Digital FPV video for drone racing
« Reply #86 on: January 22, 2019, 01:58:26 pm »
"Impounding" vehicles is a non-starter... folks have to work on/repair their aircraft between races. Props break, wires get sliced, PCB's and/or their components get dinged... it's a lot like an airborne demolition derby. Impound aircraft and the sport will cease to exist overnight.

Just disallow any power sources in controllers and drones except during racing or specifically allocated testing periods. Any time you power it on there is a non acceptable risk of interference, there is no need for that ... only want.
 

Offline Kilrah

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Re: Digital FPV video for drone racing
« Reply #87 on: January 22, 2019, 02:06:29 pm »
Back in the day when we were figuring out what existing analog VTXs to use for FPV it was common for some of them to be severely affected by vibrations indeed, to the point of having a barely viewable picture with common prop vibration.

EDIT: However, this only works if all pilots in the area switch to the new system. Otherwise the new system would be susceptible to a "old school" pure analog transmitter being powered up.
I suppose if we used the entire available ISM band for the spread the effect of a single analog channel powering up could be minimized, but realistically any SS system would have to be designed to accommodate multiple pilots simultaneously... and we're back to channelized allocation again
Why? RC radios use the whole band and can coexist just fine. OK lower bandwidth, but larger numbers too...

Just disallow any power sources in controllers and drones except during racing or specifically allocated testing periods.

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.

For the VTX there's already a system for this made by a manufacturer, when checking in to a race the contest organizer essentially disables your TX so you can power it up all you want but it'll never transmit. When the participants are ready on the start line the race director goes by with the NFC programming wand to enable them until the next power off.

But that's not practical, because people also damage/break their TXs so they need to test them, and going to everyone between races to re-enable to allow testing then re-disabling before the next would be basically impossible. But there are easy ways to make that better, e.g. a longer range link similar to the R/C system built on the TX and a central "server" station managed by race directors, anytime someone powers up a TX it queries the server for whether it is allowed to transmit and only does so if answered positively.
« Last Edit: January 22, 2019, 02:11:19 pm by Kilrah »
 

Online Marco

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Re: Digital FPV video for drone racing
« Reply #88 on: January 22, 2019, 02:43:14 pm »
Again for repair reasons you can't fully disallow any kind of power source

My intuition says that unless you create some kind of standard which allows a physical detachable interlock which referees can easily check/control to keep people off the racing bands, which batteries are in a way, I'd say the odds of interference are in the whole percentage points if someone does a powered test during a race.

So it's just a question of what you find acceptable risk ...
 

Offline Kilrah

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Re: Digital FPV video for drone racing
« Reply #89 on: January 22, 2019, 03:09:01 pm »
That's why I mentioned other solutions to prevent transmission that are reasonably tamper-proof and don't prevent aircraft testing.
« Last Edit: January 22, 2019, 04:40:44 pm by Kilrah »
 

Offline richard.cs

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Re: Digital FPV video for drone racing
« Reply #90 on: January 22, 2019, 03:46:56 pm »
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.
« Last Edit: January 22, 2019, 04:07:20 pm by richard.cs »
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #91 on: January 22, 2019, 03:55:28 pm »
With 166 nanoseconds between each symbol you can swallow a few picoseconds.

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)
« Last Edit: January 22, 2019, 04:05:37 pm by TheDane »
 
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Offline hexahedron

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Re: Digital FPV video for drone racing
« Reply #92 on: January 22, 2019, 05:13:11 pm »
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.
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #93 on: January 22, 2019, 05:34:05 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.
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.
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #94 on: January 22, 2019, 05:38:46 pm »
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
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #95 on: January 22, 2019, 05:58:43 pm »
* 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.

Metallic mesh gloves exists in different sizes and lengths - https://www.google.com/search?q=metal+mesh+gloves&source=lnms&tbm=isch&sa=X
If you're really handy, you could hack a (large) microwave oven and cut some holes in its sides.
Mount the gloves, and secure them so they do not leak RF, and you have a shielded glove box you can look into - and it opens/closes easily.

Copper tape is used extensively in EMC compliance testing, if you don't want to use/rely solely on metallic screws and unweave the glove - it should be tight and tidy  ;)
 

Online Marco

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Re: Digital FPV video for drone racing
« Reply #96 on: January 22, 2019, 06:19:55 pm »
Because you can't use inter-frame compression (due to latency)

If you use 8x8 blocks, the only necessary latency is buffering 8 lines ... 0.19 ms at 720p60.
 

Offline TheDane

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Re: Digital FPV video for drone racing
« Reply #97 on: January 22, 2019, 06:42:56 pm »
Because you can't use inter-frame compression (due to latency)

If you use 8x8 blocks, the only necessary latency is buffering 8 lines ... 0.19 ms at 720p60.

 :-//  ???
Doing a partial frame update/interlacing - is like filling a FIFO buffer, and only rendering the final picture frame when full. Great for slow/non-moving stuff.

The drone is moving (fast), rendering the data sent to the FIFO buffer 'useless' (that's what I understand anyways) as the next 8 lines will be nearly the same.
If you hit the 'right' speed, it will be exactly the same - unless you do a down-up rendering, and I think that would really mess with your perception (rolling shutter effect - https://en.wikipedia.org/wiki/Rolling_shutter)

Imho the 'right' solution is a super high frame-rate, very short packets sent on a low latency network - and 'post-processing' those packets onto a high frame-rate display.
(I have worked with NTSC, PAL, SECAM, (Videocrypt+D2-MAC), MPEG1, MPEG2, MPEG4 on various medias - so I have a bit of experience on video systems)
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #98 on: January 22, 2019, 06:56:53 pm »
Imho the 'right' solution is a super high frame-rate, very short packets sent on a low latency network - and 'post-processing' those packets onto a high frame-rate display.
That would be ideal. Some challenges:

* The camera and its transmit-side electronics must be physically small and weigh well under 100 grams. A particular camera with which I'm familiar is 74 grams and considered grossly overweight by most pilots. Total aircraft dry weight is generally around 300 grams including video system, of which the four motors alone are 100-120 grams.

* Establish a low latency network (50mS is about the max) on an ISM band (so no licenses are required) that can support at least eight simultaneously active pilots.

...and the above does not address the problem of random idiots plugging in during a race, which we've kind of been dealing with as a separate question.
 

Offline IDEngineer

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Re: Digital FPV video for drone racing
« Reply #99 on: January 22, 2019, 07:05:58 pm »
I still believe that the correct way to address the idiots in the pits who plug in during a race should be technical and not social. However, a solution that could be implemented today with existing technology would be a separate bank of receivers and directional antennas pointed AWAY from the course, and TOWARD the pits. When a race is to begin, start recording on all active channels. The nulls in the antennas should (mostly) reject the aircraft on the course. But if someone lights up in the pits, they should be nicely within the antenna pattern and be recorded, with the receiver(s) naturally selecting the then-stronger signal. Most pilots have their "handle" on their OSD's and are thus self-identifying. If a pilot reports video problems these recordings are reviewed and if an offender is found, they are ejected from the event. And maybe their equipment is doused in gasoline and burned too. (Tempers get pretty hot when someone does this. The chief official at an event a while back actually took the offender's aircraft and beat it to pieces with a hammer in front of the crowd to set an example.)

I'd still rather have a self-enforcing technical solution, but this might be a stopgap until the tech is ready.
 


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