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Dec 14, 2019, 01:06 PM
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Can anyone verify if recording via the air unit's onboard DVR increases your overall latency? How about with the goggle DVR? Has anyone tested this?

Thanks!
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Dec 14, 2019, 01:10 PM
Registered User
Quote:
Originally Posted by Five4Quad
Can anyone verify if recording via the air unit's onboard DVR increases your overall latency? How about with the goggle DVR? Has anyone tested this?

Thanks!
Tested, no latency increase from what I remember.
Dec 14, 2019, 01:12 PM
Austrian FPV Freak
schim's Avatar
Quote:
Originally Posted by yurpet
I've tried flying two different planes with the telemetry cable to the DJI air unit unplugged. Both times I couldn't go beyond 6.5 km because of various issues (battery and bad weather). But I can already see that the telemetry makes a difference in the way that the video link works. With the telemetry the bitrate stays above 20 MB/s and then abruptly falls or starts to oscillate between 20 and 0.5 at about 4 miles mark for some mysterious reason. Without the telemetry it behaves much more normally. It starts to decrease from 25 - 20 MB/s at about 5.5 km mark (at the 1,200 mW setting with the 17 dBi receiving antenna) and at 6.5 km drops to around 10 Mbs. I don't know how it will behave farther away yet, but in my opinion there is the clear change in the signal bitrate behavior.
Have you seen dustin dunhills video here:
DJI Digital FPV (1200mW Hack/Mod) LONG RANGE TEST - How Far & How Clear on 4Lte Parrot Disco? (41 min 50 sec)

He first flew 4miles with 700 and at this flight he tried 1.2watts and still had Video out at 4miles.

What do you mean by with or without telemetry?
Dec 14, 2019, 01:48 PM
Registered User
Daemon's Avatar
His video confirms that there's some sort of hard limit right at 4 miles.
yurpet is saying that he thinks the video degrades more quickly at or near
that limit, when telemetry lines are connected to the FC, than when
they are not. The implication would be that some bit of extra processing
required to deal with telemetry, messes with the link quality.. I'm skeptical.

Assuming this limit isn't being done intentionally, my hypothesis is that this happening
due to a desync in the DSSS chip stream because both ends have their clocks perfectly
synced and at that distance (about 22uS out), the chips don't line up with their expected
receive window. I think DJI is using a modified 802.xx Wifi standard, and it simply
doesn't account for the possibility of light speed delays in a long range link
or it does, but only to some arbitrary limit.
One possible way to test this would be to take a 2nd pair of goggles
and put them in Audience mode (which does not participate in the two way
conversation of the primary goggles) and switch them on while the
aircraft is already far out. It should sync its clock only based on the signal
it sees (it having no way to know the 22uS delay is already baked in). I'll bet
that signal works further out. Unfortunately, once paired, there does not
appear to be any way to switch the primary goggles to Audience mode for its
own video link.
Last edited by Daemon; Dec 14, 2019 at 02:09 PM.
Dec 14, 2019, 02:09 PM
Registered User
Quote:
Originally Posted by Five4Quad
Can anyone verify if recording via the air unit's onboard DVR increases your overall latency? How about with the goggle DVR? Has anyone tested this?

Thanks!
The FAQ on Dji's website says it increases latency by up to 5ms when recording on the air unit. I can't personally feel the difference but when i tried testing the ms numbers in the HUD seemed a bit lower with recording off.
Dec 14, 2019, 02:53 PM
Registered User

Thanks Percievedshift and _j4t_!


Thanks for your replies on recording latency!

Quote:
Originally Posted by _j4y_
The FAQ on Dji's website says it increases latency by up to 5ms when recording on the air unit. I can't personally feel the difference but when i tried testing the ms numbers in the HUD seemed a bit lower with recording off.
Dec 14, 2019, 03:57 PM
Registered User
Quote:
Originally Posted by schim
Have you seen dustin dunhills video here:
https://youtu.be/AQI7AlMLeAU
He first flew 4miles with 700 and at this flight he tried 1.2watts and still had Video out at 4miles.
The best thing, he used stock antennas. Seems like they aren't that bad 4 miles on omnis is crazy.
Dec 14, 2019, 04:33 PM
aka JoshW, damightyj
JoshW24's Avatar
Quote:
Originally Posted by Daemon
His video confirms that there's some sort of hard limit right at 4 miles.
I don't know I'd say it confirms a hard limit.. but it is definitely looking that way isn't it.

Quote:
Originally Posted by Daemon
Assuming this limit isn't being done intentionally, my hypothesis is that this happening
due to a desync in the DSSS chip stream because both ends have their clocks perfectly
synced and at that distance (about 22uS out), the chips don't line up with their expected
receive window. I think DJI is using a modified 802.xx Wifi standard, and it simply
doesn't account for the possibility of light speed delays in a long range link
or it does, but only to some arbitrary limit.
Yeah, this does track doesn't it.. hmmmmmm

Quote:
Originally Posted by Daemon
One possible way to test this would be to take a 2nd pair of goggles
and put them in Audience mode (which does not participate in the two way
conversation of the primary goggles) and switch them on while the
aircraft is already far out. It should sync its clock only based on the signal
it sees (it having no way to know the 22uS delay is already baked in). I'll bet
that signal works further out. Unfortunately, once paired, there does not
appear to be any way to switch the primary goggles to Audience mode for its
own video link.
I have doubts on this. Audience mode only works if the Player goggles are active. You can verify this by plugging in the Player goggles, then connecting with the Audience goggles, and then powering down the Player goggles and immediately losing your feed in the Audience goggles. This tells me that the Player goggles have to have a good signal all around, and in fact suggest the Audience goggles are connecting solely to the Player goggles and not the Air Unit at all. Curious your thoughts on that?
Dec 14, 2019, 04:49 PM
Thread Killer
Protonus's Avatar
Can everyone email: [email protected]
and let them know you'd like them to make replacement foam for our DJI HD FPV Goggles?

Ideally it would have a new plastic frame that fixes the side and nose light leaks, that replicates the TPU shims. And, it should be like the "comfort" foam thickness they offer for the Oculus Quest, with the optional washable soft covers too.

I emailed them a few months ago about this, and they said per the email below that they need to see interest. Also, I have a spare pair of goggles now that I would be willing to send them for fit testing if we can show enough interest. Thanks!

Sent from my VS990 using Tapatalk
Dec 14, 2019, 05:09 PM
Registered User
Quote:
Originally Posted by Protonus
Can everyone email: [email protected]
and let them know you'd like them to make replacement foam for our DJI HD FPV Goggles?

Ideally it would have a new plastic frame that fixes the side and nose light leaks, that replicates the TPU shims. And, it should be like the "comfort" foam thickness they offer for the Oculus Quest, with the optional washable soft covers too.

I emailed them a few months ago about this, and they said per the email below that they need to see interest. Also, I have a spare pair of goggles now that I would be willing to send them for fit testing if we can show enough interest. Thanks!

Sent from my VS990 using Tapatalk
Done!!

Sent from my SM-G965U using Tapatalk
Dec 14, 2019, 05:34 PM
Registered User
Quote:
Originally Posted by schim
Have you seen dustin dunhills video here:
https://youtu.be/AQI7AlMLeAU
He first flew 4miles with 700 and at this flight he tried 1.2watts and still had Video out at 4miles.

What do you mean by with or without telemetry?
I mean with Tx and Rx of the FC telemetry output disconnected from the DJI air unit. What I imply by that is that DJI might use the distance signal in the telemetry to curb on the power at the 4 miles mark.
Dec 14, 2019, 05:36 PM
Registered User
Quote:
Originally Posted by Frosty3907
Having an incredibly frustrating time trying to get this going with iNav.
I had it working with my iFlight TwinG mini but I needed more motor ports (for a servo) so I put it on my iFlight TwinG F7 and cannot get the receiver input to show up in the iNav receiver tab.
Same port setup that was working in betaflight.
SBUS receiver selected.
No changes to hardware, just flashed iNav.

Any ideas?
Or does anyone have a FC working nicely with iNav and can control a servo?
Sorry should have specified: I'm using the dji RC as well.
Anyone successfully using that with inav?
Dec 14, 2019, 05:50 PM
Registered User
Quote:
Originally Posted by Daemon
Assuming this limit isn't being done intentionally, my hypothesis is that this happening
due to a desync in the DSSS chip stream because both ends have their clocks perfectly
synced and at that distance (about 22uS out), the chips don't line up with their expected
receive window. I think DJI is using a modified 802.xx Wifi standard, and it simply
doesn't account for the possibility of light speed delays in a long range link
or it does, but only to some arbitrary limit.
But DSSS means the effective signal frequency is multiplied by a factor of 4 or more, right? Then how would the 5.8 GHz antennas still work?
Dec 14, 2019, 05:58 PM
Registered User
Oh, not if the carrier frequency is much higher than the DSSS modulation frequency. But, in any case, the bandwidth of the signal has to be at least 4x wider than the standard 20 MHz, right? Is this the case?
Dec 14, 2019, 06:00 PM
Registered User
Quote:
Originally Posted by Frosty3907
Sorry should have specified: I'm using the dji RC as well.
Anyone successfully using that with inav?
This should be possible, just specify the UART port of the FC to which the air unit's Tx is connected as the receiver port in the Ports tab.


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