DJI Air 3 No Video feed to DJI Fly

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Does that mean that’s it’s a different way of transmission compared to the VTX ?

VTX simply means "video transmitter". It's not a standard.

Occusync is a proprietary DJI standard for encoding and modulating the 2-way signal between controller and drone. It is digital, and encrypted with a changing key thats exchanged between RC and drone.

Next thing I'd try is switching to one of the Obstacle Avoidance camera views and see if that displays. That eliminates the main camera and it's interface from the equation, further isolating the image processing SoC.

If that doesn't display, send it to DJI. I guarantee you will not be able to DIY it cheaper than a DJI repair will cost, including any packing and shipping costs.

 

[Droning on and on...]

Is there a separate antennas for the controller transmission and for the video transmission, because if they’re separate it could be the video antenna.

You're wasting your time with assumptions that don't apply. This is nothing like a home-built drone running Betaflight, where the control system is completely separate from the video system.

There is only one abstract antenna (implemented as a multi-antenna array) transmitting and receiving Occusync, with the control parameters, some RID information, and other protocol bookkeeping going to the drone, and telemetry and video coming back from the drone... all mixed in the same encrypted Occusync packets formatted according to DJI's proprietary spec.

There's no separate antenna and transmitter sending the video out.