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Oct 04, 2012, 02:32 AM
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more wifi secrets


Today's discovery was that wifi cards don't successfully arbitrate when 2 packets are sent simultaneously. There are resends, but the process is much slower than believed, causing packets to get delayed for many seconds.

The delusion about fast wifi arbitration led to the camera & aircraft sending packets simultaneously, which was causing massive network collisions. The delusion persisted, even after discovering wifi had to run in 54g mode to work at all.

After reordering the packets so the camera doesn't get triggered to send until the copter is done sending everything, the network recovered, but not completely.

It still fell over after 15 minutes, but since the downgrade was constant, it was probably a firmware bug or overheating more than network collisions. This was still using 54g mode, which has serious range degradation.





By this time, the process had already begun to shift from wifi to USB for the camera. That would be a very painful process, just like debugging the USB host was.

Vicon uses hard wired gigabit ethernet for its cameras. They probably get a huge reduction in latency, compared to wifi. They probably also do all the keying & a lot of object detection in the camera. Marcy 2 just does the keying in the camera.

Wifi doesn't have a noticeable effect on latency. In a subjective test, the left eye lagged slightly more than the right eye, but both had hardly any lag. Things would still be better if the next frame didn't have to arrive in order to discover the size of the previous frame.





Finally, efforts to control the Syma by computer worked. It merely took a simple RC filter to convert PWM to constant voltage to emulate the sticks. A 10k & 1nF were good enough. The Syma remote control ended up taking linear voltage. The Syma also bases stick center position on the voltage when it powers up.

Adding the tracking LED either made it nose heavy or the stick input is now off balance.
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