That thread on GiantCod is very interesting. I wonder if Allan can pass on a question to Bodmer for me (I'm on too many forums already). I'd like Bodmer to repeat the graphical test with a Spektum DSM2 setup for us and post it up to see. Id like to see what the wave shape looks like in comparison to FrSky.
Other random thoughts after reading that thread, to be taken lightly...
My Futaba 8U's (AKA FF8) both out put a very accurate 22.2mS frame (as in both the same period). I think from old conversations with FrSky, Corona and FlyDream, they used Futaba gear mostly while prototyping. This may have lead to some rash assumptions... Why not, I did too. I thought that 50Hz rate or 20mS frames was the 'standard'. I was caught out by it with FlyDream modules when I made my PPM sweep generator (used for range tests), I found that it would not work with them till I slightly increased the frame rate to closer match Futabas.
I measured my FrSky Two-Way RX servo frame period as well tonight, and I get 18.1mS. Slightly less than the Futaba frame rate of 22.2mS. More servo pulses out per second than PPM frames in.
Could it be also that the FrSky module is just taking too long in sending a frame and also decoding the telemetry, and missing the next sync pulse on these short frame handsets ?
Given the module is also decoding the telemetry data and sending it to the comm port, it could be that it's fairly busy right after sending a frame and runs out of time on the handsets with fast frame rates. The result of missing the sync pulse would be dropping a full frame(and resending the previous one perhaps).
Maybe FrSky's engineering people need to add some debug traps into their TX module code to see if it's dropping any incoming PPM frames ?
Question.
How would you counter this theoretical problem ?
Faster RF frames, say 10mS (and 20mS servo pulse periods at the TX) ?
Syncing the TX module with the incoming PPM frame rate and RF frames also ?
That one might be hard, as I think the RF rate cannot be changed once the system is designed. The spread spectrum system demands very precise timing at both the TX and RX.
Digital filtering at the RX ?
Delay the servo pulse output by a couple of frames and look at the slew rates of them and smooth it out. Good, but that means 20-40mS latency, and just as many modelers will not like that as this issue.
So how did Spektrum, Futaba and other sort this out ? High latency ?
More questions than answers, but I'm good at that
Martin