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Old 08-04-2017, 08:56 PM   #1 (permalink)
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Default PPM input error checking

Does the 3digi have any signal filtering or error checking that would increase input delay when using a ppm signal?

I'm using a hitec optimaD receiver, and it allows me to switch between sbus and ppm output. My TX outputs a ppm signal. The receiver does the sbus conversion.

I'm trying to figure out which method of connection is likely to be the fastest.
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Old 08-05-2017, 02:38 AM   #2 (permalink)
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The CPPM is checked for validity like any other input signal when it is received.
After that the processing is the same for every input signal type.

SBUS may be a little bit faster, because its data packages are usually sent more often (framerate) by the receiver than CPPM packages.
You can check the current framerate of your input signal in the Live Data. There are 2 small boxes RC1 and RC2 at the bottom of the page. If you move the mouse pointer over them, a small tooltip will open, that shows infos about framerate and number of received channels.
So you can compare your 2 different input types.

But I donīt think you will feel much difference in flight.
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Old 08-05-2017, 01:40 PM   #3 (permalink)
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Quote:
Originally Posted by DirkS View Post
The CPPM is checked for validity like any other input signal when it is received.
After that the processing is the same for every input signal type.

SBUS may be a little bit faster, because its data packages are usually sent more often (framerate) by the receiver than CPPM packages.
You can check the current framerate of your input signal in the Live Data. There are 2 small boxes RC1 and RC2 at the bottom of the page. If you move the mouse pointer over them, a small tooltip will open, that shows infos about framerate and number of received channels.
So you can compare your 2 different input types.

But I donīt think you will feel much difference in flight.
That's a neat feature. I was wondering whether it would be possible to display the quality of the signal as "very good", "good", or "bad", instead of displaying it as a hex number. The former would be more informative for the user.
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Old 08-06-2017, 01:49 AM   #4 (permalink)
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Quote:
Originally Posted by The_crack_fox View Post
Does the 3digi have any signal filtering or error checking that would increase input delay when using a ppm signal?

I'm using a hitec optimaD receiver, and it allows me to switch between sbus and ppm output. My TX outputs a ppm signal. The receiver does the sbus conversion.

I'm trying to figure out which method of connection is likely to be the fastest.
CPPM sends (usually) 8 servo pulses in sequence followed by a (minimum of a) 4ms blanking pulse so you have a 20ms latency with a CPPM signal. The actual frame lenght is somewhat flexible some systems will send the 8 pulses and a 4ms or 4.5ms blanking pulse which means depending on the channel widths it can be shorter (the width of each channel pulse depends on the channel position from 1ms - 2ms) and some will simply fill any remaining space in the frame lenght with the blanking pulse.

Basically, CPPM is the signal that old 72mhz PPM radio systems would transmit from the radio to the receiver on an FM carrier wave. The receiver decoded the signal back off the carrier and sent each pulse to one of the reciever ports in sequence and the blanking pulse would reset it back to channel 1 again. The number of channels could vary form 2 to as many as 12, though more channels - higher latency of course.

I assume you have an Aurora 9?

If your transmitter outputs a CPPM signal to the radio module that doesn't mean that the radio module is sending a CPPM signal to the receiver, likely it's just getting the channel positions from the CPPM signal and whatever protocol exists between the radio and receiver is some sort of packet based serial digital signal and then the receiver creates whatever signal is required from the channel position data. It may or may not make sure that the CPPM frames line up at the transmitter and receiver ends. If I remember correctly the latency on the Aurora 9 is around 50ms and that would seem to indicate that the CPPM frames aren't synchronized.

Depending on whether or not the CPPM frames are synchronized at the transmitter and receiver sides sbus may or may not be faster than CPPM . However, sbus is a serial digital signal with error correction and is more resistant to electrical noise where a CPPM signal is really just 8 analog PWM servo pulses sent in sequence with a blanking pulse so whatever the case sbus is the better option.

Last edited by Atomic Skull; 08-06-2017 at 02:07 AM..
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Old 08-06-2017, 06:48 PM   #5 (permalink)
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I'm using an A9X tx. I have a maximaSL receiver which is supposed to be much faster, a true sbus link. I like the voltage telemetry on the optimaD though. I have it connected to the receiver battery. Stops me getting greedy and trying to squeeze one more flight out of a low battery.

I'll try the maximaSL and see if I can feel a difference.
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Old 08-06-2017, 11:09 PM   #6 (permalink)
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I'm using an A9X tx. I have a maximaSL receiver which is supposed to be much faster, a true sbus link. I like the voltage telemetry on the optimaD though. I have it connected to the receiver battery. Stops me getting greedy and trying to squeeze one more flight out of a low battery.

I'll try the maximaSL and see if I can feel a difference.
Are you sure then that the Aurora 9x hardware outputs a CPPM signal? I had heard that they had eliminated the separate radio module on the 9X and that it had greatly reduced latency compared to the Aurora 9 which would seem to indicate that it was no longer using a CPPM signal to feed the channel positions to the radio hardware.

The reason the Aurora 9 did it this way was that it was originally a 72 mhz radio and they just upgraded it to a 2.4ghz radio by changing the module (this is why the antenna was external and mounted on a screw in post, that was where the 72mhz antenna used to go)
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Old 08-07-2017, 03:27 AM   #7 (permalink)
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The new A9x has a menu to select the type of receiver you're using for each model. There are 3 options.

SLT, for bind and fly models with the Tx-R protocol. I've never seen one of these in an australian hobby store.

Optima/Minima mode, same as the old A9 = PPM out from tx.

Maxima mode, low latency sbus. These don't do telemetry unfortunately.
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