Espritmodel.com Telemetry Radio
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Old Aug 02, 2012, 09:58 PM
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Transmitter Resolution - What Does It Mean?

Conversations at the field really show how little I know ...

First off, I get that a Tx / Rx pair feeds the servo signals telling the servo where to position the arm between 0 and 180 degrees based on the length of a pulse (from 1 ms for 0 to 2 ms for 180). And that the pulses are transferred to the servo in a frame every 20 ms or a frame rate of 50 frames per second. So the Tx / Rx pair can tell the servo to position the arm at a specific location 50 times a second. Cool. (I understand that 20 ms is sometimes 22 ms, or 14 ms or 11 ms or 7 ms depending on the Tx in question - just using 20 ms as an example)

How small of an increment can be sent? I can see a servo that is either open or closed having to deal with just 2 signals - pulse of 1 ms and pulse of 2 ms. So either the arm is positiioned at 0 degrees and stays there or it is positioned at 180 degrees when it gets the 2 ms pulse. If the increment allows the arm to be positioned at each degree from 0 to 180 then there would be 181 different length of pulses to tell it the different positions. If you wanted 1/2 a degree positioning between 0 and 180 - then what - 361 different length of pulses?



So what does transmitter resolution mean? I have a DX7 (not SE) which has a resolution of 1024. The DX7SE has a resolution of 2048. The SE has a finer resolution. OK fine.

What does "resolution" have to do with the ability of a Tx / Rx pair sending "arm positioning pulses" to the servo?

I hope this makes sense to somebody and that that somebody can shed some light on this thing called resolution.

Thanks for sharing ...
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Old Aug 03, 2012, 03:33 AM
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Hi.
A transmitter tells the receiver where to instruct the servos to go. On a 1024 system, there are 1024 positions available. The receiver converts that number for each channel into a proportional pulse for each servo.

The '1024' number has to accommodate full stick movement, full trims and full throws. You only use all 1024 positions when everything is at an extreme. In other words, you normally use much less than 1024 steps on a 1024 radio.

The amount of servo rotation for each change in position is constant; ie each step is a fixed size. However, the amount of rotation on a 1024 radio will be double that of a 2048 radio. In other words, both may move the servo through 180 degrees but the 2048 system has finer control.

One Tx trim click is about 4 of 1024 steps but this can be adjusted on more expensive radios.

The DX7 and DX8 can have 150% travel each way. This setting gives you the potential for 1024 steps. At the default 100% settings you only get 2/3 throw (100 out of 150% max) from say position 170 to 853 with 511 the center position. So in normal use at default settings you only use about 683 steps of servo resolution with a 1024 radio.

The DX6i only has 125% Travel Adjust each way. So the servos can be made to move between positions 85 to 938. The DX6i cannot make the receiver generate pulses corresponding to positions 0-84 and 939-1023.

The numbers I've used may not be exact but hopefully help explain how the settings work and interract.
Regards, David.
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Old Aug 03, 2012, 05:51 AM
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As the pulselenght is an analog value, it can theroetically have an infinite amount of steps. This was the case at old systems, which were all analog from the poti of the RX to the servo.

Nowadays, the TX electronics are digital and therefore have some kind of resolution, which they superimpose on the pulselength.

My Jeti module, for example, has a 15bit resolution. My TX, however, has only 1024 steps. The 15bit are not really useful, but at least they provide a nice oversampling.
Modern servos again have digital electronics and sample the pulse length again, and the best I have heard do this at 4096 steps.

From what I have experienced, even on nicely built models, the rudder slop is still much bigger than the 1024 resolutions steps.

So with todays electronics, we really do not need to worry about resolution.
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Old Aug 03, 2012, 03:58 PM
k2k
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The basis for the resolution numbers has to do with the analog to digital converter (ADC) integrated circuit chip. As stated by others, the voltage from the transmitter controls needs to be converted to digital format. The simplist ADC would only be able to discrimiate 2 levels (on or off). Add another bit to the ADC and it can now discrimitate 4 levels. This is a binary device so it is based on powers of 2 ( 2^1, 2^2, 2^3 ...). So, a 10 bit ADC will have 1024 possible levels, an 11 bit 2048, and a 12 bit 4096. The higher the resolution the more closely the output matches the stick movement.
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Old Aug 03, 2012, 04:05 PM
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I answered the different resolutions over at his cross-post on RCU.

Andy
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Old Aug 03, 2012, 04:38 PM
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Here's Andy's reply ...

Transmitter resolution is how accurately the transmitter can measure your stick positions.

Receiver resolution is how the receiver can generate pulses from the transmitter. This is why you see Spektrum radios talking of 1024 or 2048 resolution. It is sending the data out to the servo in discrete steps of either 1024 or 2048 steps across the range 900 to 2100 microseconds.

Servo resolution is how accurately the servo measures the pulses coming out of the receiver.

In the real world is doesn't make much difference - the mechanical aspects of the rest of the system (in the model) rarely come anywhere close to what 2048 can deliver.

BTW, on JR and Spektrum servos 60 degrees is 100% travel, 90 degrees is 150% travel, and 150% travel corresponds to the full 0-2047 steps.

Andy
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