Show RX signal level as a percent score

[geeksville]

Currently we are showing raw received signal levels. In screen.cpp change that to scale based on the following roughly measured values to something that looks like a % value betwen 0 and 100:

• rx signal measurements -3 marginal, -9 bad, 10 great, -10 means almost unusable. So scale this into % signal strength.

[mnantel]

SNR is logarithmic, right? Also, I have seen values of +11 reported - I have not managed to test the lowest possible value.

[Professr]

Yes, I should have made that exponential (sigh). I've occasionally seen 11 too if the units are right by each other, but in practical terms anything over 10 is pretty ideal

[Professr]

Check my math here, but SNR is logarithmic with respect to 0 so we have to calculate the log of its absolute value and then return its sign. If our range from "bad" to "good" is -10dB to 10dB, we can expect the log value to be -1 to 1. Anything less than -1 is just as useless as -1, and anything above 1 is probably in unusually-ideal environments in close proximity, so I'm clamping the scaled offset to the 0-100 range:

float snrLinear = log(abs(node->snr)) * (node->snr / abs(node->snr)); snprintf(signalStr, sizeof(signalStr), "Signal: %d%%", clamp((int)((snrLinear + 1.0) * 50.0), 0, 100));

Or would the SNR already be a logarithm and I'd need to use it as an exponent (can you tell I'm bad at math)? If so, the algorithm would become: float snrLinear = pow(node->snr, 10) / 1e10;

[mnantel]

I am not much of a mathematician nor good enough a developer to critique :) I was merely passing an observation as I considered how I would have tackled this. No doubt this makes more sense - I’ll test it out later on !

[Professr]

A SNR of 10 vs. 9.75 is an 11% drop going by an exponential scale. I wonder how the inverse square law comes into play here, could the relationship between dB and distance be somewhat linear? I guess it all depends on what we're trying to measure here; the underlying data might not have a straightforward relationship to a practical percentage :\

[Professr]

The tbeam radio looks like it supports reading RSSI, which would be much easier to convert to a percentage. I don't know if that's supported across other chipsets though. If we wanted to use it, we'd have to add a field to the MeshPacket definition, and I think that's a much more involved project than expected. If we want to use a linear scale, my PR supports that. If we want to use exponential, let me know and I'll commit the changes. Otherwise, I'll close the PR and wait for RSSI support 👍