SmartAudio: VTX_MAX_POWER and RCx_OPTION = 94

twistedwings · October 6, 2021, 2:03pm

How does RCx translates into a power level ?
In other words: what is the scaling of RCx ?
I didn’t get that from the documentation.

How does VTX_MAX_POWER translates into a power level ?
Every vtx has different power levels.

andyp1per · October 6, 2021, 2:08pm

It’s a dodgy heuristic based on switch position and the value of MAX_POWER:

github.com

ArduPilot/ardupilot/blob/43e83f18bee60f5b458eb4e85ada7bc2c8b36de8/libraries/AP_VideoTX/AP_VideoTX.cpp#L362

    
      
          void AP_VideoTX::announce_vtx_settings() const
          {
              // Output a friendly message so the user knows the VTX has been detected
              GCS_SEND_TEXT(MAV_SEVERITY_INFO, "VTX: %s%d %dMHz, PWR: %dmW",
                  band_names[_band.get()], _channel.get() + 1, _frequency_mhz.get(),
                  has_option(VideoOptions::VTX_PITMODE) ? 0 : _power_mw.get());
          }
          
          
// change the video power based on switch input
          // 6-pos range is in the middle of the available range
          void AP_VideoTX::change_power(int8_t position)
          {
              if (position < 0 || position > 5) {
                  return;
              }
          
          
    uint16_t power = 0;
              // 0 or 25
              if (_max_power_mw < 100) {
                  switch (position) {
                      case 3:

twistedwings · October 6, 2021, 2:33pm

thank you for the reply.

If I got it right RCx is translated into 5 positions (ranging from RC_MIN to RC_MAX).
Than the position is translated into a power using VTX_MAX_POWER and heuristic.

I am not a SA expert but afaik the power is set by a power level.
I am using an Eachine VTX03S with 5 power levels.
0 1 2 3 4
0 25 50 100 200
If I set VTX_MAX_POWER to 200, I can switch to power level 2 (50mW) max.
If I set VTX_MAX_POWER to 1000, I can switch to power level 4 (200mW) max
but ArduPilot says “vtx power changed to 800mW”.

I assume something like betaflight vtx tables is needed to translate power levels into power.

andyp1per · October 6, 2021, 3:28pm

Should be up to 6 positions. Generally you should be able to set VTX_MAX_POWER to your actual max power and access your max power. But the mapping of level to power is fixed (SA 2.1 gives power in db so much easier).

It’s all a hack and fairly high up on my list of things to do when 4.1 is out the door. Tables is one way but takes a bunch of configuration and flash. It should also be possible to learn the values which is my preferred approach. SA 2.1 also publishes the supported table so I need to support that as well.

twistedwings · October 6, 2021, 5:00pm

I see.
At least it works for now with the exception of the misleading power information.
Looking forward for the improvements.