AGC sets a gain value that is a bit higher compared to what you can set manually. This is actually a kind of a bug, according to a post by @obj .
AGC also ends up setting the highest possible gain value. This is due to the nature of the ADS-B signals, many short messages, compared to the TV signals theses dongles are meant to receive.
It’s pretty horrible actually…
The R820T tuner has three gain stages: LNA, mixer, VGA.
If you ask librtlsdr for a manual gain value (or “maximum”, which picks the largest manual setting) there is a table that sets fixed gains for each stage based on the requested total gain.
Setting --gain -10 asks librtlsdr to use automatic gain settings in the tuner. For a R820T tuner, librtlsdr uses these settings:
LNA gain is set to be controlled by the tuner’s AGC
Mixer gain is set to be controlled by the tuner’s AGC
VGA gain is set to a fixed value
The tuner’s AGC (in hardware) samples the input power (probably at the tuner output? I can’t remember…) and adjusts the LNA/mixer gain based on that. Unfortunately it is not well suited to ADS-B traffic (which has short bursts and highly variable power levels), it’s designed more for DVB-T signals (continuous signal, stable power level). So what usually happens is that the tuner’s AGC sees what seems to be a very weak signal and so sets LNA / mixer gain to maximum.
So the net effect is that “–gain -10” actually sets a fixed gain (!)
So you should interpret “–gain -10” as something more like “–gain 55” (I can’t remember the exact value, it’s a little higher than the usual maximum of 49dB)
