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Option AGC |
The AGC (Automatic Gain Control) in the RTTY decoder is a double-edged sword. It attempts to maintain a constant input signal level so that the subsequent filters and decoding logic always operate at an optimal amplitude.
Here are the scenarios when you should turn it on or off:
Turning AGC ON is useful in the following situations:
Fluctuating signal strength (fading/QSB):
This is the primary reason for using AGC. If the signal constantly fluctuates in volume due to propagation conditions, AGC automatically compensates for these differences. You don’t have to constantly adjust the volume control or gain slider.Band scanning (tuning):
When scanning the band and encountering stations with vastly different transmit powers, AGC ensures that each station is processed immediately at the correct level.Convenience:
For normal operation, AGC is a “set and forget” feature. It ensures that the decoder doesn’t clip when a signal suddenly becomes very strong.
Turning AGC OFF is useful in the following situations:
High noise levels during pauses (noise pumping):
During transmission pauses, the AGC only detects the faint noise and attempts to amplify it to the target level. This causes the noise to become extremely loud during pauses (“pumping”), which in turn can lead to many incorrect start bits and garbled characters.Strong neighboring signals (QRM):
If a very strong station appears directly next to your desired signal, the AGC reacts to this high overall energy and reduces the level. This pushes your actual (weaker) signal below the decoding threshold. In this case, it’s better to turn off the AGC and manually adjust the level so that your desired signal arrives optimally.Strong atmospheric disturbances (rattling/static):
Short, extremely loud crackling noises (e.g., from thunderstorms or electrical devices) can temporarily “block” the AGC. The gain drops and takes a moment to recover, during which time valuable signals are lost.Using the radio’s internal AGC:
If your radio already has a very good, fast-acting AGC, unwanted oscillations or regulation effects can occur when two AGC systems are connected in series. In this case, it is often cleaner to disable the software AGC.
Summary & Recommendation
Default: ON
Most users prefer the convenience of automatic gain control.Manual OFF
If you are a professional user who wants to maximize your signal-to-noise ratio (SNR), turn off AGC, set your radio to a fixed gain (RF gain), and manually adjust the level in the app using the gain slider so that the “crossed bananas” or the tuning meter are in the optimal range.
Pro Tip
When turning off AGC, make sure you have enough headroom at the decoder input so that loud pulses don’t distort the signal.