Hi Andy, I suppose this would be confusing to someone not used to FM operation on a multi-mode rig. In all modes, ALC is a function of transmitter output power. In SSB modes we keep our transmitted digital signals clean by running our transceivers below the power level that causes ALC to be generated. In FM, however, the transmitted signal is a constant level carrier. The audio modulation from the sound card causes the frequency of that carrier to 'deviate' above and below its unmodulated frequency. What we care about on digital FM is how clean the audio at the receive end is. The ALC reading reflects only how much RF we are putting out, not what the modulation looks like at the receiving end. Remember that essentially all voice FM transmitters use some sort of peak audio clipping to limit how wide our modulation 'deviates' to keep it inside the pass band of the receive IF filters. When we run digital FM, we need to keep the audio drive low enough that we are not driving the transmitter audio section into clipping.
Setting the audio drive level for digital operation with an FM transmitter is not quite as simple as for SSB. We really must monitor the actual transmitted signal while we make that adjustment. That can be tricky without a deviation monitor but it can be done by comparing your own digital modulation to other known good signals. That said, though, a little extra distortion is not quite the problem on FM it is on SSB. On FM, audio distortion will generally not interfere with other stations. It may look bad on the water fall and may not decode quite as easily as if the level was correct but it will likely still work OK. That is why we can get away with just holding a handy talkie up to the computer speaker to operate digital modes on VHF FM. That make sense? Gary - N0GW --- In digitalradio@yahoogroups.com, Andy obrien <k3uka...@...> wrote: > > Hmmm. Well I thought it would be simple enough to transmit digital > modes on 2M FM but one issue I just ran in to is the ALC is very high > and my usual method of lowering it has no effect. I also lowered the > mic gain but that had no impact. Something simple I am not taking in > to account when using FM ? > > Andy K3UK >
