Dear Gary, LF group,

The FFT process of the spectrogram does a very good filtering job, and the
sound card audio input has a surprisingly good dynamic range, being able to
cope with unwanted signals maybe 70dB or more stronger than the wanted
signal (at least, on the sound cards I have tried, which are nothing
special). The limiting factors usually seem to be distortion in the RX IF
and audio, and audio frequency hash which gets into the sound card from the
rest of the computer; unfortunately an external audio filter is not much
help with either of these. I find that using a very narrow IF or audio
bandwidth usually worsens the effect of QRN, something in the range 200 -
1000Hz seems best. Where a very narrow bandwidth external audio filter IS
useful is for aural CW reception in the presence of QRM such as carriers or
other CW signals.

Cheers, Jim Moritz
73 de M0BMU