Hey BDS, more Peter territory than me but I agree, dot should mean out (i.e. lets not stuff with the signal here so clean..). Turning on a low pass filter will lets low freq's (~20hz - ? ~~1khz?) to pass up to whatever the filter is set to (this is turning off highs), high pass is the opposite, highs get through lows don't (depending where the cut off freq is set). Depending what the low pass filter is set to, it may make no or little difference.