I've scripted a very ugly hack for this, but I'm not very happy with it:
$insert($div(%samplerate%,100),'.',$sub($len(%samplerate%),3)) kHz
Left to say ... it works!
But I'd rather have a $frac() function to get the fractional part right of the point.
(You did note that $div() does an INTEGER division, didn't you?
