I notice by zooming in on the waterfall display (Pixel = 1) that, 
despite my computer clock being synchronised with a time server, the 
transmissions seem to start about one second before the waterfall 
line indicating the start of the time period, and end a few seconds 
before the next line.

Do others see this, or is there space before and after the 
transmission on your waterfall? Is this a clue to my unreliable 
reception (ie that my receive setup is slightly behind the 
transmitted signals)?

MIke, G3XDV