Apparently a standard FDM scheme for telephone signals is to multiplex 12 voice channels, each occupying 4kHz bandwidth, into a group signal with 48kHz in the range 60-108kHz, using subcarrier frequencies from 64-108kHz in 4kHz increments, and using only the lower sideband of each modulated signal. This all seems to make sense -- the arithmetic works. Five of these group signals are then multiplexed together into a "supergroup", again using only the lower sideband of each modulated signal, to produce a combined signal with a 240kHz which occupies the 312-552 kHz band. So far so good. The problem is, supposedly the subcarriers have frequencies from 420 to 612kHz in 48kHz increments. How does a signal with 48kHz bandwidth, modulated on a 612 kHz subcarrier, end up below 552kHz? There seems to be a downward shift of 60kHz but I can't find an explanation of this anywhere. I read this in William Stallings' Data and Computer Communications (7e), and it's repeated in numerous places on the web. Some of them are definitely just taking it from Stallings; others, well, they may be taking it from him too, but they don't make that clear. In any case, none of them explain this shift. Is this just an error that's being copied over and over, or is there a good explanation that everybody is simply failing to mention?