From http://www.nist.gov/pml/div688/grp40/wwvb.cfm
The carrier power is reduced and restored to produce the time code bits. The carrier power is reduced by 17 dB at the start of each second, so that the leading edge of every negative going pulse is on time. Full power is restored 0.2 s later for a binary “0”, 0.5 s later for a binary “1”, or 0.8 s later to convey a position marker. The binary coded decimal (BCD) format is used so that binary digits are combined to represent decimal numbers.

Anyway, I think that, probably and without too much analysis, a dedicated hardware is easier to manage than ADC and all the software needed, only to detect 17 dB of signal strength change (a lot, by the way).

Daniel