Recall a few years ago when WWV was transmitting the voice announcements on slopbucket + carrier? The announcements were barely intelligible after they replaced their old transmitters and started using SSB with carrier.
I noticed that sometime later they became intelligible once again, but just assumed they had adjusted the modulation and carrier levels to get the best compromise between modulation depth and distortion. But it looks like the went back to true DSB AM.
Modulation
The signals broadcast by WWV use double sideband amplitude modulation. The modulation level is 50 percent for the steady tones, 50 percent for the BCD time code, 100 percent for the second pulses and the minute and hour markers, and 75 percent for the voice announcements.
http://www.boulder.nist.gov/timefreq/stations/wwv.html
