I was reading they started using a new protocol modulation for newer receivers. Some really old receivers may stop working. This is lower frequency band, really low, but not as old as old stations that used to transmit signals to subs. That was audio frequency. I have not listened o shortwave wwvb in a long time. The time code could be read by the changing of tones, really a low speed, but took less than a minute, then a 1 minute sync signal.
Greg