This has been going on for over 25 years that I know of. But I don't think it was digital. AM and FM radio had a service for the blind that was a sub carrier. Sounds very close to it.
At 06:29 PM 11/12/2007, you wrote: >In the U.S. the FCC has approved a system called IBOC (In-Band On Channel) to >add digitial audio to existing AM and FM stations. In broadcast radio, there >isn't the luxury of unused channels that allow every station to have one >analog and one digital transmitter. I haven't seen any terrestrial digital >radio receivers in stores yet.
