Those who have followed my posts know that I've experimented extensively using RDS with Part 15 transmitters. Actually, RBDS - the North American equivalent.
I'm not going to go into what RDS/RBDS is here. Suffice it to say that it is a very low bitrate data stream encoded onto a high frequency subcarrier of an FM signal, that carries such information as your station name, the current song being played, the genre of the station, etc. Most modern car radios can decode the data signals and display the information on the console. Some home radios can as well, and it's becoming more commonplace.
RDS signals can be generated using either hardware or software. The hardware solution is expensive, and it ultimately has the same output as the software solution. While there are pay ware RDS programs, there are also open source (i.e., free) ones. The free ones are generally simpler, and have limitations.
I ended up modifying the source of an open source program called JMPX to overcome some of its shortcomings (namely, getting the name of the currently playing track into it from the automation software I use), and it works nicely. It also allows me to generate a stereo signal that can be transmitted with a mono transmitter (but I don't use that feature).
The usefulness of RDS/RDBS in Part 15 broadcasting is questionable, however, and this observation is a direct result of my experiences.
First, to generate the appropriate signals on the computer, you need a DAC that will sample at 192Khz. They're relatively easy to come by and inexpensive. You just have to ensure that your DAC will support that sample rate, and that it is set to that sample rate in your settings.
Second, the RDS/RDBS signals take up a portion of your overall signal from the transmitter. JMPX allows you to set the percentage - I run mine at 5 or 6%. That makes a measurable difference on your listenable audio range. Not huge, but it's there. It would be more noticeable for Part 15 rules than Canada's greater BETS rules.
And finally, the bitrate and strength of the RDS/RDBS signals are, correspondingly relatively weak and error prone. Realistically, you won't be able to receive and display them nearly as far as you can listen to your audio. I found that if my BETS signal went 1/4 mile, about 1300 feet or so, the RDS/RDBS signal was stable at about 100-150 feet. All this on my car radio, which is much more sensitive than most other radios. And that was only with the car parked - moving about introduced errors in the RDs/RDBS data stream from interference, and reduced the range even more.
So, basically you have to be almost right beside a Part 15 transmitter to reliably receive RDS/RDBS data.
Finally, I found that some transmitters had difficulty passing through the RDS/RDBS frequencies (when they convert the incoming analog signal to digital, they must use lower sampling rates). The Decade MS-100 and the Retekess TR-508 did it reliably. The Whole House 3 did pass them through (I managed to get genre data at about 150 feet), but attenuated so you had to be even closer to the transmitter to get the other data. I never did receive any data from the Broadcastvision AXS-FMT, so it's unclear if it passes any of those frequencies through.
So, in conclusion, if you are using a Part 15 transmitter, RDS/RDBS is not going to be all that useful unless you have your radio receiver extremely close to that transmitter. With BETS, it's a bit better, maybe useful throughout a house, but outside in a car it doesn't have much utility, particularly if you're moving.
It's just neat to know that it's there.