RDS Encoder

The RDS encoder takes data from the computer, like the song name, and generates an RDS signal which is then sent to the FM transmitter. I searched the web for a RDS encoder and I only found one, designed and built by a guy in the Czech Republic who runs a site called Pira.cz. I wanted to learn while I was doing, so I decided to order the PIRA32 kit. Building this kit was a new experience as I had never installed SMB (surface mount devices) before. The kit looked like this:

Here is a look at the SMD parts, and a blurry picture of my soldering job:

I studied the schematic and found that the encoder has two power voltages on the board, 5 volts for the digital portion, and 8-20 volts for the analog (signal) section. I could have build the board as specified, but then I'd have to find yet another wall-wart power supply. Instead, I realized that I have a perfectly good 5 and 12-volt power supply in my computer, which of course will be on all the time I'm transmitting, so I didn't solder the 5-volt regulator, but hooked it up to a "disk drive" style power connector. I also found a suitable case from Halted:

As it turned out, I found that getting the power from the computer is a bad idea. A computer power supply is stable enough for digital devices like disk drives, but it has too much noise to be used in a device that will connect to an audio device like an FM transmitter. After I learned my mistake, I removed my computer power cable and installed the voltage regulator and power connector as specified in the installation manual.

The RDS Encoder has two connections to the FM transmitter labeled "In" and "Out". The reason is that an FM stereo transmitter uses a 19kHz pilot signal to transmit stereo, and the RDS signal uses 57kHz. These two frequencies need to my synchronized or it could lead to noise or bad RDS reception. Few FM transmitters have a way to synchronize their 19kHz pilot signal, so the RDS encoder has to sample the 19kHz signal from the FM transmitter and synchronize its 57kHz signal to that. That is the purpose of the "In" jack.

Another fact about FM transmitters is that none have a convenient port to extract their 19kHz pilot or to inject a 57kHz signal along with the audio. So in order to use this RDS encoder, I had to modify the radio to which it is connected.