The Radio Data System: FM radio adds features to stay viable, part 4

Broadcast FM radio has extended its user-friendly capabilities through the addition of embedded, backward-compatible functionality which adds text and even graphics to the radio display.

This part concludes with additional RDS details and IC implementations.

Q: Where is the RDS signal in the FM-channel spectrum? Does it affect existing FM stereo broadcasts?
A: One of the design requirements for RDS was that it does not impact FM stereo radios in use or detract from their audio quality. The designers used a standard scheme of establishing another subcarrier in the FM channel, in addition to the one at 38 kHz one for the stereo L-R signal.

Q: How did they do this?
A: They placed the RDS subcarrier at 57 kHz, three times the stereo pilot-tone frequency (Figure 1). The RDS subcarrier is locked onto the pilot tone harmonic at the receiver, with the reasoning being that any radio with RDS is also a stereo radio with the 19 kHz pilot tone. Using a subcarrier that is at a harmonic of the pilot tone minimizes interference with the L+R and L-R audio signals.

Figure 1. Similar to the FM stereo approach, RDS adds another subcarrier to the channel spectrum, this one at 57 kHz (Image: Keysight Technologies).

Q: What is the encoding format?
A: The subcarrier is phase modulated using quadrature phase shift keying (QPSK) at a data rate of 1187.5 bits per second (bps), which is the frequency of the RDS subcarrier divided by 48. This rate enables the decoding circuits to operate synchronously, thus reducing problems with spurious signals in the decoding circuits.

Q: What are the details of the encoding of the data?
A: Data is transmitted in groups consisting of four blocks. Each block contains a 16-bit information word and a 10-bit check word as shown. This means that with the data rate of 1187.5 bps, approximately 11.4 groups can be transmitted each second.

The check word enables the radio decoder to detect and correct errors. It also provides a method for synchronization. The relatively long 10-bit check word is a concession to the realities of the poor signal conditions which often occur in cars or portable radios.

Q: Is there a single data format?
A: No. Different stations will want to transmit different types of data at different times, so there are 16 different group structures. The coding structure is such that messages which need repeating most frequently normally occupy the same position within groups.

Each type of group has to be identified so the radio knows how to decode the data correctly. This identification is provided by a four-bit code in the first four bits in the second block. The first block in a group always contains the PI code, while the PTY and TP are in block 2.

Q: This is getting complicated — is there more?
A: Yes, there is. RDS makes it possible for a station to identify what kind of program it offers. To do this, each RDS transmission also includes an RDS code designated PTY, indicating the Program TYpe. There are about 40 PTY codes such as News, Sports, Talk, Classic Rock, Top 40, Religious Talk, and so on.

Q: Are these PTY codes the same for PDS and RBDS?
A: No, they are not. In fact, the differing PTY codes are the main differences between the two.

Q: RDS and RBDS offer a lot of capabilities and options, but to what extent are they used?
A: The data is spotty and unclear, but it appears most users just use the basic indication of station call letters, frequency, song, and artist. Perhaps not surprisingly, many stations now sell part of their RBDS “space” by preceding the song name callout with a brief ad, usually from personal-injury lawyers or real-estate agents – certainly not the high-minded original intention.

RDS ICs
Q: How was RDS actually implemented in circuitry?
A: RDS followed the usual path of basic IC functionality that eventually was integrated into ICs having more functions. For example, the LC72725KVS is a now obsolete 16-pin IC from ON Semiconductor which implements the signal processing required by the European Broadcasting Union RDS standard and the US National Radio System Committee RBDS standard. It includes a band-pass filter, demodulator, and data buffer on chip. RDS data can be read out from this on-chip data buffer by external clock input in slave operation mode. Note that it is not a receiver or decoder, it is solely a demodulation and buffer IC.

Similarly, Philips Semiconductors offered the SAA6579 RDS demodulator in 1997. It provides a data signal and a clock signal as outputs for further processing by a suitable (microprocessor) (Figure 2).

Figure 2. The Philips Semiconductors SAA6579 RDS demodulator (1997) was just that, as it separates out the RDS signal but did not provide further decoding (Image: Philips Semiconductor via Digi-Key).

Q: What was the next step in integration?
A: Adding the RDS function to an FM receiver IC was the next step. Silicon Labs offered the Si4706, a CMOS IC which integrates the complete FM and RDS/RBDS data-receiver function from antenna to analog or digital audio and data out in a single 3 × 3 mm 20-pin QFN (Figure 3). Note that Silicon Labs sold this product line to Skyworks Solutions in 2021.

Figure 3. The Silicon Labs Si4706 (now from Skyworks Solutions) is a complete antenna-to-digital output FM receiver IC including RDS demodulation, error correction, and decoding (Image: Skyworks Solutions).

The RDS engine includes demodulation, symbol decoding, advanced error correction, detailed visibility to block-error rates (BLER), advanced decoder reliability, and synchronization status. The Si4706 provides complete, decoded, and error-corrected RDS groups (100 blocks), up to 25 groups at a time.

Q: What is the status of RDS receiver ICs now?
A: The automotive radio situation has changed dramatically in the past few years. Radios are no longer a distinct, independent add-in unit for the console. Instead, their functions are integrated into the overall console and user display along with many other ADAS (Advanced Driver Assistance Systems) and infotainment functions. As such, auto vendors rely on their Tier 1 suppliers who, in turn, use custom ASIC devices to incorporate as many of these console functions into as few ICs as possible.

Conclusion
RDS and RBDS make broadcast FM-stereo receivers more user-friendly by adding basic features such as station ID, song name/artist, and more. It adds features that users now expect when they use other audio sources such as podcasts, streaming, downloaded music, CDs, and more. While it requires a radio with added functionality, it does not affect or interfere with older FM mono/stereo radios which cannot handle the encoding signals.