The Radio Data System: FM radio adds features to stay viable, part 2

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 looks at other radio-receiver upgrades that preceded RDS.

Q: Is this the first time that FM radio has been “upgraded”?
A: No, not at all. In the 1970s, broadcast FM went stereophonic. It did this by setting up an all-analog encoding scheme that allowed the received, if equipped for stereo, to produce separate left and right audio channels.

Q: But did this make existing FM radios obsolete if they received a stereo-encoded signal but were only monophonic designs?
A: Again, no. The designers of stereo FM radio knew that obsoleting the millions of FM radios in use was unacceptable. Instead, they used a simple but effective encoding scheme to maintain compatibility.

Q: How did this work?
A: In brief, the transmitter encoded (modulated) a left+right (L+R) sum signal at baseband and modulated an L-R difference signal modulated around a 38-kHz subcarrier, which is the second harmonic of an added 19-kHz pilot tone which was also transmitted (Figure 1).

Figure 1. To add stereophonic capability to monophonic FM while retaining backward compatibility, designers used the baseband to convey the L+R analog signal and added a subcarrier at 38 kHz for the L-R signal (Image: Audio Misc/UK).

The standard monophonic receiver would just demodulate the baseband L+R signal, while the stereo receiver would implement an analog sum of both the baseband and subcarrier signals. The result of the addition would be a (L+R) + (L-R) = 2L left-channel-only signal. The receiver would also invert the subcarrier L-R signal and add it to the baseband signal, yielding (L+R) – (L-R) = 2R for the right channel.

Not only was this scheme fully compatible with existing mono-only FM receivers, but it was also invisible to users of either monophonic or stereophonic receivers. In a few years, FM stereo receivers were commonplace, at a modest cost increase over mono-only radios.

Q: Is this the only case of maintaining upgrade compatibility which mass broadcasting has achieved?
A: No, as perhaps the greatest such achievement in compatibility was the upgrade from monochrome (black and white) TV to color TV in the 1960s. The overriding challenge was this: color-encoded signals had to fit into the existing 6-MHz channel-spectrum allocation and appear in color on the color TV screens, of course. The images also had to appear in proper gray-scale rendition on the millions of B&W TVs in use.

At the same time, transmitted signals which were only monochrome encoded would have to be rendered in proper grayscale on the new color TVs but without color fringing or artifacts. The reason for this was that many mid-size and smaller TV broadcasters were not switching over to color transmitters immediately due to cost and many other technical and practical issues.

Q: How was this problem solved?
A: It’s a very complicated story involving interleaving color-related signals in time and frequency, phase encoding, a nominal 3.58 MHz color subcarrier, non-obvious non-RGB color concepts, the eye and color interpretation, complex synchronization for vertical and horizontal scans, advanced video-signal encoding and decoding, and more (Figure 2).

Figure 2. An extremely innovative and insightful technique was devised to fit color-signal encoding into the existing 6-MHz bandwidth of the monochrome analog TV signal while maintaining backward and forward compatibility (Image: Science Direct).

The challenges in design, creation of totally new components including color image-capture tubes (vidicons) and displays (CRTs) and the factories to make them, new topologies, and manufacturing development cannot be overstated. The work was done almost entirely by Radio Corporation of America (RCA) under its dominating leader David Sarnoff, who spent today’s equivalent of several billion dollars of their own money to realize this dream. The result was a vertically and horizontally integrated effort ranging from filming to broadcast studios to consumer TVs.

Q: All this may be interesting, but what does it have to do with RDS?
A: Perhaps not much directly, but the relatively poor market experience with DAB led developers of the RDS system to ensure that it would not adversely affect the existing FM radio base and would also be transparent to users of both the existing radios and simple to use with the newer RDS-compatible receivers. In other words, “if you like your existing FM stereo or mono receiver, you can keep it and need to pay no attention to RDS” (today’s software and system creators could learn a lesson from what those old-fashioned analog engineers were able to accomplish and how they maintained compatibility with the upgrades).

The next part of this article looks at the details of RDS.