In this installment of our RDS series, I’m going to focus on RDS injection rate and pilot synchronization. In the Oct. 20 issue, we went in depth with Program Service name scrolling on your stations. Read the first three articles at www.rwonline.com/article/99554.
RDS runs as a small 57 kHz digital data subcarrier on your FM station. When installing your RDS encoder, you have options to control its overall output level.
Get the Most Out of RDS This is the fourth in a series of articles. Read past stories at www.rwonline.com/article/99554. Past topics:
• What You Need to Know
• RadioText Send Rate
• PS Scroll This level, often compared in percentage to the overall modulation of the FM carrier, is called the RDS injection level. The National Radio Systems Committee and European RBDS/RDS standards don’t actually specify a recommended level.
The standards documents show a nominal value of 3 percent to 11 percent injection but they shy from coming up with a “recommended” value. I have looked at the injection levels used by many different stations across the country, and I’ve seen many values; I’ve seen them below 3 percent and above 11 percent. I would say that the typical value has been 4 to 5 percent; and in the past I set my own encoders in this range.
However, with the newer, portable radios, it has been my experience that 4 to 5 percent just isn’t enough for solid RDS performance. At this point I would recommend 6 percent. This is a good benchmark to aim for, in my opinion.
The iPod Nano’s 5th and 6th generations as well as Insignia NS-HD01 and NS-HD02 receivers perform nicely with 6 percent injection. Anything lower than 6 percent on the portable units results in performance that is less than optimal. As an added benefit, 6 percent really makes RDS reception solid in mobile environments, even in multipath; and I’ve noticed RDS performs much better on the edge of your coverage area at 6 percent injection rather than at 4 or 5 percent.
In my testing with Microsoft’s Zune HD, I found that the analog portion of the FM tuner has issues with injection rates below 5 percent, even in optimal reception conditions. Often, I had to increase the RDS injection rate to 7 to 8 percent to get the same RDS performance as the legacy Zune and Apple’s Nano can do with 5 to 6 percent.
I noticed that stations with injection below 5 percent typically don’t even show on the Zune HD. I performed these tests with multiple Zune HD models, with the factory-installed software and even upgraded to the latest versions.
I presented these observations to Microsoft in the fall of 2009; they acknowledged the issue, but so far they have not dedicated resources towards changing this. While it appears 7 to 8 percent is best for the Zune HD, I personally feel that this injection rate is far more than what should be necessary, seeing how every other radio I’ve worked with does well in the 4 to 6 percent range.
Also note, this issue with the Zune HD isn’t relevant if your station is running HD, as the HD PAD data will be used instead of analog RDS. But with only about 1,600 FMs authorized by the FCC for HD operation, that leaves this problem applicable to approximately 8,150 analog FMs should they elect to encode for RDS. (The figures are as of Sept. 30, 2010, the latest available from the commission.)
When setting your injection level, I feel that it’s best done with a modulation monitor that supports RDS injection rates. Often, I’ve run across stations that didn’t have the right measurement equipment when setting this up, and they have no idea what their injection rate actually is.
I’ve seen RDS injection rates below 3 percent, where it fails to register on most receivers. I’ve also seen it well over 10 percent, which is unnecessary and, assuming that you’re keeping your station’s total modulation within FCC limits, robbing you of main channel modulation, which can affect loudness.
It’s important to be precise about your modulation and injection rates, so make sure you have the proper test equipment. For those stations that don’t have this equipment in-house, see if you can borrow it from a sister station, or buy lunch for a colleague who has access to the gear.
An example from the Inovonics 730 user manual of a properly synchronized RDS subcarrier with the pilot, as shown from an oscilloscope. This step can get overlooked when you’re installing an encoder. Precision is worth the effort. You should revisit your injection rate if you didn’t do this when you deployed an RDS encoder on your station.
Likewise, if you make any changes to your system such as adding or replacing an STL, exciter, RDS encoder, audio processor or other Subsidiary Communications Authorization generators, revisit your RDS injection and other modulation parameters.
Be sure to follow the instructions for your RDS encoder to synchronize and align the RDS signal with the 19 kHz pilot. I’ve seen stations that don’t have their encoders synchronized, and this has caused issues with RDS reception.
Also, keep an eye on the sync status of your RDS encoder; some encoders call this “free run” when it is not synced. On some units the synch status is displayed as an indicator on the front panel. On other encoders, this is done using software, such as a computer program, serial/telnet commands or Web configuration.
Watch this over time to make sure you’re always synced. I’ve run into situations where an encoder was going in and out of sync and it turned out the input level of the 19 kHz sample wasn’t sufficient.
When the encoder was going in and out of sync, it would cause RDS reception errors, creating a bad user experience of delayed RadioText reception and skipped Program Service scrolling frames. The added benefit of this process is that a properly synced RDS encoder in quadrature will reduce slightly the modulation peaks of the subcarrier without reducing their actual levels, giving you more room for your main channel modulation (see Fig. 1).
I’ve been getting good feedback from people interested in these details of RDS. Feel free to comment as we continue this discussion. My e-mail is firstname.lastname@example.org. In our next article in the series, we’ll start exploring the RT+ tagging standard.
Alan Jurison is a regional IT manager/broadcast engineer for Citadel Broadcasting in Syracuse, N.Y. He holds several SBE certifications, including CSRE, AMD, DRB and CBNT. Opinions are the author’s own.