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Live Tests Explore Future of HD Radio

Culminated with demo showing 12 independent audio signals from one FM transmitter

The author is a research engineer at Nautel.

One possible, and bright, future path for HD Radio is all-digital operation.

This year, in the months leading up to the NAB Show, Nautel had the chance to conduct live tests of our HD Multiplex technology, in association with PILOT and Xperi, and with the support of Beasley Media Group’s KKLZ(FM) in Las Vegas. (PILOT is NAB’s coalition of innovators, educators and advocates dedicated to advancing broadcast technology and cultivating new media opportunities.)

Our tests of all-digital FM radio culminated with a live demonstration at the spring NAB Show, where we were able to show 12 independent audio signals from a single FM station’s transmitter over the air.

Our initial concept for HD Multiplex was presented three years ago at the 2015 NAB Show. Using HD Multiplex, a combination of up to 15 audio streams and data services can reside within 600 kHz of signal bandwidth, or up to nine audio streams may be accommodated in 400 kHz of signal bandwidth. For more information on how HD Multiplex is implemented, visit Nautel’s HD Multiplex page at

It is important to note that this technology is still experimental at this point and has not been authorized for use. For our 2018 tests, Beasley Media Group obtained experimental authorization from the FCC.

Our 2018 live test was composed of three components: The first was a proof of concept for the Nautel HD Multiplex transmission scheme in which multiple IBOC digital radio signals are effectively stacked immediately adjacent to one another to achieve additional audio and/or data throughput.

The second, and main focus of the tests, was an evaluation of hybrid and all-digital transmission utilizing the MP5 mode, a mode that has always been an element of the HD Radio system but never authorized. The MP5 mode supports both hybrid and all-digital operation. (The most common hybrid IBOC Service Mode in use today is MP3, which permits use of up to five audio services but typically is used for three to four.)

The third component was our live demonstration at the NAB Show of a modified form of Nautel HD Multiplex, which allowed normal KKLZ analog operation as well as the transmission of four digital sidebands.

Two test periods were conducted prior to the show at KKLZ. The station was kind enough to suspend its analog carrier in some overnight periods to allow us to fully test all-digital transmission with up to 15 HD channels. During the testing period, we switched KKLZ from their normal MP3 hybrid transmission to MP5, adjusting the TPO to keep signal parameters within safe peak power limits both in hybrid analog/digital and all-digital tests.


In daytime hours and at the NAB Show, the analog carrier was present so that normal programming could be aired; however, KKLZ suspended its regular HD Radioprogramming during the tests to allow us to conduct our experiments. KKLZ’s analog carrier is located at 96.3 MHz. The four digital sidebands used for our tests were located at 96.0, 96.2, 96.4 and 96.6 MHz, frequencies that are not normally associated with standard FM radio stations in the U.S. Fig. 1 shows a spectrograph of the transmission.

Our first goal, “proof of concept,” worked very well. We tested HD Multiplex signals in both MP3 and MP5 modes. In MP3 mode, we determined that, given that the receiver can tune to the desired sidebands, up to 15 audio services were possible on 96.2/3/4 MHz.

After testing reception of all audio services at the KKLZ studio, two HD Multiplex configurations were selected for drive tests around Las Vegas. The 400 kHz signal was chosen as it could provide tuning frequencies at 96.1, 96.3 and 96.5 MHz available on all standard HD Radio receivers and which fit within today’s channel allocations. A modified version of the 600 kHz mode was also tested, but was reduced to 500 kHz in order to contain the signal within the channels authorized for experimental use.

The drive tests results for the 500 kHz signal are shown in Fig. 2. The receiver was tuned to center sideband that included the power typically allocated to its lower sideband partner. Green areas represent solid signal reception, and blue areas indicate expected periodic audio loss due to the experimental nature of the test signal.

Only two areas of signal drop were noted. As expected, there was signal loss in the airport tunnel and in the shadow of Lone Mountain on the far side of Las Vegas as indicated by yellow dots. On a radial test run, HD Multiplex was also received 64 miles out near the Arizona border (not shown). The tests prove that HD Multiplex is a viable signal for broadcast, and once a real-time modulator has been implemented, more scientific coverage tests can be performed.


The second goal of our pre-NAB Show testing was to not only show “proof of concept” but to provide signal measurements comparing MP3 and MP5 along a number of driving routes around Las Vegas to assess long distance/loss of our test signals.

We sent vehicles out on Interstate 15 and U.S. Route 95, and also drove around two loop routes encompassing the majority of the Las Vegas metro area as well as throughout the Las Vegas Strip. For our tests, we superimposed a 25 Hz tone on the audio program material. In each test vehicle, audio derived from the vehicle’s subwoofer was input to a computer for the purpose of data collection. Xperi’s “Prospector” receiver correlated the GPS position of each vehicle with the status of reception of the desired signal, giving us a “worm trail” for our reports.

Coverage results were promising, as depicted in Fig. 3: Hybrid MP5 was very robust, outperforming the hybrid MP3 mode. In MP5 hybrid mode, the analog carrier does not detract from the digital signal at all. All-digital tests yielded similar promising results. The receivers in our test vehicles (Ford Explorers) had no issues in receiving the all-digital MP5 transmissions. While some handheld receivers may not be able to accommodate MP5, it appears that the majority of HD Radio receivers will have no problem.


HD Multiplex is not yet available and requires more testing. However, some in the industry are already proposing a sunset date for the FM carrier sometime in the future. The intent of the KKLZ test was to start a conversation in the industry on what an all-digital transition may look like.

Our tests showed that a broadcaster can maintain its present-day hybrid FM+IBOC operation while adding 200 kbps for dedicated all-digital data casting applications. When the value equation for the spectrum tips in favor for digital transmission, the FM carrier can be replaced to provide an all-digital configuration, which will increase coverage area and open carriers for wider application on existing HD Radio receivers.

With multiple services, stations can more effectively compete with satellite and streaming services, or lease HD channels to third parties for over-the-air “streaming.” Additional data services such as weather, traffic and artist experience could also be implemented.

This important field test could not have happened without the enthusiastic participation of Beasley Media Group and their Las Vegas station KKLZ, and our thanks go out to Beasley’s Susan Crawford and Mike Cooney for their support, along with station engineers Steve Griesbach and Ray Fodge. Thanks also to our partners, David Layer and NAB’s PILOT test crew, and Russ Mundschenk and his team at Xperi.

View Philipp Schmid’s presentation on this topic from the Nautel Users’ Group meeting at the 2018 NAB Show at