Philipp Schmid is a research engineer for Nautel. This is one in a series of Q&As with industry professionals about their presentations at the upcoming NAB Show in Las Vegas.
Radio World: You’re involved in two sessions. We’ll start with “Developing HD Radio SFN (Booster) Standards,” (Sunday, April 17, 5:30–6:30 p.m.). It is well known that FM single-frequency networks are a challenge to make work well. Why would a broadcaster opt to deploy an IBOC signal SFN for HD Radio?
Philipp Schmid: With translator frequencies becoming sparse, on-channel boosters may be today’s best and only option to increase your coverage; and besides it enforces your station’s frequency branding. Here is the challenge: Any on-channel transmission causes interference in the overlap region of the two transmitters. The IBOC signal is resilient to multipath interference and with proper planning allows a broadcaster to extend seamless coverage between adjacent transmitter regions. When the IBOC signal is closely aligned in time, I can demonstrate a “hand off” from one transmitter to the next. This is not possible in FM, since an interferer with as little as 100:1 received signal power (or 20 dB desired/undesired) will degrade the FM reception quality. The effect can be minimized with close time- alignment, but not entirely mitigated. HD Radio can tolerate interferers of up to 4:1 received signal power (or 6-7 dB D/U). This greatly reduces the potential interference zones and even better the time differential of the signals is relaxed within the interference zones. In short, IBOC SFNs can be used to seamlessly fill gaps in coverage or extend coverage, for example if the entire length of a highway is to be covered.
RW: What is the state of IBOC SFNs today?
Schmid: iBiquity Digital Radio (now DTS) already demonstrated the feasibility of IBOC SFNs on WD2XAB, Baltimore, in 2009 and found that if signals are properly time-aligned in the interference zone, HD Radio coverage can effectively be extended proving that modulation synchronization can be achieved across independent modulators. At that time iBiquity introduced additional fields in the communication protocol between the studio-side equipment (the exporter) and multiple modulators (the exgine) at each transmission site. iBiquity proposed a method for aligning the signals that proved to be not feasible in the majority of deployed modulators as it required absolute time knowledge. Nautel has developed a proof-of-concept system using the iBqiuity-introduced sync fields that does not require absolute time knowledge and relies on a GPS sourced one pulse per second (PPS) signal only. This system is applicable to new and already deployed Nautel transmitters, widening the potential application field for this technology.
RW: What is the point of an industry-wide standard?
Schmid: Our proposed solution will work with Nautel gear, including new transmitter models, such as the GV and VS lines, as well as older transmitter lines, such as the NV line. All communicate with the Nautel ExporterPlus. If a station is already using Nautel as their main transmitter, this is a great advantage. However, it is a tall order for a station to switch out their main transmitter for the sake of a small booster to fill gaps in coverage. Therefore, it would be nice to standardize the communication protocol in such a way that vendor interoperability could be achieved between exciters and exporters. DTS/iBiquity is the ideal body to standardize this modification of the existing E2X protocol as they already control this interface. The hope is that with this technology IBOC SFNs now become practical.
RW: What are the key points of such a standard?
Schmid: Our proposed system time tags audio all the way from the capture at the exporter to RF out, effectively fixing the HD1 throughput delay to within very tight tolerances. SFN tolerances need to be tight in order to account for signal flight time to the interference zone. A nice side benefit of this technology is that the digital audio throughput delay on HD1 is now fixed to under one audio sample. This should be of interest to any broadcaster currently broadcasting hybrid FM+HD and chasing after their diversity delay. Provided their FM throughput delay is fixed, they should have no problems under this system.
This technology is strictly confined to aligning the IBOC signal. But now that we have fixed HD1 delays, this opens up the possibility of using HD modulation monitors that can measure the delay between the FM and HD1 audio and controlling the FM SFN delay by using the HD1 as a reference. Today this method allows us to control the FM delay to within 22 μs (or one audio sample). This is good enough for FM synchronization of mono FM stations or in places with sufficient terrain shielding. Our challenge to the modulation monitor manufacturers is: Can you give us subsample accuracy on your measurement to fine tune the FM SFN? The big upside to the broadcaster is that this method works no matter the method of FM audio delivery to each transmitter and no longer requires expensive STL synchronization equipment and now also takes exciter delays into account.
RW: How successful have trials been and are we close?
Schmid: I will be presenting lab results of the system that show we can align two modulators to within 2 microseconds as shown in the oscilloscope plot below:
Time-Aligned IBOC Signals on an Oscilloscope
We are presently working with a station on a field install of the technology and we will be reporting on the initial results during the panel discussion. Initial results are promising, but we have not yet had a chance to conduct extensive drive tests. With the completion of successful tests we are hoping the industry will follow our lead and adopt our proposal as a standard and IBOC SFNs will gain more traction.
RW: You’re also helming your own, somewhat related session, “Digitizing Terrestrial Radio,” (Saturday, April 16, 3:25–3:55 p.m.). Tell us about that one and how it relates and could be a solution for the SFN-HD problem.
Schmid: While we are broadcasting hybrid FM+IBOC, we will be limited by the FM being the weak link in an SFN. With some markets approaching 20% of cars on the road today with HD Radio (New York: 18.6% Dec 2015), now is the time to plan for a transition to all-digital broadcasting and leave the shackles of FM behind. My second session as part of the SBE Ennes workshop entitled “Digitizing Terrestrial Radio” covers how we can achieve an all-digital transition strategy based on our HD Multiplex concept that is backward-compatible with today’s receivers. I can demonstrate up to 15 audio services emitted from a single broadcast transmitter that can be picked up using off-the shelf receivers. HD Multiplex will make optimal use of IBOC SFNs allowing broadcasters to tailor their coverage precisely to the target area they are interested in. For example, AM broadcasters translating into the FM band could build out the same coverage area as the parent AM station. The neat thing about HD Multiplex is that it is composed of independent IBOC signals, therefore, each one can be part of a different SFN.
RW: Could an all-digital broadcast system really happen (anytime soon)? Or, while possible from an engineering standpoint, do the business and economics conspire against it?
Schmid: While the technology exists and receivers are available today, it is a challenge to turn this vision into reality due a congested FM band in urban centers. Therefore, rural areas may be the first to benefit from HD Multiplex. I can see broadcasters unleashing more content in rural areas at a fraction of the transmission cost associated with FM. Consumers would buy HD Radio receivers if exclusive and compelling HD content were available. Cars with HD Radio receivers approaching 20% is still early in the game, but I could see broadcasters in urban centers considering switching to HD Multiplex and increasing their total population reach across all services beyond their single service today. Sports radio will seize this technology and bring multiple franchises under one umbrella and deliver more game play-by-play for both home and away. Multicultural programming will flourish and will no longer require specialized SCA receivers.
The key economic aspect to address is: how do we turn today’s single-purpose broadcast environment into a shared purpose broadcast system? Once we are able to address this question we will all benefit and bring hundreds of audio services to the FM band.
RW: Are more stations really what we need? How else can a station differentiate themselves?
Schmid: With today’s on-demand culture, I do believe, there is a desire for more available content, which HD Multiplex can deliver. For example, it is rather straight forward to create an “on-demand” news and traffic channel by simply looping your top of the hour news and traffic on a side channel continuously. If listeners missed the news, they could tune to HD2 anytime; that would differentiate a station and is doable today.
Detail: Coupon radio demonstration using QR codes
Click on the Image to Enlarge
Another creative way to differentiate your station is by offering a headphone-optimized simulcast on a side channel. I have experimented with binaural audio and the HD Radio system seems to carry the surround sound effect rather well. Today, it would be ludicrous to suggest a dedicated broadcast for headphone listening, but with enhanced channel capacity, this may just be what sets your station apart. Imagine your station sponsoring a live concert. All you need to do is record the event with binaural microphones and use a temporarily allocated side channel to deliver the on-stage concert experience. We will have binaural microphones on our booth for show attendees to see.
“Coupon Radio” may be another means of differentiating your station and retaining listenership. The graphical capability of HD Radio allows a station to broadcast bar codes and QR codes to receivers for specific store discounts applied via bar code scanners at point of sale. These codes can be captured on a smartphone app; the more you listen, the more you save. “Coupon radio” provides marketing feedback to the advertiser and added revenue for the radio station.
RW: As you know, there are a lot of interesting sessions and fascinating equipment out on the show floor to be seen, so why should show attendees take time out of their schedule to attend one or both of your sessions?
Schmid: I admit, I am a dreamer; in my sessions I hope to share with attendees an exciting vision of what the future of radio could hold. When you are browsing the show floor, you will see what is available today and this coming year. I am hoping to look beyond that, but at the same time I want to point to the puzzle pieces available today in order to incubate tomorrow’s ideas.
Come visit the Nautel booth (N2522) and I will show you HD Multiplex in action, I will demonstrate binaural audio and we can talk single-frequency networks. Mark our HD Radio event on Tuesday April 19 at 3 p.m. on your calendar.
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To see more show Q&As visit our NAB Show 2016 web page.