Industry experts shared their views recently about how stations will integrate IBOC/HD Radio into existing analog AM and FM facilities. Here is a sampler of their remarks, made during this fall’s NAB Radio Show in Seattle.
Note that due to the potential for interference, the FCC’s interim approval of digital radio as outlined in the report and order of Oct. 11 did not authorize the use of separate antennas for digital FM radio. Comments below referring to the use of separate antennas were made prior to the report and order; and although these implementations may be feasible technically, they are not allowed by the commission at this time.
Daryl Buechting, senior manager of radio products for Harris Corp., said the transition to AM digital requires a close look at hardware.
“You need an AM transmitter that’s capable of IBOC. That means that it has inherently wide bandwidth in the design, about 50 kHz of audio bandwidth in the analog channel … and about 100 kilohertz capability for the phase channel.”
What about the antenna? “You need Hermitian symmetry to 5 kHz,” he said. “What that means is that you want equal but opposite sign reactance, if you’re plus at 10 kHz on one side, you should be minus at 10 kHz. on the other.”
Also required is low VSWR out to about 15 kHz. “If you don’t have this, there are some possible fixes in AM, there are broadbanding networks that can bring everything into symmetry; phase rotation is important for best performance,” he said. “You may need a new computer-aided-designed antenna coupling unit or phasor.”
On the FM side, the transmitter has to be linear. “We can do both separate amplification or separate antennas and we can also have transmitters up to 15-20 kilowatts that can do common amplification of IBOC plus FM only,” Buechting said. “You should check your antenna to make sure that you are flat +/- 200 kHz. If you are on a combined antenna, you should check with the manufacturer of your multiplexer to make sure that it is compatible with IBOC.”
Other digital configurations are possible, such as separate amplification.
“If you’re doing that, you need an IBOC-only transmitter. If you decide to go with a separate antenna then you still need an IBOC-only transmitter,” he said.
“If the antenna that you have up for the separate is the same gain as your existing, then it’s 20 dB less, but a lot of people don’t have the real estate to put up the same gain, so you may only wind up with a unity gain antenna.” He also said the antennas should be located on the same tower, with one mounted above the other.
Bill Gould, broadcast products manager for Harris Intraplex products, said studio-to-transmitter links for IBOC will require a different approach.
“Delivering 20 kHz digital audio will require a 44.1 sample rate to the transmitter site. We need to look at a migration path to all-digital radio which will include other services.”
He said the audio must be delivered not only to the IBOC chain through the IBOC processor, but it also needs to be delayed before it goes to your existing audio processor, exciter and transmitter.
“This delay is accomplished as part of the IBOC system, but it lines up the analog or delays the analog so it is in sync for blending later on. Whenever possible, keep your chain digital and at 44.1 all the way through.”
Eric Wandel is director of product development for Electronic Research Inc. He noted several approaches to integrating a digital IBOC signal with an existing analog FM signal.
“The main method of achieving the high-power analog/digital combiner is the injector method that typically uses the 10 dB combiner,” he said. “The 10 dB coupler is used to combine existing analog FM and IBOC transmitters into one antenna. It’s very flat, profiling of good match, good isolation and flat across the band.”
He said these devices are wideband and, regarding frequency and bandwidth, are “off-the-shelf, one size fits all.”
“These are flat across 20 MHz; this will allow you to use an existing antenna and existing combining facilities. The disadvantage of this method is that a lot of power is thrown away. Ninety percent of the digital power is converted to heat due to the losses. The analog transmitter has to have some headroom.”
He listed several points to consider including power output of the existing FM transmitter, available room on the tower, antenna type and the requirements for physical space in the building.
Other options mentioned included using separate antennas that are vertically separated, interleaved antennas, dual antennas with separate inputs for analog and digital, and a dual-input antenna where all of the antenna elements are radiating both analog and digital.
“If you’re building new analog facilities now, you need to think if what you’re building will be upgradable in the future,” Wandel said.
Richard Hinkle, director of RF engineering for Broadcast Electronics, said, “When you’re running a separate amplification system, the peak-to-average ratio defines the size of that transmitter. The IBOC signal has a very high peak-to-average ratio. There is significant loss in the RF output path.”
He stated that about 90 percent of the IBOC signal and 11 percent of the analog are dumped into a load. This equates to a 10 dB loss in the IBOC path.
“In addition you have the reject load here, which you have 90 percent of your IBOC power and 10 percent of your analog power dumped into it. You need 11 percent overhead on the analog transmitter with little or no impact to existing antenna installations.”
The site will require additional space for the IBOC transmitter, combiner and reject load; allowance needs to be made for heat dissipating from the load.
“As we move to separate amplification, separate antenna, you can see the significant difference in that you get rid of the 10 dB loss,” Hinkle said. “Its very significant because in the same implementation where at 20 kW FM I needed a 2,000 watts IBOC power going into that 10 dB injector, now I only need 200 watts of IBOC going into the antenna.”
Another advantage of separate amplification is that you don’t have additional 0.45 dB (11 percent) loss in the analog path. The biggest consideration in going with separate antennas is “do you have enough tower space?”
The third configuration is common amplification.
“In the common amplification approach, you have one transmitter, it passes both the FM and the IBOC signal,” he said. “You need about 30 percent overhead in an existing FM Class C unit to accommodate the common amplifier approach.” There is less site space required in this configuration and the combiner losses are eliminated.
Dave Chancey is broadcast technology director for Moseley Associates. He also visited the topic of STLs.
“We have the Starlink series; it’s based on a common platform. We sell it in an RF and T-1 mode,” he said. “It has a common chassis, audio cards and a 2 Mbps multiplexer. The audio cards have AES and analog audio in and out on both ends.”
From the front panel it can be adjusted from 16 to 32 to 64 QUAM; the higher the rate, the more spectrally efficient it will be.
“We sell the Starlink in two different forms. The data rate of the Starlink can max out at E-1 data rates,” he said. “In the 4S mode you can run two linear stereo pairs through the 500 kHz, 950 MHz channel.”
Bob Surette, manager of RF Engineering for Shively Labs, was asked, “Why can’t you use existing balanced combiners systems to combine the digital and analog?”
Theoretically, it can be done, he said.
“You’re basically back-feeding your digital signal through your existing broadband combining system. If you take the concept of back-feeding the balanced combiner modules, now you have two outputs, you have a digital output and an analog output.”
Surette went on to describe a combining system using left-circular polarization for analog and right-circular polarization for digital that essentially would improve the isolation between the two transmitters.