IBOC end to end
There is no question that the transition to digital technology will be the biggest change to over-the-air radio broadcasting since its inception. Ultimately, the end-to-end air-chain not only will be capable of delivering higher-quality digital audio within radio's current infrastructure, but it will also enable new services such as datacasting and supplemental audio service over the existing FM channel.
Nevertheless, the benefits will come at a price ranging from a low estimate of $35,000 for AM stations with an IBOC-capable transmitter, satisfactory antenna system and digital infrastructure in place, to $200,000 or more for stations needing a new transmission system, STL and complete studio upgrade. Table 1 outlines typical anticipated station costs.
Given the expense and the lack of a mandated transition time frame like the FCC established for television's conversion to digital, it is likely that most stations will gradually convert to digital. The question is how? Even if a time frame for initiating full-digital broadcasts has not been set, there are steps a station can take now to ensure that the most intelligent and cost-effective analog-to-digital migration path is pursued.
Table 1. Per-station anticipated costs. Information courtesy of Ibiquity.
When it comes to converting to digital, one plan does not fit all — or even some. Your station's plan will be unique, influenced by budget and by your answers to a few critical questions:
At what point in the air chain will the signal become digital? Earlier is better.
Where is the studio in its equipment-replacement cycle? With digital, listeners will hear legacy equipment noise.
What processing changes will be required? IBOC issues range from diversity delay to where the processor is located.
Will the current STL make the grade? Composite systems will play havoc with the signal; the path may need to support data or supplemental audio and communications.
What is the condition of the current transmitter site? More equipment means less room, and greater power consumption and cooling requirements.
Does the transmitter have the bandwidth for IBOC? It needs to be wide and flat with sufficient headroom.
Is the antenna system up to par?
Has a digital business model been established? You will be able to offer new services.
To better answer these questions, the following 10 steps can help you prepare for the IBOC transition.#1: Once the signal is in the digital domain, keep it there
Multiple digital-to-analog (D/A) and analog-to-digital (A/D) conversions, sample-frequency rate changes and varying data compression/decompression (codec) schemes can distort a signal, adding undesirable artifacts and noise. An all-digital studio facility will deliver the best overall signal with the lowest noise level and widest frequency and dynamic range.
Innovations in wireless technology can provide systems with broadband capabilities. Connections can carry several types of audio and data at once.
Stations running an analog studio will also need to maintain strict level control. If too high of a signal goes into an A/D converter, the converter may overload or run out of bits, causing ugly and unpredictable audio. If the levels run rampant, consider placing a brick-wall limiter in front of the A/D converter.#2: A new digital console will help you, now — and later
If you have to replace an aging console now, you will find that many new digital consoles offer features that will improve your performance and streamline your operation today and pave a solid path to the future. Even better, many digital consoles cost less than their analog counterparts. An all-digital console that can accept multiple input sample rates and various digital formats as well as analog inputs is becoming common. Even the analog transmission will be improved when the signal originates from an all-digital studio.#3: Don't ignore the wiring, cabling and clocking
The AES digital audio signal must be as clean as possible. While proper wiring, termination, grounding and quality cable and connector replacements have always been important, they will be required to get the most out of a digital path.
Many recent installations are using CAT-5 cable instead of digital audio (110Ù) cable. Shielded CAT-5 provides an RFI-free, quick and simple installation with a smaller footprint.
Digital clocking is important. AES audio must be synchronized to a common clock to avoid digital level changes, clicks and pops on the air. A small studio may be able to get by with a digital console with an internal clock that allows for silent switching and routing of various inputs. Multi-studio and other complex systems may be timed via GPS or other synchronizing systems, especially if signal routing is used. Some digital consoles available today include self-timing inputs or a complete networkable routing system that includes distribution of digital audio synchronizing signals.#4: Plan for diversity delay
IBOC transmission includes a 6 second to 8.5 second delay from the time the digital audio leaves the console until it is heard by the listener. Off-air monitoring of the digital signal will not be possible. Stations with a talk radio format most likely are prepared for this because of the profanity delay system.
During the IBOC transition, the hybrid mode will contain an analog and digital signal. The analog signal will be delayed accordingly to achieve a smooth blend when the receiver switches between the two signals. This will occur during initial signal acquisition or in areas of low signal strength.
The station will need to change its off-air monitoring practices to one of post-console/pre-IBOC signal monitoring for on-air talent. Most stations will also want to confidence-monitor for RF or audio loss with an automated no-carrier/no-audio detection alarm or have off-air personnel monitor for quality. For on-air personalities who want to hear the fully processed sound of their voices, a mimicking processor can be inserted into the real-time monitor loop. This unit can closely simulate the transmitter processor or produce a custom signature sound. In addition, communication provisions to the studio (other than monitoring the delayed air signal) should be made for personnel on remote location feeds.#5: Locate the audio processor where it will do the most good
With digital, even the location of the audio processor will come into question. Should it be located in the studio or at the transmitter site? The heavy compression and processing that most stations currently use on their analog signals will not be compatible with creating high-quality, artifact-free IBOC signals. In general, stations should consider moving their existing audio processing equipment to the transmitter site and placing it in the path of the analog transmitter only at a point after the diversity delay.
A separate audio preconditioning system designed for the complex requirements of a data-compressed digital audio transmission system can be added in front of the IBOC exciter. This preconditioning system uses psycho-acoustic algorithms that preserve optimal signal quality at a lower bit rate, freeing part of the bandwidth in the channel for other uses. For example, the preconditioning system can actually enable primary FM and a supplemental program service to be broadcast simultaneously on the same FM channel.
Ensure that there will be sufficient room to house the equipment necessary for IBOC operation. Combiners and filters may be housed in a separate room.
This processing technology can clone the analog sound signature and apply it to the digital transmission chain without generating the artifacts associated with hard pre-processing.#6: Maintain quality and consider streamlining communication at the STL
Early IBOC adopters have found that inadequate STL systems — STL systems that fail to maintain high linearity across the bandwidth and use compression algorithms, especially after a studio codec — can cause big problems. In fact, as IBOC service is initiated, STL issues are second in importance only to the transmission system.
Whether the station is replacing an STL or installing its first system, pay close attention to the input and output capabilities. The station may be adding a data stream or supplemental audio to its hop. The STL system should support IBOC's 44.1kHz sampling rate and be expandable to handle frame relay and IP transmission. New IBOC-ready STLs and upgrade kits for late-model STLs are now available.
This may also be the time to consider streamlining the station's communication systems. Multichannel STLs can carry multiple audio streams. Multiplexer systems can be used with wired or wireless communication systems to provide audio, data, telephone, LAN and control paths.
If the station can not replace its composite STL system, a conversion system can be used to meet the AES input requirements of the exciter. Some stations are converting the composite signal back into discrete audio before the A/D converter. However, multiple alternations to the audio should be avoided. This arrangement is not recommended and should be considered temporary.#7: Evaluate the transmitter building sooner rather than later
Most importantly, is there enough space at the transmitter site? Another transmitter may only be the start. Allow room for a combiner, a mask filter (depending on the transmitter manufacturer), a transmitter/antenna switch, a UPS and a second equipment rack. Now is the time to determine what equipment will be added. Once it is specified, allow an extra two to three feet of clearance on all sides to avoid surprises.
Most likely, the power (and associated backup generator) requirements will also increase considerably. Once the power requirements for new equipment have been calculated, add 20 percent for headroom and minor equipment additions.
Combiner losses, reject load-generated heat or less efficient power amplifiers will generate heat and require additional airflow. If the building can not handle additional heat, consider locating the combiner or load in a separate “hot room” or an outside enclosure.#8: Grounding and protection are even more critical with digital
Stations using a phone line or other dedicated hard-wired link for remote control or monitoring should ensure that all connections are properly terminated and isolated. Stations planning to provide datacasting services will need a phone/data line surge and lightning protection throughout the serial system.
Like PCs, some digital equipment must boot before it is ready for service. If these devices are in the station's critical chain, power must be routed through a UPS to prevent the agonizing 30-second to two-minute lapse of dead air or noise. The UPS should be large enough to support all critical components for the required amount of time. Power conditioners will also help to safeguard against line voltage fluctuations and ac line noise.#9: Understand the transmitter options
AM stations have only one transmission option: low level combining of the analog and digital signal through one transmitter. Determine if the current transmitter will be suitable for IBOC transmission.
For the low intermodulation distortion (IMD) that is necessary, the AM transmitter should provide audio bandwidth of 50kHz at the modulator to amplify the 30kHz audio component and 100kHz phase modulation of the carrier. Because high linearity is also essential, the transmitter should also provide low incidental quadrature modulation/incidental phase modulation (IQM/IPM) specifications (between -35dB to -45dB).
To date, no known tube transmitters are capable of reaching the -45dB IQM figure. Stations that own a solid-state PDM transmitter should talk with the manufacturer to evaluate whether it is suitable for IBOC, investigate what modifications will be needed, and how much they will cost.
FM stations have more options — but most likely will need a new transmitter no matter what the implementation path. If the station's current FM transmitter is not capable of passing the IBOC signal because of narrow bandwidth, non-linear class C operation or little headroom, but it is still fairly new with up to 10 years of service remaining, separate amplification — high-level combining with a new IBOC transmitter — will probably be the best choice. Most tube transmitters will not be able to pass the IBOC signal without substantial and cost-prohibitive modifications. Also keep in mind that to maintain its existing analog coverage, the current transmitter will have to boost its output power by 10 percent to 11 percent above the current total output power to overcome any combiner losses.
On the other hand, stations that operate a lower-power FM station with a late-model transmitter running 30 percent or more under its maximum power may be able to upgrade to IBOC at a fairly reasonable cost. In this case, the FM analog and digital signals would be low-level combined in the same transmitter, using the common amplification method.
A third FM option — space combining — is currently being tested. At present it is only allowed under special temporary authorization (STA) from the FCC.#10: Don't overlook the antenna and transmission line
There are four key reasons to evaluate the final elements: to prevent interference, to maintain coverage, to prevent equipment damage and to conform to safety regulations. Given the importance, consider a site survey to ensure that the station can properly pass the digital signal — especially for AM.
The survey will sweep the feedline, tower and antenna to determine frequency response across the band, impedance bandwidth and VSWR symmetry. These figures will become more important than ever with digital because poor numbers will translate into increased bit error rates that will reduce coverage.
For most modern AM transmitters, IBOC requires that attention be paid to the load and that it be flat and symmetrical, exhibiting Hermitian symmetry out to 5kHz with a VSWR of 1.2:1 or less at 10kHz and 1.4:1 or less at 15kHz either side of carrier. Hermitian symmetry occurs when the upper and lower sideband frequencies at a given offset from carrier exhibit reactance of equal but opposite sign. For example, a load impedance that is 50Ù+j0 at carrier may be 46Ù-j5 (capacitive) at 5kHz below carrier, and therefore should be about 46Ù+j5 (inductive) at 5kHz above carrier. The symmetry requirement actually applies to the output of the PA combiner and not necessarily the transmitter output connector, so phase rotation occurring in the output network must be taken into account if measurements are taken at the output connector.
Most FM antenna systems are proving to be IBOC-capable with little or no alignment. VSWR needs to be no greater than 1.1:1 at analog center carrier frequency (F
The digital infrastructure will provide capabilities for new services and revenue streams that were undreamed of in an analog world. This data could be used as a source for additional program-related data (PAD) or for independent services. Other possibilities include subscription services and Internet broadcasting. FM stations will be able to offer a supplemental audio channel.
Although the transition to digital will be complex and costly, it can represent more than an enhancement to the station operation. I believe that it is a defining moment in radio — a moment when the future will be limited only by our imaginations. Now, when even AM can broadcast “in color,” the future should bring new forms of information and entertainment over the airwaves that have served us well for so long.
Mullin is a technical writer and instructor for Harris, Broadcast Communications Division.
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