Trends in Technology: HD Radio Transmission Update

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Trends in Technology: HD Radio Transmission Update

Nov 1, 2014 9:00 AM, By Doug Irwin, CPBE DRB AMD

Andy Skotdal, co-owner of KRKO (1380) and KKXA (1520) in Everett, Washington, took part in �All-Digital AM IBOC Testing� on KRKO and KKXA October 2nd through the 6th of 2014. Andy was kind enough to share his thoughts on the experience.

The test vehicle fleet

During the tests

Preparing the vehicle fleet for test runs

The effort was grueling. We had five test cars and two stations to map from Friday to Tuesday. I drove approximately 1,230 miles. I think the average was 900 miles on each car. Many nights ended at 1 or 2 a.m., only to return for more at 9 a.m., so this was a grind for everyone. I''m exhausted today and I''m glad the road signs aren''t moving out to the middle of the road any more; or at least that is what my brain started to interpret!

We had a core of five people and additional volunteers to keep feeding the beast. This was the only 50 kW site, the only diplex site, the largest market, and only the third daytime test. It was actually only the second test on a commercial station (well, two stations here) because one of them had been on a test transmitter.

David Layer of NAB Labs in one of the test vehicles

David Layer of NAB Labs is taking a very disciplined approach to all of this and he was a treat to work with.

One moment of excitement came with a loud �POP� as the KXA transmitter blew its rectifier stack. The show must go on though, so we cannibalized my KRKO transmitter in order to enable the KXA tests to happen as planned. We ran at 12 kW on KRKO until parts arrived on Tuesday. The rectifier failure had nothing to do with the testing, but it was fortunate that the Nautel factory engineer was on site to say, �Hey, it''s your rectifier stack.� Ok!

Jeff Welton of Nautel adjusts the HD Radio exporter

A little about the HD receivers

I learned that there is a huge amount of variability in receiver capability. The Ford factory radios are really nice and keep a good lock. My Kenwood aftermarket radio, not so much. That could also be a function of the antenna in the vehicle, but I''m more inclined to think it is a receiver variation.

Monitoring KKXA during the tests

I also learned that some receivers locked on an all-digital signal that drops will never reacquire that signal again if they stay on that channel. There seems to be a timeout function where if the signal is gone for too long, the receiver assumes it will never come back and stays silent rather than continuing to sniff for the signal. You have to tune to another channel and then tune back.

Takeways from the testing

There is no equipment available to stations to allow them to see what the HD error correction level is in the field. This would give you an indication of what the HD carrier looks like leaving the transmitter site and give a field snapshot as to whether transmitter site modifications are making an improvement or not. If a receiver is working 2x as hard to decode your signal, it would be nice to know because that causes you to lose forgiveness when you get to overpasses and power lines, for example, whereas if the receiver doesn''t have to error correct, you have more headroom for correction when other variables creep in like power lines and bridges.

I learned that our gear was not programmed to allow for what I''ll call all-digital demo mode, meaning if I want to go all-digital on Saturday nights by contact closure to promote the technology and encourage people to buy receivers, if the FCC allows that someday, the system won''t allow that the way my exciter and exporter work right now. Nautel thinks they can make that correction, but it wasn''t contemplated that stations would do something other than hybrid or all-digital.

Some of the engineers involved in testing: David Layer, NAB; Stephen Lockwood, Hatfield & Dawson; Mike Rhodes, Cavell Mertz; Tom McGinley, CBS Radio Seattle; and Andy Skotdal and Buzz Anderson, KRKO(AM)/KKXA(AM), Seattle

The Everett all-digital AM HD Radio tests were the last of nine carried out by NAB Labs. Results of the tests will be processed and submitted to the Federal Communications Commission in 2015.

Continue reading for Doug's analysis of HD Radio transmission options for FM...

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Trends in Technology: HD Radio Transmission Update

Nov 1, 2014 9:00 AM, By Doug Irwin, CPBE DRB AMD

Back in April, we looked at transmitters with a particular emphasis on LPFM. This month, we''re going to take a look at recent developments in HD Radio transmission. It''s capital budget time after all, and if you''re looking at new transmitters next year, read on.

GatesAir has several lines of FM transmitters. Their solid-state transmitter line is known as the Flexiva, and is available at power levels up to 80 kW. Flexiva''s RF amplifiers are designed around LDMOS FETs, which provide for high density and high efficiency. As with most modern solid-state transmitters, the design uses switch-mode power supplies that achieve an efficiency of around 96 percent. These power supplies are hot swappable and auto-ranging, an important feature for those of us at the end of long, long power runs up a mountainside. The Flexiva is completely broad-banded with no mechanical tuning required so it can easily fulfill the role of an N+1 transmitter at an auxiliary site.

Key features include:

  • � Identical power blocks provide for scalability; 5 kW or 10 kW power blocks use 16 rack units.
  • � Common dual 1800 Watt Power Amplifier and IPA modules for all power levels.
  • � Single-phase or 3-phase power, Delta or Wye configurations; 190 to 464 VAC.
  • � 1:1 power supply to power amplifier module ratio.
  • � Hot-pluggable, hot-swappable power amplifier and power supply modules.
  • � Distributed hardware based control architecture uses analog circuits to control critical transmitter functions. Critical functions are not reliant on a microprocessor.
  • � Variable speed fans optimize cooling, maximize efficiency and minimize noise.
  • � Redundant internal cooling fans draw air from front to rear with ducted air options available.
  • � Operates in the following modes: FM, FM + HD Radio, HD Radio only or DRM+.
  • � Maintains power up to 1.5:1 VSWR, with proportional fold-back for safe operation at reduced power into marginal loads, such as an ice-bound antenna.
  • � Global monitoring and control via IP; graphic user interface (GUI) works with any PC, tablet or smartphone.
  • � Full SNMP support; in-depth diagnostics and setup via front-panel control.

Let''s take a look at the output power capability of the Flexiva line.

One of the lower-power Flexiva line transmitters will easily fulfill the role of �exciter� in this case. The Flexiva �exciter� uses direct-to-carrier digital modulation. It features multiple AES inputs, composite program inputs, and analog inputs with failover switching. An internal stereo generator is included. Flexiva will also accommodate digital MPX via AES192. The Flexiva has its own complete remote control capability with an embedded web interface, SNMP support, and parallel GPIO. A GPS receiver for single frequency network synchronization is available as an option. You will need to add the optional Gen4HD board to the �exciter� in order to transmit HD Radio. This internal expansion board provides hybrid crest factor reduction, which is GatesAir''s peak-to-average power reduction technology, as well as linear and real-time non-linear pre-correction. Other key features of the Gen4HD board include an RF spectrum analyzer display, asymmetrical sideband power control, and Modulation Error Ratio (MER) monitoring.

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Trends in Technology: HD Radio Transmission Update

Nov 1, 2014 9:00 AM, By Doug Irwin, CPBE DRB AMD

Broadcast Electronics is also a fan-favorite of many Broadcast Engineers. Like others in the �big 4� group of transmitter manufacturers, they have an extensive line. We''ll examine the FMi-T, which is their high-power, vacuum tube series of HD transmitters. One of the most important facts to take away about the vacuum tube PA is that the cavity is a half-wave design, which eliminates the plate blocking capacitor and sliding contacts. These components are one typical failure point of high-voltage tube amplifiers. BE''s peak-to-average power reduction method is called �Vector Power Enhancement.� The technology increases the amount of HD power output while maintaining good overall RF efficiency. The FMi-T series also makes use of adaptive pre-correction.

Other features of the FMi-T series are as follows:

  • � Large PA tube used, providing for effective cooling, even in environments with high ambient temperatures
  • � Tuning and loading controls at zero volts DC potential
  • � Broadband neutralization eliminates the need for re-neutralization during tube changes
  • � Produces full rated power into 1.8:1 VSWR, capable of operating into higher VSWR with automatic power reduction; Protected against open and short circuit.
  • � Microprocessor-based controller allows remote access to monitor all operating parameters.
  • � RF efficiency 67 percent or better at rated power; overall AC-to-RF efficiency 57 percent or better at rated power.

Taking a look at the FM-T line power vs. HD injection levels:

The latest exciter design from BE (used for the FMi-T line of transmitters) is the STXe. With the addition of Vector Power Enhancement, this new exciter can operate in elevated HD injection level applications, or in DRM+ installations that desire an FM analog and DRM+ transmission from a single transmitter. STXe combines BE''s peak-to-average power reduction scheme and real-time distortion pre-correction to yield higher transmitter efficiencies and output power. It has multiple program inputs with silence sense and failover, a built in RDS generator, an FSK IDer for translator applications, and a built-in audio delay for use with single frequency networks.

STXe has its own built-in controller with an updated GUI. If your transmitter site has IP connectivity, you will have access to all of the STXe parameters via a PC, laptop, tablet or smartphone. SNMP is also supported for remote monitoring and control.

Nautel''s most recent design is the GV series. As with all Nautel designs, it is a solid-state transmitter. The GV series uses LDMOS FET RF amplifiers and redundant hot swappable power supplies. Common module types are used across the GV line and the NVLT series. This reduces spare requirements, assuming you have multiple sites with Nautel transmitters.

A 15 percent increase in RF efficiency has been gained, in part by the introduction of Nautel''s �Spectrum/Efficiency Optimizer� which optimizes RF amplifier parameters dynamically to meet the demands for analog output power at higher HD injection levels. Nautel calls their peak-to-average power reduction technique �HD PowerBoost� This feature is standard in the GV.

Take a look at the power output vs HD injection levels for the GV Series:

The GV Series features an integral direct to digital exciter, with an optional redundant exciter. The exciter supports adaptive pre-correction, and an optional Exgine card for HD Radio.

Monitoring and control is through Nautel''s Advanced User Interface (AUI) via a 17� front panel touchscreen, web interface, or local �backup� control panel. The AUI offers comprehensive display of all transmitter parameters and detailed logging.

Transmitter parameters are set in �presets.� These presets include the frequency, power level, operating mode, audio input options including failover, and a number of other settings. Remote GPIO control is also available. Using the transmitter''s internal GPIO and AUI, the GV provides a built-in, fully integrated monitoring and control mechanism for external site-related equipment. This could include things such as air conditioning and generator status. The AUI can also notify station personnel via email if any monitored parameters transition to a fault state.

Functions accessible via the AUI include:

  • � Dynamic RDS scrolling, integral
  • SCA generators
  • � New oscilloscope instrumentation
  • � MER instrumentation
  • � Livewire support
  • � MPX over AES input
  • � Shoutcast and Icecast streaming inputs
  • � RF and audio spectrum analyzers
  • � Audio input silence sense with smart
  • source switching and return
  • � USB audio storage and playback system
  • � Optional Orban Inside multiband
  • digital audio processor
  • � SNMP support
  • � Email notifications

Real-time measurement of MER (Modulation Error Ratio) provides the ability to diagnose issues related to HD Radio transmission, even down to measuring the MER performance on specific sets of subcarriers.

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Trends in Technology: HD Radio Transmission Update

Nov 1, 2014 9:00 AM, By Doug Irwin, CPBE DRB AMD

Continental Electronics continues to produce their familiar line of high-power FM transmitters�the 816R series. Power levels range from 11 kW to 21.5 kW using the 816R-2C and up to 40 kW with the 816R-7C. A single tube is employed as the final amplifier�a 4CX15000A, 4CX20000E, or 4CX25000C depending on the power level. All of the transmitters are self-contained, except the 816R-7C, which uses a separate power supply cabinet and external harmonic filter.

The �HD-ready� 816R-HDR is a high-power combined amplifier for IBOC applications. You can buy it initially as an FM-only transmitter, while planning to add the Exgine equipped 802Ex digital FM exciter and an exporter for IBOC operation later on. Continental''s D816HD Series Transmitters range in combined TPO from 18 kW to 56 kW at HD power levels of -20 dBc to -10 dBc. These combined transmitters use only one tube each, the final power amplifier tube. The D816HD Series Transmitters are also completely self-contained, including the harmonic filter. The 70 kW version has two high-voltage rectifiers and plate transformers located in two separate chassis. D816HD transmitter systems work with Continental''s 802Ex digital FM/HD exciter to provide forward-looking fully adaptive pre-correction, system and HD performance monitoring, and on-channel spectral analysis.

Transmitter purchases are among the most expensive you''ll ever make and really can have a significant effect on your peace and tranquility. While some of us have our �favorite� brands, it''s always a good idea to become familiar with what is available from other manufacturers as well. Take some time to find users of the transmitter lines you are considering; ask about their experiences, good and bad. Ask about customer support in particular. �Service after the sale� is of utmost importance. While HD Radio might not be in your immediate plans, it''s a good idea to plan for the future. Consider a transmitter that can be upgraded to HD should the need arise.

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