Performance evaluation

December 1, 2007


Once an HD Radio system is operating in a stable fashion, the various parameters and data streams associated with the MPS and the SPS (and perhaps even another SPS) must be monitored. It used to be that all a station really needed to do was watch the peak modulation level, and in addition with FM, the pilot level and once in a while the L-R level (for the occasional out-of-phase song). Now with HD Radio, it's important to monitor and measure the time alignment and level alignment between the analog modulation and the MPS, and stations will likely want to monitor the RBDS on the analog channel along with PAD data for both the MPS and SPS. Fortunately, there are now multiple monitoring devices made specifically for HD Radio, and each of these devices gives the end user options about which parameters are to be measured, and how the device is to alert the user once a problem is detected. Do you want a contact closure to signal your legacy remote control? Or do you want an e-mail about an out-of-tolerance parameter? Want to browse into the monitor itself via the Internet? Well then, you're in luck: That's a common feature now.

Here's a rundown of several major monitoring products available today.

Belar Electronics offers the FMHD-1, a combination analog and HD Radio monitoring system. This radio has three antenna inputs: one for off-air reception, and another group of two to be used while the device is located at the transmitter site. One input of that group is used for high-level RF from an analog transmitter, the other is used for high-level RF from the HD Radio transmitter. The monitor receives and demodulates the analog and digital signals simultaneously, displaying them on a 640 × 240 color LCD display. (A rotary knob/encoder completes the basic user interface.) The display is used to show HD Radio status, data, time alignment, and configuration information; for the analog transmission, it displays total modulation, pilot, L, R, L+R and L-R. Additionally, the display can be used to show a spectrum of the analog and digital signals.

The time alignment scales are ±375ms (±16384 samples) and ±5.8ms (±256 samples).

The FMHD-1 has eight user-assignable audio outputs on its rear apron that can accommodate the analog versions of the FM signal plus the MPS, SPS1 and an SPS2. The unit also has three user-assignable AES-3ID outputs (75Ω unbalanced). There are two demodulated composite outputs and two LVDS outputs (RJ-45, 100Ω).

This monitor can be controlled by a computer via its Ethernet capability or the RS-232 serial connector. Wizard for Windows is the Belar software that allows the user to remotely monitor the FMHD-1. And finally, there are four user-assignable relay contacts to indicate alarm conditions.

Audemat-Aztec offers three different versions of its HD radio monitoring system: the Goldeneagle HD FM, the Goldeneagle HD AM, and the Goldeneagle HD FM/AM.

Audemat-Aztec Golden Eagle AM/FM monitor

Audemat-Aztec Golden Eagle AM/FM monitor

These units will constantly monitor a station's (or stations') parameters and generate alarms should any of the selected parameters fall outside their pre-programmed tolerances. The unit will display all RBDS and PAD for HD Radio. It has an embedded Web server (used with client software supplied by Audemat) to study the detected parameters from afar via the monitor's Ethernet capability. Another feature of the Web server is that of streaming audio: The end-user can listen to a stream of embedded left and right audio, or alternately, the left channel can be a stream of the analog audio while the right channel is a stream of the digital audio. This obviously facilitates the time alignment adjustment. Of course if you are in the same room with the unit, the audio is available on a headphone jack, or via balanced analog audio, or a balanced AES version.

Alarms are reported via e-mail or optionally by voice or contact closures that signal an in-place remote control.

An optional FFT spectrum analyzer module enables the end-user to see the real-time spectrum of the complete RF signal. Thresholds can be set with respect to a spectrum mask so out-of-tolerance conditions can be detected and reported. With the spectrum analyzer option comes what Audemat refers to as the Digital Demodulator, which provides representation of total modulation, pilot and RBDS injection, and the analog and digital levels displayed in a bargraph format.

Day Sequerra has a wide line of HD Radio monitoring products such as the M4.2R (an updated version of the M4). This monitor will tune both AM and FM bands and has the capability of storing up to 20 presets. It will detect and display RBDS data (Program Service and Radio text fields) for the analog modulation, plus PAD data (station long and short name, program type, song file, artist, album, genre and comment fields) for each HD channel.

Day Sequerra M4.2R monitor

Day Sequerra M4.2R monitor

It has six alarm relay contacts that can be assigned to represent loss of RF carrier, audio, OFDM, RBDS, PAD, multicast or time alignment. These same alarms can be reported to the end-user via e-mail. Day Sequerra's remote control software is called Remote Dashboard and communicates with the M4.2R via its Ethernet port. There again, if you are in the same location as the monitor itself, you can listen to the demodulated signals via the front headphone jack or via balanced analog (+4dBU) outputs on the back. A transformer-isolated 110Ω source SP/DIF digital output comes from a rear-apron XLR-type connector.



Coaxial Dynamics 91090 Digital Wattchman monitor and alarm system

Coaxial Dynamics 91090 Digital Wattchman monitor and alarm system

I have the power

If you've installed an FM band HD Radio transmitter then you likely relied upon the unit's power meter (calibrated at the factory) to set the digital power level. You may very well want another means, though, to monitor digital power. And since the in-line wattmeters we've all used over the years are not adequate for measuring the power out of an HD Radio transmitter (because wattmeters associated with these transmission line sections are calibrated to read average power, and don't work well when there is a high peak-to-average ratio as there is with the IBOC technology we are using) you'll probably want to consider the purchase of a new wattmeter.

Coaxial Dynamics offers the 91090 Digital Wattchman monitor and alarm system. This unit is similar in function to its more familiar 81090 series, but Digital Wattchman makes use of two digital plug-in elements so it can monitor forward and reflected power simultaneously. (Using the digital plug-in elements allows the user to measure the power of complex modulations schemes such as those used in DAB and DTV.) The 3RU display unit includes two 4.5" mirrored analog meter movements. Accuracy of the power measurement is specified as ±5 percent; alarm response time for the unit (including the time for necessary for the relay contacts to move) is specified as 15ms. Amplified dc outputs are also included so your remote control can be connected directly.

Bird Electronics BPM-E

Bird Electronics BPM-E

Bird Electronics offers the BPM-E (Broadcast Power Monitor-Enhanced) for the measurement of complex RF waveforms such as those encountered in the transmission of HD Radio. The BPM-E comes mounted to a Bird Thru-line section, and has the simultaneous forward and reflected monitoring capability expected today. It also includes an integral RF test port that allows for convenient testing of the spectrum of the RF mask. Both RS-232 and Ethernet communications means are included for remote monitoring via a Web interface. The unit interfaces with a Bird 3129 display unit (1RU) so power can be read locally at the unit.

Analyze this

After installing and commissioning an HD Radio transmitter, a proof of performance needs to be run. A spectrum analyzer is needed for this, and perhaps you'll be lucky enough to have one at the ready. Fortunately there are several relatively inexpensive analyzers out there that fill the bill.

Agilent Technologies N1996A CSA spectrum analyzer

Agilent Technologies N1996A CSA spectrum analyzer

First is the Boonton 9102, which is a hand-held device that measures from 100kHz up to 4.0GHz (optionally up to 7.5GHz). The display features Resolution Bandwidth from 100Hz to 1MHz, and video bandwidth from 10Hz to 1MHz; and one of its power measurement capabilities is known as channel power — probably the most accurate way of measuring the HD Radio power spectrum. The 9102 also has an Ethernet interface that allows TCP/IP communication via a LAN or WAN.

Another instrument manufacturer with a real legacy in broadcast engineering is Agilent Technologies (formerly known as HP). One of its newest offerings is the N1996A CSA spectrum analyzer. This unit covers from 100kHz to 3GHz (optionally to 6GHz) with RBW in 10 percent adjustable steps between 10Hz and 200kHz, and then in steps of 250kHz, 300kHz, 1MHz, 3MHz and 5MHz. VBW can be set between 1Hz and 8MHz. This unit can also perform the channel power type of measurement, and features Ethernet connectivity as well as USB connectivity.

LP Technologies LPT-3000 spectrum analyzer

LP Technologies LPT-3000 spectrum analyzer

LP Technologies offers the LPT-3000 spectrum analyzer. This device covers from 9kHz through 3GHz, measures channel power, has the appropriate resolution and video bandwidth settings, and even has pre-programmed Ibiquity-specified RF masks. It also has storage space for up to 900 waveforms and it can convert its measured data to an Excel file format. Communication with the unit can be made via serial or USB, with Ethernet as an option. LP Technologies also makes a rackmount version of the same analyzer (LPT-3000R) that is specifically designed to live at a remote site. This unit has an optional four-input RF switcher that gives it the ability to multi-task. It is also priced at less than half the cost of most other analyzers.

HD Radio is a modern technology that requires the station engineer to learn about new techniques in RF communications. Let's face it: AM and FM technology date from the 1920s and 1930s, and share very little technologically with digital radio. The inclusion of HD Radio necessitates new test and monitoring capabilities; fortunately the proliferation of digital transmission has attracted many manufacturers to build the necessary equipment, thus keeping the prices reasonable. That benefits us as broadcasters, and by extension, our listeners as well.


Irwin is the chief engineer of WKTU-FM, New York City.

Resource Guide

Manufacturers of test equipment

Agilent Technologies
800-829-4444
www.agilent.com
Boonton
973-386-9696
www.boonton.com
Audemat-Aztec
305-249-3110
www.audemat-aztec.com
Coaxial Dynamics
800-COAXIAL
www.coaxial.com
Belar Electronics
610-687-5550
www.belar.com
Day Sequerra
856-719-9900
www.daysequerra.com
Bird Electronics
866-695-4569
www.bird-technologies.com
LP Technologies
316-831-9696
www.lptech.com


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