HD Radio: Proving Performance with Modulation Error Ratio

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Inovonics, while not new to the field, recently introduced an IBOC receiver with great features. The INOmni 632 (a 2013 Radio magazine Pick Hit) is a 1/3 rack-width FM and IBOC radio that will not revert to the associated FM in the event that it loses its assigned signal. If you have it set to an HD2 channel, it will mute when it loses the signal. (If you installed silence-sensors on older-style HD Radio receivers you know how important this is.) The 632 has alarm tallies for carrier loss, digital loss and audio loss in the form of open collectors. It has rear-apron balanced analog outputs for left and right audio in addition to an AES out. On the front panel you are given a display of the monitoring assignment, a quality indicator of the digital signal, and other PSD such as name, type, artist and title. Tuning and menu access are accomplished through a small jog wheel. Also found on the front panel is a headphone output so you can use one of the most important pieces of test equipment -- your own ears.

The time-alignment between the analog FM and the digital simulcast (usually known as HD1) is an aspect of IBOC transmission that requires attention because in practice there are components of the transmission path that can generate slight changes in the overall delay of the system. The net effect on listeners can be anything from slightly annoying (time delay slightly out) to instant tune-out (time delay not set at all) as their radios blend back and forth from analog to digital.

To make this easier for those who deal with time alignment issues, DaySequerra has introduced the M4DDC Diversity Delay Control. This is a 1RU, stand-alone device (for use in both AM and FM IBOC systems) that features DaySequerra's TimeLock algorithm for automatic alignment of analog AM or FM audio and HD1. According to DaySequerra, TimeLock can maintain the time alignment down to one sample. The M4DDC has some other particularly useful features in addition to that: For example, it will generate e-mails corresponding to loss of time-alignment, Program Audio, Carrier, OFDM Lock or (optionally) LevelLock. It has five rear-apron tally outputs corresponding to those same conditions. The M4DDC has an embedded Web server that can serve as a remote confidence monitor since it will generate audio streams of the decoded audio and use common browsers for access. Additionally the unit has balanced analog outputs (via XLR) for the decoded audio that can also be set for split mode, with the digital simulcast audio on the L output and the analog audio on the R output. A headphone output on the front of the unit has access to the same audio.

Belar's FM/HD Radio product is the FMHD-1. This is a 2RU device that can be used as an off-air receiver or at the transmitter site, since it has two high-level RF inputs (one for analog and one for IBOC only). The FMHD-1 decodes the HD Radio signal and analog FM signal simultaneously displaying HD Radio status, data, time alignment, and configuration information, as well as total, pilot, L, R, L+R and L−R metering and RF spectrums. On its front the unit has a 640x240 color LCD display and rotary-encoder for the user interface. It can monitor multiple audio streams with an optional second plug-in HD decoder, and its eight user-assignable analog audio outputs; three assignable AES-3ID outputs provide support for a wide variety of broadcast scenarios including multicasting of course. It has both RJ-45 10/100BaseT Ethernet and RS-232 computer interfaces: When used in conjunction with the Wizard for Windows software the FMHD-1 can be viewed remotely. The unit also provides four user-assignable relay closures used to indicate alarm conditions.

Audemat's GoldenEagle HD FM/AM is a monitoring system really suited for radio clusters. The GEHD will sequentially monitor a list of stations (up to 10) for performance aspects such as the analog RF and audio levels and RDS data, along with HD RF levels, audio levels, and time alignment. The GEHD supports SMTP and will send messages out regarding error conditions that it finds (all user-configurable, by the way). Error-condition logs can be generated as well, with results stored up to 30 days. The proprietary user-interface must be downloaded by an authorized user (with appropriate credentials) using standard Web-browsers; afterward the user-interface is installed on the user's computer. All configuration and subsequent monitoring is done with the U.I. It has balanced outputs (on XLR connectors) corresponding to the decoded audio as it scans through its (user-configured) presets, and it also supports streaming, so the remote user can listen in to any selected analog or digital signal. The GEHD also has the capacity for up to five (optional) GPIO boards -- in essence it becomes a remote control in addition to a monitor. The user can schedule up to 10 daily recordings of audio that can be downloaded later for listening and analysis.

IBOC transmissions are a couple of orders of magnitude more complex than the AM and FM transmission schemes we've all become familiar with throughout our careers and for that reason the monitoring of said HD Radio transmissions is far more complex as well. There are many anecdotes in the trades about HD Radio not working and the vast majority are examples of misinformation. How many of these came from listeners' first impressions when they encountered a system that wasn't set up correctly? Whether or not you believe HD Radio has a future (I do) I implore you to make your best effort now to optimize your systems.

Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at doug@dougirwin.net.

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