HD Radio: Proving Performance with Modulation Error Ratio

July 1, 2013


Using MER

In February 2010, iBiquity released a paper on techniques for measuring the quality of IBOC transmissions. A quality-metric known as MER (Modulation Error Ratio) was introduced by the National Radio Systems Committee as a standard. If you don't read the entire document, it's important to note that stakeholders in the United States have agreed to this methodology.

After three years we are now seeing the introduction of MER metering for IBOC transmitters. Nautel recently made this available for its NV and NX series transmitters; Continental also has MER metering in its current HD Radio exciter, the 802Ex FM/HD. Nautel recently presented a webinar on its MER techniques.

According to information presented in the webinar, Nautel has implemented the FM MER technique described in the iBiquity paper in its NV series transmitters. There's no standard for AM, however, so Nautel has implemented what it refers to as a textbook standard methodology for its AM transmitters MER measurement.

So what are the implications of a low MER value, and how is MER measured? Conceptually (at least) it's pretty straightforward. Recall that the IBOC subcarriers are in groups (or partitions) consisting of both QPSK (for the data) and BPSK for the reference carriers. See Figure 1.

Figure 1. The FM HD Radio spectrum. Click to enlarge.

Figure 1. The FM HD Radio spectrum. Click to enlarge.


In an ideal situation, when QPSK carriers are demodulated the result would be perfect placement inside the constellation, resulting in easy error-free detection, (Figure 2). In the real world though, when the QPSK carriers are demodulated they show up close to the right (reference) spot, but not exactly right on.

Figure 2. The QPSK constellation showing error-free detection.

Figure 2. The QPSK constellation showing error-free detection.


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MER measures how off the actual readings are from the reference points, as Q1 through Q4 show in Figure 3 after demodulation.

MER example
Figure 3. MER example

Figure 3. MER example


The farther off the readings, the more difficult it is for the receiver to correctly detect the signal. If they are too far off then they can't be detected well, resulting in bit errors. Too many bit errors leads to a total failure of the demodulation and decoding process.

It is interesting to note that peak-to-average power reduction actually generates noise in the constellation, but that particular noise ultimately has little effect on the ability of the receiver to properly demodulate the data. David Hershberger, senior scientist for Continental Electronics, wrote in his paper "IBOC Signal Quality Measurements" that "Peak-to-average (PAR) reduction algorithms may introduce enough deliberate distortion to digital signals that the effects of a transmitter on MER are obscured. Special measurement methods can separate the effects of a transmitter from that of PAPR reduction." And further: "The PA(P)R reduction noise is large compared to other OFDM systems. A textbook evaluation of MER ... will result in the PA(P)R reduction noise dominating the measurement. For this reason NRSC has proposed several modified MER measurements which result in metrics which are closer to the true system performance." Both the Continental and Nautel implementations of MER effectively subtract noise generated by PAR from the measurement results.

The Nautel implementation of MER uses an RF sample from the transmitter output to derive the MER. (It's the same sample used for the adaptive pre-distortion.) The measurement is fairly granular in that it allows you to see the MER of each partition (or group of subcarriers) and therefore how each partition is being affected by some external influence. Some examples will demonstrate this.

In Figure 4, we see a partition fairly far removed from the center frequency, with a high MER. In Figure 5, we see a partition closer in, with somewhat of a lower MER.

Figure 4. Click to enlarge.

Figure 4. Click to enlarge.


Figure 5. Click to enlarge.

Figure 5. Click to enlarge.


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Low MER can actually decrease the coverage area of the IBOC transmissions, and especially for AM (according to the webinar) it can make the receivers in the field take longer to lock on the IBOC data, as Table 1 shows. Interference from MER and noise add linearly at the receiver. A standard Exgine signal has MER of 17-18dB using iBiquity PAR.

Data Carrier MERReduction in Service Contour
24dB0.05dB
22dB0.09dB
20dB0.14dB
18dB0.22dB
16dB0.31dB
14dB0.48dB
12dB0.74dB
10dB1.13dB
8dB1.79dB

Table 1. The effects of MER on FM reception.

An MER of 14dB or better is the standard for what is considered acceptable by iBiquity.

The MER of the inner partitions can be negatively affected (i.e., reduced) by components in the analog transmission as well. Stations using the MP3 mode (which you would use to broadcast HD3 for example) could inadvertently have their HD coverage compromised to some extent by the inclusion of high frequency SCAs. Figure 6 shows this.

Figure 6. Effects of increased modulation index. Click to enlarge.

Figure 6. Effects of increased modulation index. Click to enlarge.


Note that the MER for the inner partitions degrades as the baseband is loaded with more signals. A stereo subcarrier alone will affect IBOC quality. Traditional subcarriers primarily affect the MP3 partition, but can affect the MP1 as well.

Harris Broadcast will add MER measuring capability inside its new line of Flexiva exciters and transmitters after the introduction of the Generation 4 Exgine this coming December.

Broadcast Electronics provides an MER output via its IP interface for its VPe system; however, the company is still in the process of verifying how it compares to the NRSC standard (derived from the iBiquity paper). It is also in the process of developing software to verify MER per the NRSC standard, which is independent of the modulation equipment.

Aside from the more complex high-level measurements taken at the transmitter site, you will need devices at your studio location (or some other spot where reception is optimum) to keep track of the day-to-day aspects of your IBOC transmissions. Fortunately there are quite a few devices available.

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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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