HD Radio: Proving Performance with Modulation Error Ratio
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 MER||Reduction in Service Contour|
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.
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.
- continued on page 4
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