-------------------------------------- AM DIGITAL SIDEBAND POWER With all the discussion regarding the true RMS power in the digital sidebands for the iBiquity AM IBOC system, I decided I needed to conduct some research. It has taken me a while to post because I needed to consult with some experts and run some tests to verify our findings. To me this is basically an academic exercise, albeit a valuable one: the AM IBOC system will work, or not, regardless of our calculations or discoveries here. TOTAL TRUE RMS DIGITAL POWER I have learned that calculating the "total true RMS digital power" in the AM sidebands utilizing the iBiquity IBOC digital transmission scheme is more complex than it first appears. The reason for this is the presence of the multiple OFDM carriers within the measured passband. Typically we deal with a single carrier, or compare a single signal to another single signal and express that as a ratio in dB. A spectrum analyzer will conveniently display the relationship between signals in dB, making this a fairly simple matter. However for the purposes of calculating the total true RMS digital power, given the multiple individual OFDM carriers involved, it is necessary to calculate the aggregate power of the multiple carriers, integrated across their full passband. This is NOT the power you will observe directly on a spectrum analyzer display when setting up the system, or measure directly in the field with a field intensity meter (which is a voltage device not a power meter). IBIQUITY DIGITAL POWER LEVEL SPECS. The NRSC and iBiquity standards give the maximum AM digital sideband power as 27.8 dB below unmodulated carrier (dBc). When we set up our AM HD transmitters we adjust the digital sidebands so that they fall at least 27.8 dB below the carrier as measured on a spectrum analyzer with 300 Hz resolution bandwidth, as per the iBiquity spec. This is the figure we have been concerned with in the "real world" when deploying these systems, since this is the spec we need to meet. And we always certify that with a spectrum analyzer each time we launch a system. We have purchased quite a few spectrum analyzers for the purpose of setting up AM and FM digital systems. They are also helpful of course for verifying occupied bandwidth and looking at spurious emisisons. BANDWIDTH CORRECTION FACTOR. But in the case of "total true digital RMS power" these numbers, and the spectrum analyzer display, don't tell the whole story. To compensate for the difference in the 300 Hz measurement bandwidth used for spectrum analysis versus the total bandwidth occupied by all the digital carriers in the sidabands, a "Bandwidth Correction Factor" must be employed to integrate the power across the band of interest. The Bandwidth Correction Factor ("BWC") can be expressed as 10 log(4360.5 Hz / 300 Hz), or +11.6 dB. This represents integration of the power of the static, individual OFDM digital carriers across 4360.5 Hz. ADDING IT ALL UP So we add the BWC of 11.6 dB to the measured (required) digital sideband signal level of -27.8 dBc (what we read on our spectrum analyzers) and we get a total sideband power (for the upper or lower digital sidebands) of -16.2 dBc. Given this fact, the interfering true RMS power/energy of the IBOC digital sideband to a first adjacent station for a 50 KW AM station is 1200 watts, which comports closely with Barry McLarnon's figure for true RMS power of the combined digital carriers, not 500 watts as I stated in an earlier post (the 500 watts was not integrated or bandwidth corrected). There was no deception intended. It's all a matter of calculations that are applied to the observed measurements, taking into account the nature of the signal we're measuring and the bandwidth of the tools we're using. Self interference to the host analog station is 3 dB worse, since both sidebands are present within the host envelope. The total RMS energy in the two digital sidebands, combined, is 13.2 dB below unmodulated carrier within the host channel envelope, or 2400 watts for the 50 kW AM example. IS IBIQUITY WRONG? But iBiquity says each sideband is 27.8 dB below carrier. Accounting for both digital sidebands, the digital signal is still only spec'd at -24.8 dBc. What's wrong here? Are we all exceeding legal power? No. -27.8 dBc is the correct level observed on a spectrum analyzer for each of the primary digital sidebands when setting up the system. It's not the total true RMS power integrated across all the digital carriers, which is what is being discussed here. It's also worthwhile to note that when we speak of a 50 KW analog station, that too is not the power integrated across the full audio passband when modulated with a complex waveform. Additionally, with 100% modulation the analog power will actually be 75 kW (150% of unmodulated power), but the digital power never changes. What does all this mean? It means both numbers (-24.8 dBc and -13.2 dbC) are correct. This is just about definitions and measurement methodology. CUT TO THE CHASE: EXPERIMENT AND RESULTS. Ok, this is a lot of theory. To confirm these numbers, a transmitter manufacturer conducted a test at my request. Using a waveform independent, true-RMS power meter, total power in a properly adjusted IBOC AM transmitter was measured with unmodulated carrier and all digital carriers turned OFF, and again with all digital carriers turned ON. The tests confirmed the math above: RMS power on either sideband was 16.2 dB below unmodulated carrier, and RMS power within the full band was 13.2 dB below unmodulated carrier. This was also repeated using a calorimeter, which is truly bandwidth agnostic, and the same results were observed. So I'm fairly confidant that -13.2 dBc for on band digital to analog, and -16.2 dBc for first adjacent digital to analog, are the right numbers for true RMS power of the digital carriers in a properly adjusted AM IBOC system. Just remember these refer to an unmodulated carrier. Also all of this is for the HYBRID mode, that is, analog and digital co-existing on channel. Pure digital transmission is another matter. WILL IT WORK? SORRY I DON'T HAVE THAT ANSWER. None of this guarantees that AM IBOC signals won't create interference to other AM analog stations in the field, nor does it guarantee that they will. While the system authorized by the FCC today has been extensively tested and verified by the system designers and the NRSC, I'll be the first to admit that those were for the most part (except for some field tests) laboratory conditions. As they say "the proof of the pudding is in the eating." Over 750 AM and FM stations have been converted to digital as of today. For the reasons I and others have mentioned in previous posts such as array and amplifier nonlinearities, pattern bandwidth, and impedance asymmetry, it is certain that there WILL be cases of interference. This will be something we as engineers will be asked to correct. And the transmission system designers and receiver manufacturers will also continue to work to improve the system. As the owner of more AM radio stations, in markets large and small, than any other broadcaster, you can rest assured that Clear Channel is very concerned about the potential for interference in the AM band. Whether our stations are the cause, or the victims of interference, we will work diligently to correct it. We have more to lose than most. Thanks, Steve Davis Senior Vice President, Engineering Clear Channel Radio