[IRCA] MW-IBOC carrier power

["Dan Strassberg" <dan.strassberg@xxxxxxx>]

12 dB down from 50 kW is NOT 625W! It is 1/16 of 50,000W or 3333W. That's
assuming we're talking dB power and not dB voltage. Although dBs are a way
of expressing the logarithms of dimensionless ratios, they were originally
defined for power ratios. dB = 10 log (base 10) P2/P1. Since the power
dissipated in a resistor is poportional to the square of the voltage across
the resistor (P = V^2/R), it was a very small step to dBV = 10 log (base
10) V2^2/V1^2 = 20 log (base 10) V2/V1. But don't mix dB and dBV within a
calculation or formula because the numeric results will be wrong!

I suspect that I came up with 15 dB, where the correct answer was apparently
12 db, because, in all likelihood, the 12-dB number applies to the ratio of
the main-carrier power to the combined power in both digital sidebands. The
power in both sidebands is twice the power in one of the sidebands. A power
ratio of 2:1 = 3 dB.

Most likely, we're also talking about power at zero modulation, for both the
main (analog) carrier and the digital sidebands. Although the FCC allows
125% modulation of the AM carrier on positive peaks, I believe that most
stations average modulation index is considerably lower--probably a bit less
than 40%. In the ridiculous extreme of modulating an AM carrier with an
audio-frequency square wave and hitting 125% on the positive peaks, the
power on the positive peaks would be almost exactly five times the nominal
carrier power (2.25^2 = 5.0625) and the average power would be about 2.5
times nominal carrier power. Since, when broadcasting normal program
material, modern highly efficient 50-kW AM transmitters usually have a power
input of about 85 kW (that is, 1.7 times the nominal carrier power), it is
pretty clear that the normal average modulation level is substantially lower
than 0.5*(125%) or 62.5%.

I think that the reason the MW-IBOC digital sidebands sound so much louder
than their relatively low power (~1600W per sideband for a 50 kW AM)
suggests, is that the average index of modulation is much higher than that
of the main (analog) carrier. iBiquity's HD Radio systems are designed
around a system called OFDM (orithogonal frequency-division multiplexing).
OFDM uses multiple carriers. The HD Radio digital sidebands thus contain
multiple subcarriers, each of which conveys information via a complex
digital-modulation scheme--at least BPSK (bipolar phase-shift keying), but
more likely QPSK (quadrature phase-shift keying) or 16 QAM (16-level
quadrature amplitude modulation).

If I'm wrong about any of this, please don't hesitate to correct me.

--
Dan Strassberg, dan.strassberg@xxxxxxx
eFax 707-215-6367





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