World Broadcasting Unions Digital Radio User Manual - page 48
DIGITAL RADIO GUIDE
TERRESTRIAL TRANSMISSION SYSTEMS - HD RADIO
48
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Waveforms and Spectra
The HD Radio system provides a flexible means of transitioning to a digital
broadcast system by providing three new waveform types: Hybrid, Extended Hybrid,
and All Digital. The Hybrid and Extended Hybrid types retain the analogue FM
signal, while the All Digital type does not.
All three waveform operate well below the allocated spectral emissions mask as
currently defined by the United States Federal Communications Commission.
The digital signal is modulated using orthogonal frequency division multiplexing
(OFDM). OFDM is a parallel modulation scheme in which the data stream
modulates a large number of orthogonal subcarriers, which are transmitted
simultaneously. OFDM is inherently flexible, readily allowing the mapping of logical
channels to different groups of subcarriers.
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Hybrid Waveform
The digital signal is transmitted in sidebands on either side of the analog FM signal
in the Hybrid waveform. The power level of each sideband is approximately 23 dB
below the total power in the analog FM signal. The analog signal may be
monophonic or stereo, and may include subsidiary communications authorization
(SCA) channels. See Figure 4.10.
Figure 4.10: Hybrid spectrum allotment of FM HD Radio system
Extended Hybrid operation involves use of us to four Extended Partitions in
addition to the 10 Main Partititions.
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FM Extended Hybrid Waveform
In the Extended Hybrid waveform, the bandwidth of the Hybrid sidebands can be
extended toward the analogue FM signal to increase digital capacity. This