■ FIGURE 2. AM band spectrum of HD radio.
some of the key problems that still plague
analog radio. Noise and static are the bane of
AM radio. And fading is common, especially
at night when long distance skip conditions
come into play. FM pretty much is noise free
because of its inherent insensitivity to amplitude variations. But operating in the VHF
range ( 88 to 108 MHz), the signal is subject to
multiple paths due to reflections. This also
causes fading. Digital techniques pretty much
take care of the noise for sure. Fading is also
somewhat mitigated on FM, as well. However,
fading on AM is not especially improved.
The greatest potential benefit of digital is
greater fidelity. Most AM stations only trans- - 15 - 10
mit analog frequencies up to about 5 kHz and
many cut off lower than that. This is definitely
low-fi but most of AM radio is talk anyway, so
we don’t really notice this. FM radio transmits
frequencies up to 15 kHz, truly hi-fi. On a good receiver with
decent speakers, you can hear the fidelity. Digital is supposed
to make this better. HD radio claims that AM will sound more
like FM and FM will sound more like CD quality.
Figure 2 shows how HD radio adds the digital information to AM. The digital data appears in new sidebands
above and below the regular analog sidebands which take
up the 5 kHz segments directly above and below the
carrier. HD radio extends the bandwidth of the AM signal
from 10 kHz to a total of 30 kHz. If you recall, it takes quite
a bit of bandwidth to transmit digital information. That is
why the digital signal is compressed first.
There are actually two sets of digital sidebands. The primary sidebands extend from 10 kHz to 15 kHz above and
below the carrier. The modulation technique is orthogonal
frequency division multiplexing (OFDM). OFDM divides
the compressed digital audio into multiple parallel streams
of lower speed digital data and modulates them on up to 81
different carriers using 64 QAM (quadrature amplitude
modulation). The secondary sidebands from
5 kHz to 10 kHz also use OFDM, but
employ 16 QAM modulation. There are
even some tertiary sidebands that are
“under” the analog sidebands meaning they
are in quadrature (90-degree shift) to the
station carrier). What you end up with at
the receiver is a 36 kbps digital bit stream
for stereo AM. The maximum theoretical Primary
bandwidth is about 8 kHz. That is better
than the 5 kHz of the analog signal, but few
can notice any difference on voice.
The biggest problem with HD AM radio
is that the digital sidebands sometimes spill
over into adjacent channels. The AM spectrum was laid out for 10 kHz station spacing.
Nearby stations were not assigned frequen-
■ FIGURE 3. FM band spectrum of HD radio.
cies that close together but most radios can still pull in distant stations that may be operating on an adjacent channel.
The digital sidebands produce a whining type of interference
with the station, sometimes making it unintelligible. This is a
horrible problem at night, so much so that the FCC has
banned HD radio transmission in the AM bands after 6 pm.
The FM spectrum of HD radio is shown in Figure 3. It,
too, places the compressed digital signal in extended upper
and lower sidebands. The OFDM sidebands occupy the
space from 130 kHz to 200 kHz range above and below
the carrier for a total bandwidth of 400 kHz, double that of
a regular analog FM broadcast. The resulting bit stream at
the receiver is 96 kbps for stereo which theoretically should
give CD quality audio. As for adjacent channel interference,
it is less of a problem in the FM band especially if local
stations are widely spaced. VHF FM signals rarely travel
more than about 100 miles anyway so the interference
problem essentially does not occur.
OFDM is a bloody complicated digital system. There
August 2006 13