excellent. In my area (Austin, TX),
there are many HD FM stations. Most
of them have two or three auxiliary or
multicast channels. I was really surprised at this. Given all the multicast
options, the HD basically doubled the
number of stations in my area. Cool!
As for AM, there were only two
with HD. I suspect that will be the
case in most areas, but you never
know. Check it out for yourself.
Incidentally, HD on the FM band is
24/7. On the AM band, HD is only
available during daylight hours. The
FCC prohibits night time HD broadcasts because of the very long range
possible because of skip propagation.
It is common for one distant AM station to interfere with another on the
same frequency at night. You can still
hear the AM long distance, of course.
Quality-wise, the sound is very
good for a small radio. I could actually tell the difference between the
analog and digital signals. Describing
how audio sounds is pretty much like
trying to describe how a particular
wine tastes or whether vacuum tube
audio is better than solid-state audio.
Subjective to the max. In my case, the
analog is more of a softer or muffled
sound while the digital, to me, is a bit
more crisp. Greater clarity like you get
with a CD or MP3 player. If you are
used to those, you will find HD radio
to your liking.
Technically, the HD100 uses a
standard “can-type” AM/FM tuner.
The IFs come out at 455 kHz for AM
and 10. 7 MHz for FM, as has been the
case for decades. The IF is then sent to
be digitized by an analog-to-digital
converter. Several HD radios use a
chip made by Texas Instruments.
Referred to as the AFEDRI82001, it
uses a 12-bit pipelined A-to-D converter sampling at 80 megasamples
per second (MSPS). This chip then
filters the digital signal with DSP filters
and decimates the signal down to a
lower data rate. The outputs are the
in-phase (I) and quadrature (Q) signals
in digital form needed by the DSP
chip to do its FFT.
The DSP is Texas Instruments’
TMS320DRI250 digital baseband. It
does all the decoding, demodulation,
filtering, and other processing stuff
that is part of the iBiquity protocol.
The digital outputs then go to
DACs where the original signals are
recovered. The audio power amps —
either class AB or class D — drive the
two speakers. A separate amplifier is
used for the headphone output.
Overall, the HD100 is an excellent buy. I have tested several other
HD radios and they are all comparable but I like the sound on this one the
best. You can get it directly from
Radiosophy at www.radiosophy.com.
For a detailed look at more HD
radio stuff and a good section on how
HD works is available at the iBiquity
site mentioned earlier. If you want
the real gory details, go to the Texas
Instruments website at www.ti.com
and click on DSP or search for HD
radio. Look for HD radio as an option
on new cars in the not-too-distant
future. Until then, after market HD car
radios are available. NV
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October 2007 81