Improving AM: Some Tough Decisions Ahead
Much of the
discussion and comments to the FCC about AM revitalization have
centered on interference and noise. These are serious issues standing
in the way of the AM band being once again viable. But are we also
missing the third side of the problem triangle, the receiver?
A major problem with
AM is the inferior sound delivered to the user. There has been talk
about an across-the-board power increase. This would, of course,
provide some relief from the noise simply by overpowering it in some
cases. But there is a downside to that solution, and it’s a matter
of money and questionable overall gain.
Take a simple
two-tower AM directional operating at 1 kW.
If granted a power
increase to 5 kW, the signal-to-noise on a typical radio might
improve, but this would require not only a new transmitter at about
$35,000, but a new phasor as well at about $25,000 to handle the
additional power, and of course, the tuning units for each tower, and
maybe some new coax.
So with labor and
engineering fees along with equipment, we are easily talking an
investment of close to 100 grand and a much bigger power bill. But
the audio will still be telephone-quality in the primary contour of
the station— still inferior to all other entertainment audio
because of the poor response and audible distortion of the typical AM
The issue of
bandpass for AM radios has been debated for years.
Unless you’re a
rather young engineer, you remember all the hooplah over the
implementation of the NRSC pre-emphasis and cutoff that was touted as
the way to make AM close to FM with an audio response to 9 kHz on the
radios that were to be built to complement that curve.
While the NRSC curve
became mandatory for transmitters, the receivers never came en masse.
And despite the new push for AM improvement, sadly, I think the
receiver issue may be the immovable object that stops revitalization
in its tracks.
The elephant in the
room is IBOC. All of us engineers know that you can’t improve
bandwidth and fidelity on AM radios and accommodate AM IBOC at the
same time. We are going to have to make a choice.
IBOC on AM is just
not cutting it. Can we finally admit that?
noise makes it impossible to open the bandwidth of a radio front end.
Even the narrow radios of today emit hiss and noise from IBOC
operations. Nighttime operation is an even bigger problem, with
adjacent-channel skywave service made unlistenable. Yes, a lot of
money has been poured into it, but it’s a massive failure. Major 50
kW stations using it report poor receiver lock, even in prime signal
areas during the day.
If we are going to
make AM viable in the ears of the listener, we are going to have to
kill AM IBOC and finally mandate a new receiver standard that
requires a reasonably flat audio bandpass of 6 kHz. (Ironically, this
is still less than that of a typical car radio in the 1950s!)
Optionally, this bandpass could automatically narrow for weak
At one time I was
among those who said reducing bandwidth to 6 kHz was blasphemy. But
the current allocation plan requires it to fight adjacent-channel
interference, and truth be told, 6 kHz sounds very good if the
receiver is well-designed. But this will only happen with a
rulemaking to mandate acceptable response and distortion.
transmission side, the FCC should mandate a maximum negative
modulation level of less than 95 percent for AM. Such a limitation
would eliminate some distortion issues present in existing
transmitters and receivers right away.
FCC decades ago mandated television standards to force inclusion of
UHF tuners as well as FM standards that mandated stereo FM reception.
This leveled the playing field and made both UHF TV and FM
competitive. It’s time for similar action for AM.
manufacturers need to stop whining about needing narrow operation to
fight noise. Most AM listening is done at signal levels above 2 mV/m,
plenty of signal to allow wider bandpass without letting in excessive
noise. But that can only be true if IBOC is gone. Maybe 100 percent
digital is eventually the answer, time will tell; but we can’t have
hybrid digital operation and improved quality analog at the same
If the FCC is really
serious about improvement, they are going to have to “get some
stones” to fight the iBiquity lobby, suspend AM hybrid IBOC for now
and implement a long-overdue receiver standard that could be applied
to new AM radios with a simple change in the chipset used in the
receiver front end. Manufacturers need to realize that AM radios
don’t need to be built for DX operation where extreme selectivity
is required at the expense of sounding good on local stations.
Manufacturers will make the change if mandated. Look how fast
the expanded band was implemented on AM receivers years ago, and no
one complained about any cost increase, because there was none.
If we don’t make
changes that result in drastic improvement to the actual listening
experience of the end user, we are just fooling ourselves on
revitalization. And the only way to be that drastic on the current AM
band is to improve the noise floor and the high-end response
and distortion to at least be in the ballpark of other media.
You can get rid of
all the noise and interference, change contour protection and
increase power levels, but if what comes out is still rolling off at
2 kHz, you will never make a dent against FM and iPods. It’s really
just that simple.
Larry Langford is
owner and chief engineer of WGTO(AM) and W266BS Cassopolis, Mich.
Reach him at email@example.com
Comment on this
or any story to firstname.lastname@example.org.