The 5 kHz AM Reality
Guy Wire Says: C'mon, Boys, Wake Up and Smell the Radio
By Guy Wire
Guy Wire is the pseudonym of a well-known engineer in the radio
industry who prefers to remain anonymous. This is an opinion column;
opinions expressed are Guy's own.
 I'll
never forget my first experience installing a Harris MW-1 transmitter
in the fall of 1978. This solid-state thoroughbred with a new Optimod
9000 sounded great but was real picky about antenna bandwidth. The
rig would shut down on the night antenna during modulation peaks.
My five-tower in-line array just didn't like it.
Broadbanding that pencil beam looked pretty hopeless, although
we did make some improvements with it later on. Decreasing the VSWR
protection trip-point rewarded me with a pile of dead PA transistors.
I initially found relief by rolling off much of the high-end boost
and reducing modulation. But the best final fix included maintaining
pre-emphasis from 2 to 5 kHz and installing a high-quality 5 kHz
lowpass filter at the Optimod input. Modulation was then cranked
up with no problems.
I could hear the obvious difference on the wideband Belar modulation
monitor using the filter and first thought a 5 kHz cutoff sounded
dreadful. This was during the pre-NRSC era, so reducing 12 kHz audio
down to only 5 kHz on a music station was going to be brutal and
would never fly with the PD. Or so I thought.
Originally I was going to use a relay triggered by day-night mode
switching to engage the 5 kHz filter for nighttime only. But after
listening to non-directional daytime performance on a few of my
own radios with the hardwired filter running, I was surprised how
bright the station sounded. So I decided to install the relay and
do some additional testing switching the 5 kHz filter in and out
while several of us monitored closely. After listening on dozens
of radios of all flavors, it became apparent the difference was
so minimal that it was almost shocking. On most car radios there
was NO difference.
With the MW-1 cut off at 5 kHz, not only did the high end that
was present sound cleaner, but I could also maintain higher average
loudness and positive peak modulation. Obviously the antenna was
part of the culprit at night, but listeners benefited even without
the antenna limitations.
THE MAKING OF BAD RADIOS
How had the typical AM radio gotten so pitifully awful passing
high frequencies? It wasn't widely known back then why radio manufacturers
had chosen to roll off AM response starting around 2 kHz on so many
designs. The band had filled up with lots of new signals over the
preceding 20 years and noise was becoming a real nuisance. But the
dirty little secret fully emerged in the 1980s as FM was stealing
multitudes of AM listeners. AM stereo had been adopted and was struggling
to restore competitiveness for AM music formats.
The NRSC can be credited for flushing out the complete truth. Radio
manufacturing companies had been focusing all of their effort on
improving FM receiver performance. FM had replaced AM as the listeners'
preferred choice for music listening on a huge scale. Between 1970
and the early '80s, the entire industry had flipped. AM had been
relegated to news-talk in order to survive and didn't need wide-bandwidth
performance. AM stereo didn't have a chance and essentially died
with an inaudible whimper.
The other excuse for rolled-off AM radios always given by the manufacturers
was the added "benefit" of noise reduction. Car dealers
were not happy when the buyer of a new car returned it complaining
how noise and interference had spoiled the radio reception.
MAKING THINGS WORSE
Noise interference had grown insidiously over the years, but a
new source was not the fault of power lines, light ballasts or motors.
Few wanted to admit it, but AM broadcasters provoked some of the
increase in receiver high-end rolloff themselves. As AM music stations
lost share to FM, PDs insisted on more brightness to compete.
New-breed, higher-performance transmitters and audio processors
sporting multiband compression and EQ controls gave AM engineers
a doubled-edged sword to battle back. As modulation density and
the highs got turbo-charged, adjacent channel interference and noise
across the band increased dramatically. The receiver boys responded
by further reducing bandwidth and high-end response to cut down
the annoying interference. AM has never been the same, and never
will be again in the analog world.
Too many forget that prior to 1970, AM radio was largely a de facto
5 kHz transmission service. Phone line STLs, wimpy plate-modulated
transmitters with only peak limiters and flat EQ all added up to
a dull sound. But this combination also produced limited adjacent
channel interference. Most receivers had long before rolled off
everything above 5 kHz to eliminate 10 kHz carrier whistles at night.
It's prophetic we have now come full circle.
NRSC TO THE RESCUE
The NRSC held lots of meetings and commissioned studies on how
to best cope with the problem. If only the receiver makers would
spend some effort building a better AM section and open up the high
end again. After considerable hand-wringing and worrying about how
reducing transmitted bandwidth would make AM even less competitive
with FM, the NRSC recommended to the commission a standardized pre-emphasis
curve with a 10 kHz brick-wall limit. It seemed like the best compromise
at the time and did reduce second-adjacent interference.
A few of the engineers who participated in those proceedings saw
the larger picture and favored a 5 kHz limit instead of 10 kHz.
Quite a few more, including Bob Orban, preferred 5 kHz for nighttime.
That was around 1989. They knew the bandwidth battle for better
AM had already been lost to the receiver boys. Europe had long before
chosen a 4.5 kHz limit. The world was filled with AM radios that
couldn't hear much above 4 kHz, let alone anything higher.
There have been a few notable exceptions, of course. The Sony SRF-100
AM multi-mode stereo receiver, now a collectors item, was my favorite.
The C. Crane radio and GE SuperRadio III both sport wide-band AM
response. But sadly, only radio engineers and some fans of "AM
Coast to Coast" and a few other talk shows have bought and
use them. The best feature of those sets is the superb RF front
end. A non-technical user chooses the mode that yields the least
interference, and that's almost never wide band.
A few car radio models still have better AM response than the vast
majority of OEM or after-market radios installed in cars over the
past 35 years. Some of the Chrysler/Mitsubishi products are among
those. GM/Delco/Delphi have always been the most pathetic. But those
that feature any response beyond 5 kHz are few and far between.
Anyone who argues they can hear the difference between 5 kHz and
10 kHz transmission on most radios - including those using sharp
low-pass or cheap ceramic IF filters that produce ringing artifacts
- and think it's of any importance to the AM listening public are
simply self-delusional.
Hindsight is always 20/20. The NRSC really gave the safe-keepers
of high-fidelity AM a half-baked concession. Only a tiny minority
of AM listeners was using or ever would use a radio that could take
advantage of audio above 5 kHz. Everyone else would just be subjected
to the higher probability of unnecessary interference.
CONSPIRACY THEORIES
Let's face it: The laws of physics have always conspired against
transmitting wide bandwidth for AM broadcast simply because of the
9 and 10 kHz channel allocation schemes used. No other service in
the entire electromagnetic spectrum was conceived and authorized
to allow adjacent-channel stations to deliberately interfere with
each other. You've got to wonder what the founding fathers of the
FRC and FCC were smoking.
The only way to ensure that such interference is mostly eliminated
is to reduce transmitted modulation bandwidth to 5 kHz. While this
constraint is less important for interference-free daytime service,
it is crucial to keep next-door neighbors out of each other's hair
at night.
With all of the foregoing revelations, it's not hard to understand
why my good friend Jeff Littlejohn at Clear Channel along with Cris
Alexander at Crawford Broadcasting have taken the bold and courageous
step to voluntarily limit all of their AM talk stations to 5 kHz
and music stations to 6 kHz.
It's all about facing up to reality and deferring to good common
sense. A few of us saw the benefits of doing this some time ago
and made the switch on our own stations, in some cases without first
informing the PD. The only responses I've heard about were either
nothing at all or in a few cases: "Hey, I think we're sounding
a bit louder and cleaner. Did you change the processor settings
lately?"
In the middle of this controversy we are now hearing from those
who see the self-imposed 5 kHz limit as a deliberately planned tactic
of the vast pro-IBOC conspiracy. It's all being done to "dumb
down" AM performance so when HD Radio is added, nobody will
hear any difference on analog radios while HD will sound "spectacular."
Newsflash: Nobody hears it now anyway. Wake up and smell the radio.
Only a few die-hard engineers still think analog AM can be or should
be high fidelity. The public never thought it was. You'll get your
precious wide response plus dramatically reduced noise interference
by going HD Radio.
AURAL TRUTH
Here's the truth very few engineers who insist on high-fidelity
standards for AM broadcast audio are either loathe to admit or don't
understand: The human ear and the psychoacoustic processing of the
brain are much more prone to respond negatively to a listening experience
degraded by noise or interference long before it is degraded by
reduced frequency response.
The same applies to degradation by distortion. The typical listener
is much more tolerant of even moderate levels of harmonic, intermod
or quantizing distortion than they are of noise-related interference
degradation that competes with their ability to easily understand
or appreciate the programming content.
Some of the truth of this revelation has been hammered home in
recent years by the wide acceptance of low-bit-rate MP3 audio. Audiophiles
continue to wretch at the thought of giving away over 90% of the
original information inflicted by lossy compression instead of acknowledging
the actual sound of the results is remarkably impressive. While
purists still complain about nuance degradation, consumers voted
the MP3 as CD-quality long ago.
GAINS AND LOSSES
Since Clear Channel deployed the 5 kHz limit, we are hearing reports
of listeners being able to hear many of the 50 kW clears at night
for the first time in years in areas where nearby first-adjacent
stations have made the switch to 5 kHz. Sadly that capability will
be lost again as stations begin adding HD Radio. Everyone realized
that making the hybrid HD-AM mode work involved some delicate compromises.
But the gains will most certainly far outweigh the losses.
I have stated here many times in the past that opening up HD-AM
at night is going to be painful for some stations. There will be
complaints and a number of stations will be forced to reduce HD
Radio carrier levels. A few may not be able to run HD at night at
all. But I'm betting most of the complaints will come directly from
station owners, managers and engineers themselves rather than those
generated by listeners.
Only a small percentage of all radio listening occurs on AM at
night. The NAB says 19% of that is AM, so less than 5% is nighttime
listening. Ibiquity says only about 1% of that total will be challenged
by skywave-induced digital interference to analog in protected coverage
areas. We'll have to wait and see if that's accurate, but it seems
reasonable the figure will be very low. Most of the coverage losses
real listeners will experience as fulltime HD-AM ramps up could
be reminiscent of what happened when AM stereo was "lost."
Folks inside the industry were very disappointed, but the public
barely noticed.
How much longer do we have to listen to the small band of purists
and hobbyists who loudly insist that analog AM radio can compete
for the long haul in an era thoroughly dominated by so many other
full-fidelity digital media choices? Put away your buggy whips,
boys, and accept the reality of 5 kHz AM while the hybrid HD Radio
transition phase paves the way to an all-digital AM future.
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