[BC] AM Stereo

[Greg Ogonowski]

For anyone interested in taking a trip down memory lane, you can download
the original Proposed AM Pre-Emphasis technical paper that Bob Orban and me
wrote back in 1983.

http://www.indexcom.com/pdf/A_Recommended_Preemphasis_and_Deemphasis
_Characteristic_for_AM_Broadcast_ .pdf

This is how it happened:

Every year, broadcasters would increase high frequency EQ, and the following
year the car radios would get narrower.
To stop this madness, as a Chevrolet Camaro owner at the time (comments not
necessary), I picked up the phone and started a dialog with Delco
Electronics regarding this.
I then picked up the phone again and called Bob Orban, a competitor at the
time, and told him to pack his bags, "we're going to Kokomo, IN."

After the visit to Delco Electronics, which was very impressive I might add,
the Proposed AM Pre-Emphasis Paper was written, submitted to Delco and other
auto radio manufacturers, eventually published in the Society of Automotive
Engineers (SAE) Journal, and became the beginning of the NRSC, which
eventually dismissed our pre-emphasis proposal in favor of a single-order
solution that didn't work as good, and then receiver manufacturers didn't
implement the standard.

AM radios in U.S. cars, back in the early 1980's, were just appearing on the
market with ceramic filters. Prior to this, they were usually designed with
a set of double-tuned IF transformers which gave the receiver a nice
3rd-order Bessel/Butterworth roll-off that was fairly easy to equalize using
our recommended pre-emphasis curve.  They could be made to sound very nice.
Ceramic filters, although they are available with Bessel/Butterworth
characteristics, are generally used with Chebyshev characteristics, to get
better adjacent channel rejection without regard for audio quality. This
gives the AM receivers a nasty "ringy" sound with a steep rolloff that is
difficult to equalize. Furthermore, this added characteristic to the "mix"
of radios "out there," makes it very difficult to broadcast with a good
compromise EQ/pre-emphasis that works good on all radios now. Delco, Ford,
and Chrysler radios were some of the best radios available at the time. The
manually tuned radios (MTR) versions, as opposed to the electronically tuned
radios (ETR), also used variable inductors for tuning, instead of variable
capacitors. Variable inductors keep the bandwidth constant from one end of
the band to the other, so it is not necessary to compromise the RF section
of the receiver, and adversely affect the audio from one end of the band to
the other. This is an important receiver detail that many are unaware of.
Today the ETR receivers from Delco are double-conversion, up to 10.7MHz,
then down to 450kHz, using a Philips chipset. They provide great RF
performance at any frequency on the band, but with the 3 kHz ceramic filter
required to filter the HD Radio sidebands, they pretty much give you an AM
radio that is impossible to make sound good, especially for music.
Furthermore, in doing the Orban Optimod 8600 FM research, I learned that
they managed to even screw up the FM sections with a -7dB rolloff at 15kHz,
not to mention the terrible audio signal processing for the FM blend! That
being said, only a CD sounds good on these automotive sound systems. I
finally added a third-party AUX input to plug in an iPhone to enjoy HE-AAC
streams in full fidelity using an App called StreamS HiFi Radio. That pretty
much blows everything away, provided the stream is audio processed nicely.

AM radio today is what it is. I don't think it will ever get audio fidelity
back. AM HD Radio requires a 5kHz analog filter for reliable HD, which
pretty much finishes off analog AM for music.

And, for your dining and dancing pleasure, here is an excerpt from one of
the only known three CKLW AM Stereo airchecks in existence. This was
recorded off-air with a Sansui receiver and the correct de-emphasis. It was
processed using a Gregg Laboratories 2540 S/N 010008 with the Harris AM
Stereo system. May Ed Buterbaugh rest in peace.
If only AM HD Radio sounded this good, but it can't since the codec is not
very good.

http://www.indexcom.com/audio/CKLW_AM_Stereo_Reel_01_0982_de-emph_2.00.zip

This is a 2:00 excerpt in linear PCM .wav. 20MB
Enjoy.

-greg.

_________________________________________________________
Greg Ogonowski
VP Product Development
orban
Diamond Bar, CA USA
http://www.orban.com

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