The question has been raised in the past. What broadcaster was Portland's first to promote itself as a "High Fidelity Station"?

This would have been KOIN 940 KC.

Found this information during KXL research. The Oregon Journal, November 18, 1934 page 10 ad:
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You've Heard a lot ABOUT High Fidelity --
Now You Actually HEAR High Fidelity!
KOIN-The Journal -- Always the FIRST to test and
adopt advanced and improved Radio Engineering
principles -- now announces the perfection of--
High Fidelity Transmission.
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Both Ron Kramer and Jeff Miller agree the ad announcement was to call attention to KOIN's new transmitter but was not technically "High Fidelity" for the time period. High Fidelity broadcasting was still experimental. Just six months earlier on May 25, 1934 the Federal Radio Commission approved the first four High Fidelity stations:

W1XBS 1530 KC Waterbury, Conn.
W9XBY 1530 KC Kansas City, Mo.
W6XAI 1550 KC Bakersfield, Calif.
W2XR 1550 KC Long Island, N.Y.

At the time, the AM Band ended at 1500 KC. High Fidelity stations were placed away from the standard AM Band and positioned at 20 Kilocycles for possible interference from adjacent High Fidelity stations.

KOIN's huge transmitter, which was put into service at Sylvan on November 11, 1934 must have been quite impressive none the less. Why else would Ed Craney, owner of KGIR (later aka KXLF) Butte, Montana drive 700 miles just to take notes and get ideas for his own transmitter upgrade, as reported in The Journal, November 26, 1934. [side note: Ed Craney would later become part owner of KXL in 1937]. The KOIN transmitter must have been built well and very efficient. Ron Kramer tells me it was still in use in the late 1960's.

The KOIN transmitter was designed and construction supervised by Louis S. Bookwalter, Technical Director. Others involved in construction: Victor C. Carson (formerly KXL Senior Technician) KOIN Chief Engineer; Earl Denham, Chief Assistant, and Technicians: Lloyd Weddell, Byron Paul, Bud Rasmussen & L. Mathews.

As a side note to all of this, KOIN's sister station KALE also promoted itself as a High Fidelity station when its transmitter was replaced with a home brew apparatus. The Oregon Journal, December 17, 1934 ad:
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Radio's Latest Advancement
+HIGH FIDELITY+ Transmission
KALE 1300 K.C.
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And this later ad on December 20, 1934:
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What Is HIGH FIDELITY TRANSMISSION?
The Natural Reproduction of Speech and Music.
Listen To KALE 1300 Kc.

I remember KEX use to have a show called SUNDAY NIGHT IN HI FI...It was all "beautiful music"

Wasn't KKEY's original calls something like KHFS, for HiFi Station? I seem to recall that when Ralph Weagant's dad built the station he chose those call letters.

In this case, it sounds like "high fidelity" was just a marketing gimmick, since there were no standards for what constituted "high fidelity." However, I do remember reading in a 1930s broadcast engineering book that by the 1930s, transmitters were being built that were capable of 50 Hz-10 kHz bandwidth with a flatness of +/-1 dB or better.

I also assume that locally originated programs would have been where the "high fidelity" transmission on KOIN would have really made a difference. The network feeds had a bandwidth of only 5 kHz or so. Out of morbid curiosity, I wonder what kinds of microphones KOIN used at its studio back then.

Brade: Yes! The KHFS calls stood for "Kilowatt High Fidelity Station" when it signed on the air July 3, 1954.

Alfredo: I don't know what KOIN was using at the time for microphones.

.............but I just bet that it didn't hold a candle to another new-fangled invention just barely over the horizon, courtesy of our friend Major Armstrong......."FM Radio" I think they called it, I know it's been a little while, my memory in my 20+ years of old age isn't what it used to be but.......

Incidentally would this concept really be able to float nowdays, despite the whole IBAC movement (I call it more of a "fad" than a "Movement...")

Possibly bump the X Band up from 10kHz spacing to 20? 'Twould do quite a bit of justice for the fact the FCC requires all AM-X band stations to be in AM Stereo, I think. I really can't see too many compatibility troubles with a system like this, except likely the skip problem at night-time.

Assuming (or rather "hoping") AM IBAC (Ibiquity system specifically, I don't know that much about Kahn's system's viabiity yet) never takes off but FM IBAC does, this would be a perfect upgrade to AM (possibly even across the ENTIRE AM BCB, not just confined to X!!) especially for fellow non-or anti-IBACers who don't have IBAC FMs and want to indeed hear improved fidelity on AM................