AM-Stereo!!
Let's not forget the long lost luxury of
AM-Stereo which appeared on the top line GMT400 stereos through ~1991 (the '91 brochure lists AM-Stereo as an option, but I believe it was canceled by M/Y 1992). This feature is a real historical curiosity (or oddity) of early GMT400's and other GM vehicles of the era. Read at the bottom of this post how to sample this feature.
Historical perspective:
Younger owners should realize AM stations were super popular in the 50's-early/mid 70's. Massive stations could 'skip' across the upper atmosphere and be heard for thousands of miles and some could be heard across the entire USA! Wolfman Jack radio show out of Mexico, then CA could be heard across the country on many nights (i.e. American Graffiti). Their popularity and selling nationwide ads was big business! If you were in the midwest, you'd know the names Larry Lujack, John Landecker, Tommy Edwards, The Boogie-check radio show, etc in the 70's, from WLS (Chicago), another powerful station that every radio/music enthusiast in the midwest (and states across the USA) tuned-in to hear their DJ antics and the latest rock music of the day. And there were many other powerful stations around the country, with popular DJ personalities. This was the era of Route 66 too, baby boomers, car culture, and all that. Things changed in the 70's. By the 1980's, FM's clearer delivery and broader dynamic range had taken over in terms of music delivery. There were [then] many fewer but more regional FM stations. That's how people got their music and info back then. AM-Stereo was an emerging technology advertised to be the 'next big thing' with the 'promise' it might be able to breathe life back into the decimated AM relm (due to it's longer range). It took quite a while for the FCC to settle on AM-ST approval, which might have been part of the problem with AM Stereo proliferation.
Back then, hi-fi and stereo systems were like phones today: Everyone had to have a good one, and as technology and features improved leaps and bounds, people bought a newer/better stereo systems ...home and car. The electronics industry sold the 'dream' of AM Stereo pretty hard among hi-fi enthusiasts at the time. I don't know if the average-joe really even knew it was coming down the pike, which is why GM probably only put it on the highest end stereos -- audiophiles were more likely to splurge for AM Stereo. In fact, I remember buying a new aftermarket car stereo around 1990-91 (for a car), and the main reason I stepped up to buy a TOTL Realistic unit was BECAUSE it
had the 'future-proof' AM-Stereo feature (and the 'must-have' Dolby-C cassette NR feature ...which never panned out either, lol!). I did a lot of highway cross country driving back then, across the Great Plains where sometimes you couldn't find much on FM or you were in an AM-only zone ...and used the AM-Stereo function fairly often late at night. It sounded pretty good, but not as good as FM (to me). But in the end, it was more of a curiousity; not enough stations converted over, or would only broadcast music in AM-Stereo during certain times of day (evenings usually). You couldn't really count on which station might or might not be broadcasting in AM-Stereo, and when. And it never overcame static issues during weather events, like it was sold it eventually would. It was probably better suited to local (regular) AM radio listeners with a compatible and better home AM Stereo unit ...like a regular WLS music listener. Had the technology come 8-10 years earlier, and improved, who knows, it might have taken off in a much bigger way.
There are still stations which DO broadcast in AM-Stereo even to this day. Many stations are still, in fact, owned by small time operators, real radio-enthusiasts who appreciate AM-ST. There might be one in your area. We have a couple around us which broadcast in AM-Stereo ..at times. I had forgotten about the AM/ST button in my tk and tried it a couple months ago on a bonafide Am-ST station. And, yeah, it still works! You can hear the difference. It doesn't sound as good as FM (still periodic static), but yeah, I've read if a guy has a real good home hi-fi stereo (with AM-ST), under the right listening conditions, it's quite effective.
In actual GMT400 use:
The feature was only available on the top-level (optional) ETR hi-fi system in our pickups which also included the cassette player
with the 5-band equalizer (note: the cassette deck has a Cr02 (high bias) and scan/repeat feature too!). If your radio has Bass & Trebble buttons on it's face, it doesn't have the AM-ST feature (according to the manual & sales brochure). Look for an actual AM-ST button on the radio, that's how you know. The technology GM used was C-QUAM (Motorola), the most common at the time, but not the
only AM-ST technology at the time. So our radios won't pick up stations broadcasting in another AM-ST technology, only C-QUAM. The feature is 'always-on' therefore the radio will
automatically detect and switch to Stereo in the AM (or FM) mode w/o any listener input should there be a suitable AM station in your area broadcasting in C-QUAM stereo. If you find a station broadcasting in AM Stereo, the green 'Stereo' letters will light up on the radio's fluorescent display, and you should hear the stereo separation.
This next little fact is important: Pressing the 'AM-ST' button actually
cancels AM-Stereo and puts the radio into Mono mode. It's somewhat counter-intuitive use of the button. "Non-Stereo may be desired in areas of high noise level." per the manual. So if you want to try it, turn on the radio, flip through the AM stations in your area (probably better at night while they're playing music) and look for the 'Stereo' light to come on, or call/website your local stations to see if they have the capability and when they use it. Sometimes it takes a few seconds after an AM station is locked-on for the 'stereo' light to illuminate, so go slow. The above information was per a 1989 C/K owners manual: pg 2C-24, any my experience.
Summary:
AM Stereo, while obscure today, was at one time a promising technology when radio was king. It is an interesting historical curiosity that made it's way into well-appointed early GM OBS pickups and cars. There doesn't seem to be a very good database of AM stations which continue to broadcast in AM-Stereo, but some still do. So if your tk
has the AM-ST button and you want to sample it, you'll have to be persistent and search out stations in your region; if and when they broadcast in AM-ST, or scan through the AM band manually. Pushing the "AM-ST" button
cancels AM stereo (goes mono), but you can toggle on/off to hear the difference if you find an AM Stereo station. You probably won't be overwhelmed by the quality by today's audio standards, but interesting to try nonetheless. It might be an interesting "Gee-wizz" topic amongst your later-year OBS buddies, of the 'somewhat rare AM Stereo feature' ...lol. Sorry for the long (and perhaps boring) post....
Cheers!
The Official AM Stereo Web Site is a free resource for AM Stereo radio broadcasters and listeners.
www.amstereo.org
meduci.com