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Hi Tom.  The answer to your two questions depends on what is meant by "what 
happened"?

1.  If you're thinking about the fact that there's no apparent 'high end' in 
AM audio, that's about 99.99% due to the restrictions designed into today's 
radios.  Receiver 'bandwidth' has been deliberately narrowed, in an attempt 
to filter out the electrical noise and adjacent-station interference 
inherent in the AM radio band.

About 25 years ago the broadcast industry developed a pragmatic approach to 
the problem.  We recognized that, due to band crowding and the awesome rise 
of electrical interference, it was no longer possible to transmit "wideband" 
audio as had been done a generation earlier.  Any station that generated 
'flat, wide-band audio sounded "dull" on the new generation of AM radios 
because of their narrow filtering.

So the National Radio Systems Committee (NRSC) got behind a new AM 
"pre-emphasis" transmission standard, that reduced the very top frequencies, 
but provided an artifical 'boost' in the audio that would add more 
'presence'.  (Energy climbs toward 11 khz and then drops sharply.)  The idea 
was to encourage the receiver industry to build a radio that would track 
this new curve.  Failing that, the boosting of the apparent high-end audio 
would still sound better on the narrow radios that would a station without 
the boost.  Broadcast trade groups (NAB) pushed a new receiver standard 
("AMAX") but by then the manufacturers were no longer interested in what 
they considered a dying market.  Had the radio-makers responded, AM radio 
would sound very good, even in today's noise environment.

Today just about every AM station follows this pre-emphasis curve and, while 
it is a pragmatic solution, the subjective impression one gets from such 
audio...when heard on a decent AM receiver...is quite good.

2.  If you're talking about the lack of dynamics in audio and the overall 
"squashed" sound of many stations, broadcasters discovered that the harder 
and 'louder' they could make the signal, the better they could beat down the 
aforementioned noise problems.  New digital audio processors allow stations 
to really smash the audio to the point where it no longer sounds natural.

Ironically, due to the imperatives of AM digital and the investments in new 
antenna systems as stations do power upgrades, the actual audio from the 
towers is considerably better than it was a dacade or two ago.  But the 
rising noise and interference levels and the Physics behind AM broadcasting 
make it a far-from-perfect transmission medium.

And every time we screw in a new CFL bulb in a living room lamp, we add to 
the problem!

Good health to all!

Mark Durenberger


----- Original Message ----- 
From: "Tom Fine" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, March 21, 2009 9:27 AM
Subject: [ARSCLIST] AM radio sound quality


>I recently had in the studio some old AM radio aircheck tapes, circa late 
>60's and early 70's, covering several AM stations in metro NYC area. These 
>were half-track 7.5IPS tapes made on an old Ampex belt-drive consumer deck 
>from the early 60's, probably an "A" type model. I was told the tuner was a 
>Scott tube AM tuner from the 50's. These tapes sound GOOD, not full 
>fidelity but certainly more frequency range and much better clarity on 
>voices and musical instruments than modern AM I receive in my vehicles (I 
>don't have an AM tuner at home, perhaps there are still decent-fidelity 
>tuners made but I can't get any AM stations worth listening to witha a 
>clear signal in my neck of the woods). I also know from 
>point-of-origination and network-line sourced radio transcriptions that, 
>back in the day, there was the potential for very good sound quality headed 
>out to AM transmitters. So here's my question -- what happened to modern AM 
>sound? Is it the broadcast itself or modern AM radios?
>
> -- Tom Fine