[BC] Analog AM bandwidth: tail wagging the dog?

[Robert Orban]

At 09:45 PM 10/2/2007, you wrote:
>Radio World reports that the NRSC has revised its voluntary standards to
>accomodate reduced-bandwidth analog AM audio, and has also officially
>retired the failed "AMAX" wide-bandwidth AM Stereo receiver guidelines:
>
>http://www.rwonline.com/pages/s.0101/t.8737.html
>
>Based on a study of 30 receivers, the NRSC recommends an analog AM audio
>bandwidth of 7 kHz.  But what if they did a study of FM reception on car
>radios?  Would they recommend FM stations drop to mono because it would
>improve coverage and because the stereo-blend in most modern car radios is
>so aggressive that the signal practically needs to be melting the antenna
>to be heard in full-separation stereo?  Not to mention that many radios no
>longer have an "ST" indicator for FM?
>
>Anyway... for the sake of argument, let's say that 15% of AM radios in use
>today have some kind of useful audio response above 7 kHz.  (Which I think
>is actually a low guess, since AM tuners are being made cheaper than ever
>today, and cheaper means less selective, and thus wider bandwidth.)
>The NRSC is telling us we should forget about that 15% and just cater our
>broadcasts to the majority of receivers.

I believe that a significant reason that the final preferred 
bandwidth was 7 kHz was that broadcasting wider bandwidths puts more 
stress on the audio processor. If the processor clips material 
between 7 and 10 kHz (to which a vast majority of AM radios are 
essentially deaf), the resulting clipping distortion products *will* 
end up in the 0 to 3 kHz frequency and *will* be heard on typical 
radios. So limiting the spectrum to 7 kHz and below prior to the 
processor's peak limiter section makes for a cleaner signal as heard 
on the average radio, which is down 3 dB at 2.6 kHz if one assumes 
NRSC pre-emphasis in the RF signal generator driving the receiver.

Bob Orban