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

[Kevin Tekel]

Bob Orban wrote:
> 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.

That's all the more reason why audio above 6 kHz or so should be given its
own band in the processor's multiband compressor/limiter structure, like
your 9100 did, so that most of the HF gain control can be achieved before
the audio hits the clipper(s).

The best would be to have adjustable crossover frequencies between the
bands, so the user can tailor the processing structure to suit either
full-bandwidth audio (even beyond 10 kHz is allowed in some countries) or
restricted-bandwidth audio (like continental Europe's dreadful 4.5 kHz).
The Ariane and CRL Audio Signature pre-processors let you do this, but
this capability seems to be rare in "all-in-one" AM/FM audio processors.

> 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.

Those measurements are essentially invalid for analog-tuning radios, which
most people tune for the "best sound" to their ears, not necessarily the
exact center of the signal -- and on many radios, this yields a
significant improvement of treble response.  Of course this is also a
bugbear for IBOC, as precise fine-tuning is often necessary to quiet down
the annoying hiss.

p.s. If you want a surprisingly wideband AM tuner, check out any
co-branded "Philips-Magnavox" radio with digital tuning.  During the late
'90s to early 2000s era of this dual brand name, they used a high-quality
European-
made AM/FM tuner board which used only tuned coils for AM selectivity, not
ceramic filters, and the tuner board even has "AM ST" pinouts for adding a
C-Quam decoder daughterboard.  The audio response is so wide that it would
be nice if they had included a 10 kHz "whistle filter"!




       
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