[Elecraft] AM broadcast HD noise on K3 synchronous reception mode

[amfmtv at aol.com]

I have been experimenting with the K3's synchronous AM feature 
and was  wondering about why I am hearing a background "roar"
whenever I tune in an AM  broadcast station that is running
HD.  The "roar" is not something  supersonic, but is heard right through 
the audio that runs from 30Hz to  5kHz.  I am a broadcast engineer dealing 
with AM transmitters, and am  familiar with the AM HD transmission techniques.  
The HD sidebands are  located approximately 6 to 17 kHz removed from the 
carrier, are about 20db down  from the carrier, and when detected on an AM 
envelope detector, are heard as a  "hiss".  I was thinking that if the K3's 
bandwidth was narrowed 
to less  than 6 kHz, using both the 6 kHz roofing filter and
the DSP filtering, the  digital HD sidebands would simply be
unheard since they would fall outside of  the bandwidth of
the receiver.  Using the K3 in standard AM mode, thats  just
what happens, you hear no "roar" underlying the detected audio.   
However when switched to either synchronous
mode or USB or LSB in SSB  mode, those HD sidebands appear to 
be causing that "roar" in the detected  audio.  That also got me
to wondering what effects other spurious  signals near an AM
(or SSB) signal would be audible or detectable as "noise"  or
any other artifact in the detected audio.  Has anyone else 
found  this to be happening on their K3's in synchronous AM or
SSB modes?
 
Ted   W8IXY
_AMFMTV at AOL.com_ (mailto:AMFMTV at AOL.com)