[Elecraft] New K3 SN#207 built but question on AM Filter

[Joe Subich, W4TV]

> Even with the 10 kHz channel spacing used in the USA, AM broadcast
> stations do not have 5 kHz audio bandwidth.  The FCC requires a guard
> band between stations.  As I recall, rgulations require that the audio
> start to drop off at about 4 kHz so that it can be down 20 dB 
> or so by 6 kHz (the passband edge of the adjacent station).

I don't think that's right ... or wasn't the last time I was around 
an AM station (I spent most of my career in TV).  I remember the AM 
guys doing proof to 10 KHz. 

Admittedly, many of the directional stations could not maintain 10 KHz 
through the phasors and the high end got trashed at night but the old 
allocation systems generally kept first adjacent situations far enough 
apart that 10 KHz could be obtained on groundwave during the daytime.  

"In the day" most receivers would start to roll off somewhere around 
6 KHz and the better ones had a 10 KHz notch for nighttime conditions. 

Given the DSP demodulation in the K3, it's a shame that there isn't 
an "offset" option to do "vestigial sideband" demodulation (offset the 
AM filter to the upper sideband or lower sideband) and demodulate 
carrier and one sideband for better fidelity.  This would work quite 
well if the carrier were placed at the -6dB point on the composite 
filter passband since it would keep the proper ratio between carrier 
and sideband.  Alternatively, the carrier could be moved to 1 KHz 
from the -6 dB point and the DSP could equalize out the 6 dB boost 
in audio below 1 KHz from the "opposite" sideband. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Alan Bloom
> Sent: Sunday, January 13, 2008 10:48 PM
> To: David Woolley
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] New K3 SN#207 built but question on AM Filter
> 
> 
> On Sun, 2008-01-13 at 02:49, David Woolley wrote:
> > Paul Webb wrote:
> > 
> ...
> > The basic reason has already been explained, however, there 
> are very few 
> > cases were exceeding 3kHz audio bandwidth is useful.  HF broadcast 
> > stations use 5kHz channelling, which would only allow them 
> 2.5kHz with 
> > brick wall filtering at both transmit and receive ends, 
> although they 
> > probably do expect to suffer significant adjacent channel 
> interference.
> > 
> > MF broadcast stations use 9kHz channelling in Europe and 
> 10kHz in the 
> > USA, but I suspect that adjacent channel interference is less 
> > acceptable.  As they were designed to be received with LC 
> IF filters, 
> > with poor shape factors, I suspect they don't even make use 
> of the full 
> > channel, and if they did, they would probably be required to have 
> > filters which put the adjacent channel into the filter stop band.
> 
> Even with the 10 kHz channel spacing used in the USA, AM broadcast
> stations do not have 5 kHz audio bandwidth.  The FCC requires a guard
> band between stations.  As I recall, rgulations require that the audio
> start to drop off at about 4 kHz so that it can be down 20 dB 
> or so by 6
> kHz (the passband edge of the adjacent station).
> 
> So there's not much point in the receiver audio being wider 
> than 4 kHz.
> 
> Al N1AL
> 
> 
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