[Elecraft] RE: AM bandwidth, the rest of the story :=)

[Joe Subich, W4TV]

> Somehow they have to limit their occupied bandwidth to less 
> than 10 kHz.  If they don't there will be severe adjacent-channel 
> interference. 

No they don't.  Allocation practices do not place adjacent channel 
stations in the same service area.  The rules only impose limits 
at +/- 10 KHz, not +/- 5 KHz.  I am sure the transmitters contain 
brickwall audio filters that allow flat response to 10 KHz while 
still meeting the standard at 10.5 KHz. 

73, 

   ... Joe, W4TV 
  
 


> -----Original Message-----
> From: Alan Bloom [mailto:n1al at cds1.net] 
> Sent: Tuesday, January 15, 2008 5:40 PM
> To: Joe Subich, W4TV
> Cc: elecraft at mailman.qth.net
> Subject: RE: AM bandwidth, the rest of the story :=)
> 
> 
> Joe,
> 
> Somehow they have to limit their occupied bandwidth to less than 10
> kHz.  If they don't there will be severe adjacent-channel 
> interference. 
> That means at most 5 kHz audio bandwidth, assuming a 
> brick-wall filter.
> 
> Using a real-world filter, it has to be substantially less 
> than 5 kHz. 
> You can argue whether the right number is 4 kHz or 4.5 kHz, 
> but for sure
> it isn't 10 kHz.
> 
> 73,
> 
> Al N1AL
> 
> 
> On Tue, 2008-01-15 at 07:22, Joe Subich, W4TV wrote:
> > > OK< I looked it up.  According to Title 47, part 73.44 of the FCC
> > > regulations, 
> <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> 
> > > the modulation of an AM broadcast station must be down 25 dB at 
> > > 10.2 kHz from the carrier.  
> > 
> > The information at 73.44(b) only applies to out of band emissions. 
> > You will note that the section applies additional limitations at 
> > 20 KHz from carrier, 30 KHz from carrier, etc. 
> > 
> > > Assuming a 3-pole low-pass filter (e.g. a pi-network), the filter 
> > > attenuation is 18 dB per octave, which implies a cutoff frequency 
> > > of no more than 3.9 kHz.  The -3 dB bandwidth would be a little 
> > > higher than that.
> > 
> > Any AM station that used a simply pi-network filter would have 
> > real problems since they would sound "muddy" an lack punch.  I 
> > think you will find that they are using filters much more complex 
> > than a simple pi-network - usually with a sharp cut-off ("brick 
> > wall") response to maintain response out to 10 KHz. For example, 
> > the audio response specification for the Harris AM transmitters 
> > is: +0.2/-0.8 dB, 20 Hz to 10 kHz. Ref. 1 kHz at 95% modulation.
> > See www.broadcast.harris.com/radio/transmission/analog.asp and 
> > look at the various specification sheets for the Analog AM 
> > transmitters.   
> > 
> > 73, 
> > 
> >    ... Joe, W4TV 
> >    
> > 
> > 
> > > -----Original Message-----
> > > From: Alan Bloom [mailto:n1al at cds1.net] 
> > > Sent: Tuesday, January 15, 2008 1:50 AM
> > > To: Joe Subich, W4TV
> > > Cc: elecraft at mailman.qth.net
> > > Subject: AM bandwidth, the rest of the story :=)
> > > 
> > > 
> > > OK< I looked it up.  According to Title 47, part 73.44 of the FCC
> > > regulations, 
> <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> the
> > > modulation of an AM broadcast station must be down 25 dB 
> at 10.2 kHz
> > > from the carrier.  Assuming a 3-pole low-pass filter (e.g. a
> > > pi-network), the filter attenuation is 18 dB per octave, 
> > > which implies a
> > > cutoff frequency of no more than 3.9 kHz.  The -3 dB 
> > > bandwidth would be
> > > a little higher than that.
> > > 
> > > That's about what I remember from my broadcasting days many, 
> > > many years
> > > ago.  If you think about it, a double-sideband AM signal 
> can't have a
> > > bandwidth greater than 1/2 the channel spacing without 
> > > interfering with
> > > adjacent channels.  And it has to be somewhat less than that given
> > > real-world filters.  So there is not much point in having 
> a receiver
> > > with much more than 4 kHz or so audio response (8 kHz or so RF
> > > bandwidth).
> > > 
> > > > I remember the AM guys doing proof to 10 KHz. 
> > > 
> > > Right, in order to confirm that the modulation is down 25 
> dB at 10.2
> > > kHz.
> > > 
> > > Al N1AL
> > > 
> > > 
> > > On Sun, 2008-01-13 at 20:40, Joe Subich, W4TV wrote:
> > > > > 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 
> > > >  
> > > 
> > > 
> > > 
> 
> 
>