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:[EMAIL PROTECTED] > > Sent: Tuesday, January 15, 2008 1:50 AM > > To: Joe Subich, W4TV > > Cc: [email protected] > > 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 > > > > > > > > > _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
