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
