> 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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Alan Bloom > Sent: Sunday, January 13, 2008 10:48 PM > To: David Woolley > Cc: elecraft@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 > > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > 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 > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
