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Posted to rec.audio.tech,rec.audio.car,rec.radio.shortwave,ba.broadcast
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HD RADIO is no worse than DAB or DRM radio
"Earl Kiosterud" wrote in message newst%Yi.403$CI1.60@trnddc03... "Robert Orban" wrote in message ... In article , says... "SFTV_troy" blabbed: ... this new receiving technique would not improve the sound (it would still be limited from 100-6000 hertz), but would only reduce interference. At least in the States, AM & FM broadcasting is limited to 50 Hz to 15KHz. There is no low frequency limit for either AM or FM; 50 Hz was the minimum performance standard that would meet the now long-deleted FCC Proof of Performance measurements. The effective HF limit on FM is about 18.5 kHz; this leaves a +/- 500 Hz guard band for the stereo pilot tone. Again, 15 kHz was the minimum spec that would pass a Proof of Performance, not a limit on bandwidth. Currently, the legal FCC-mandated HF limit on AM in the US is a hair less than 10 kHz, which almost completely protects second-adjacent stations from interference. This was changed around 1990 as a result of work done by the National Radio Systems Committee (NRSC). More recent work by the NRSC has indicated that 7 kHz is probably the optimum compromise between causing interference and loss of audio quality on typical AM radios (which are down 3 dB at about 2.6 kHz). However, limiting bandwidth to 7 kHz is voluntary. Robert, Was AM radio ever allowed audio to 15 KHz? I read many years ago that it was, perhaps before the NRSC recommendation was adopted by the FCC. I presumed that the stations either were allowed to overlap 5 KHz (doubtful), or that stations in a given area were separated by at least 30 KHz. -- Regards from Virginia Beach, Earl Kiosterud www.smokeylake.com I was a broadcast engineer in the late 1970s to the late 1980s. At that time (before NRSC) AM was required to transmit a minimum 5KHz bandwidth, but the maximum modulated bandwidth was not really defined. There were limits on "spurious" emissions, caused by audio distortion products and carrier harmonics. I don't recall the exact mask, but 15KHz was legal at that time. Our studio transmitter link was a Mosely PCL-505, which was flat to 15KHz, and we employed no artificial band limiting, so the station was flat to at least 12KHz. Our tower was the limiting factor for bandwidth. It sounded just like monophonic FM on the modulation monitor. During the day there was no overlap, because stations were allocated on second alternate channels in most markets. Local stations that did overlap usually worked out a solution amongst themselves if the interference was objectionable. At night it got quite a bit noisier as distant stations would skip into the area, but it wasn't generally sidebands that caused the problem, it was the carriers themselves, each whining away at 10KHz. That is still a problem, even today. The real problem was that in the late 1980s, AM stations began adding proprietary "pre-emphasis" -- high frequency boost to make their station sound brighter on typical pathetically band-limited AM receivers. This can and did cause severe interference in some congested markets. Partially to address this, and to standardize the pre-emphasis, NRSC limited AM sidebands to 10KHz in the early 1990s. Since most AM radios do not even come close to being flat to 5KHz, 10 KHz is still two or three times more bandwidth than most listeners can use. |
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