iyiyi said:
My personal interest in AM HD is to provide a robust, stable platform for digital broadcasting. When I understood that IBOC would employ ISB (as opposed to the 8VSB of HDTV), I jumped aboard the IBOC bandwagon. The current IBOC modulation scheme supplies a rock solid foundation to construct a viable lattice for digital data in the MW band.
I needed a good laugh! Stable?! You've got to be kidding.
At the risk of being labelled a "hater" or a throwback with too much Neanderthal genes or something - I'll describe the stability of HD AM from a listener perspective, and compare it to the previous system C-Quam.
I am an engineer by profession, and therefore am thoroughly familiar with the scientific method. My test methodology was to select two test stations, and compare the viability of stereo reception at a distant location pre and post HD conversion. My listening location was partly dictated by reports from another poster that mountainous terrain caused problems with C-Quam. I also selected a location remote enough that any coverage problems caused by C-Quam would be immediately apparent.
- Test receiver was a Sony SRF-A1 portable, selected because I was going to hike all over the mountainous terrain to ascertain if there was any effect from it.
- Test stations were KMKI 620 and KAAM 770 from the Dallas area.
- Primary test location was a rest area in a canyon just east of Crosbyton, TX, on highway 114. This is about 290 miles distant from the test stations.
- Test time was daytime. It is unwise to hike a wilderness area infested with rattlesnakes after dark.
C-Quam results. Both stations were surprisingly clear, although they had some static. A portable loop cleared both to local quality, but wasn't used for the rest of the test. I would characterize them as completely listenable. KKOB interference was noted on 770, although not enough to affect the C-Quam stereo decode. After a few hours of hiking around up one wall of the canyon and the other, I observed no dropouts or effect on the stereo decode at all. It was rock solid stable over several hundred feet of vertical climb on both walls of the canyon.
Because the test revealed no weaknesses in the C-Quam system at all, I move the test location to a location in the West part of Lubbock, TX, about 330 miles distant. KMKI static was noticably increased, but C-Quam still decoded. It would not be what I would call listenable, although the portable loop again brought it in with almost local clarity. KAAM was about the same, but KKOB interference was much worse, and the portable loop brought both stations in with almost equal strength. Nevertheless, KAAM C-Quam still decoded properly with no hint of any problem.
Post HD conversion results:
Both stations were markedly weaker in the primary test location. Static had increased to the point that I would characterize both at unlistenable. KKOB interference was worse on 770. A small loop did help the situation, but could not clear static completely. As the SRF-A1 is not an HD radio, I did not get HD decode, of course. Because the signal strength was so poor, I did not conduct hike tests. At the secondary west Lubbock location, KAAM was virtually covered with KKOB, which seemed to be the dominant signal on the frequency. KMKI could still be largely cleared, but even with the loop was noticeably weaker. The effect was also noticed by other DX'ers in the area who previously enjoyed the station but had largely given up in favor of streaming.
I next found a listening location nearer to Dallas to find what the coverage was of HD AM. I used a Sangean HDT-1 as my test receiver. I selected my home, of course, which was less than 10 miles from both test stations. KMKI stereo decode was good, but KAAM at 9.6 miles would not decode at all. I moved my test location to Van Alstyne, TX. KMKI HD decoded in mono, but stereo was very finicky with the supplied loop.
Because the HDT-1 also has FM, I did a quick test of Dallas HD stations. All decoded in HD with only a dipole antenna about one meter off the ground. Distance was about 70 miles. Another DX'er has reported reliable HD decode on Houston stations 84 miles from the towers with a dipole.
The only conclusion possible is that C-Quam is a much more robust AM stereo system. Of course the demographics of my listening locations were characterized by rattlesnakes and cattle, nobody cares about coverage out there for a few ranchers. But - diminished signal strength translates directly to diminished building penetration were the station do care. And the dramatic decrease in signal strength is bad news for any AM station wishing to penetrate office buildings.
The only thing "robust" about AM HD appears to be the persistence of the HD sidebands. I have previous reported the distinct presence of HD sideband pairs on Chicago stations at a rest stop near Grenville, NM on highway 64 / 87 in the daytime, two different times of the year - with nothing but a factory car radio. This is very alarming, because it means the sideband propagate very much farther than intended, over a thousand miles in this case - long after the carrier and analog signal have gone. Since there are so few AM HD stations on the air, the source of the sideband pairs is pretty obvious. Those sideband pairs were gone by Flagstaff, AZ.
Far from being a "hater" - I own three HD radios. Not out of any particular love for HD, but because the radio situation in my home town of Houston is so horrible that the only formats I have any interest in are all on HD-2. I do what it takes to get my preferred format, whether it is DX, stream, or HD. IF HD has the format I want, I'll use HD - and live with the lack of robustness in the system. It is too bad that HD is yet another aspect of DX'ing, only this time I need to DX on local signals to get the system to work at all.
