I would like to know if someone actually has documented findings that HD radio interferes with adjacent frequencies. I would think that the HD technology would make the 50,000 clear channel stations even stronger at night but I am not a physics expert.
Happy to oblige!
Since AM is still not authorized for nighttime operation, daytime is the only example of interference except for those occasions when it was being tested at night, or somebody got careless.
Case 1: KKLF 1700 interferes with local TIS station on 1680 from DFW airport, making it difficult for airport travellers to get gate information.
Case 2: KLIF 570 interferes with KLBJ 590 Austin in the Dallas / Ft. Worth area. KLBJ is a blowtorch, giving local signal quality in the DFW area.
Case 3: WOAI 1200 interferes with KFXR 1190 Dallas near sunrise and sunset - producing an audible increase in noise.
Case 4: KLIF 570 sidebands produce noticable noise on KLVI 560 in Houston. KLVI, although licensed to Beaumont, is listed as a Houston local and shows up in their ratings. KLIF noise is especially strong in northern suburban areas of the Woodlands and Conroe.
In spite of reduced power on the sidebands, the nature of the modulation makes them much more robust than the analog signal. Just as the carrier is much more robust than modulation (transatlantic heterodynes are relatively common, while actual reception rare). In careful DX tests I have performed, I have received digital sideband pairs from stations so distant that the analog modulation is not even detectable. Sidebands from stations like KOA, KMKI, WOAI are clearly audible 300 miles from the station, obliterating semi-locals even 300 miles away (a 630 in Lovington, NM is no longer receivable in Lubbock, an 860 from Hereford is no longer audible in Dumas to Amarillo on US 87.)
All in all, I would say that the situation before was one where the first adjacent is difficult to listen to when a station runs 10 kHz bandwidth audio. But that would make the effective bandwidth only +/-30 kHz under worst case circumstances. And - outside of the city grade signal contours, the extended frequency response seldom caused any problems, because most musical and speech energy is below 5 khz anyway. When a station goes IBOC, the 5 to 10 kHz sidebands are equal in amplitude and translated down to 0 to 5 kHz on the first adjacent frequency. On the second adjacent frequency, they mix with the 10 to 15 kHz sidebands to create primarily high frequency hiss from 5 to 15 kHz. First and second adjacents are VERY unpleasant to listen to!!! Much, but not all, of the second adjacent hiss can be mitigated with an extremely narrow IF - the type that used to be common in AM radios before the advent of single IC / single ceramic filter designs.
More recent AM radio designs use a single, really cheap ceramic filter which does not have much stop band rejection. Therefore, the radio is wideband by its nature and the 10 to 15 kHz sidebands are clearly audible even if the radio is tuned carefully to center frequency. Even a little off frequency, that the phase modulated 5 to 10 kHz sidebands become amplitude modulation and their contribution to the audio is considerable. Given poor mechanical design and poor mechanical lash of most tuning mechanisms, the average listener will be subjected to this noise.
