Propagation software developer V-Soft Communications has a new module in its Probe 4 software, one intended to predict interference between FM HD Radio stations, or from FM HD signals to analog stations. The IBOC module now is part of all Probe 4 programs
And jumping directly into the fray, V-Soft applied its module to the case of southern California station KATY(FM), owned by All-Pro Broadcasting, which has claimed it receives significant interference from first-adjacent KRTH, owned by CBS.
V-Soft stated in its newsletter that, using a mobile radio receiver configuration and looking at points within the normally protected 60 dBu contour of KATY, its Probe 4 IBOC model predicts that KATY "experiences a significant amount of interference."
The first map shows interference to KATY when KRTH runs IBOC at –20 dBc. The second shows the areas of interference received by KATY without KRTH's IBOC.
"The two stations meet the FCC's minimum 73.207 separations," V-Soft continued, "so, as the FCC sees it, there is no problem; however, due to the hilly terrain, the stations' interference contours actually cross well into each other's protected contours. In fact, the FCC calculated interference contour of KRTH crosses well over the KATY-FM transmission site, as is shown on the map. This is one of a number of problems with the way the FCC assigns commercial channels in hilly areas without regard to contour-to-contour interference."
V-Soft said its new Probe 4 IBOC model works as part of the standard "FM Interference" study.
"The model is designed to be as flexible as possible to properly predict numerous IBOC-to-IBOC and IBOC-to-analog scenarios. The user can choose to study one receive mode (analog, digital, or hybrid blend) at a time. Each transmitter in the study can be configured to use a different IBOC power level."
The screen image example below shows desired/undesired ratios. "The D/U ratios are defined with reference to one particular power level and the program can adjust them when it encounters stations using IBOC power levels that are different from the reference. For considering interference to the digital part of a hybrid signal, the program will apply a sliding D/U to compensate for dual first-adjacent interferers."
The second map is an interference analysis of KATY without the presence of KRTH's IBOC signal. "As it plainly shows, there is still some interference caused to the KATY-FM signal from KRTH, however it is much reduced," V-Soft concludes.
"CBS Radio Disputes KATY's IBOC Interference Claim" (July 2010)