HD Interference: Not Just For AM Anymore

Radio World Engineering Extra dropped a bomb this month with a very provocative cover story: “What Are We Doing to Ourselves, Exactly?” Written by Doug Vernier, the man who authored the technical specifications for an ongoing Corporation for Public Broadcasting-sponsored HD Radio interference analysis, the report is the first of its kind to document interference between FM-HD stations around the country.
Using anecdotal reportage, some sophisticated contour-mapping, and presumably “early data” from the CPB study, Vernier’s article conclusively proves how stations running in hybrid HD/analog mode can (and do) interfere somewhat significantly with not only themselves, but their neighbors on the FM dial.
Most interesting tidbits:
1. First-generation FM-HD transmitters cannot produce a signal that can fit the National Radio Systems Committee’s most recently proposed digital radio spectral emissions mask (in plain English: stations that run first-gen HD analog/digital signals cannot conclusively avoid causing interference to their neighbors).
2. Adjacent channel interference, which Vernier dubs “grunge,” can be severe: one FM station caused “significant second-adjacent interference to a distant station owned by the same company at a reception point within 3 to 4 miles of the (HD) transmitter….In another similar case, a station’s grunge placed excessive energy on the first – and second-adjacent channels, which wiped out the receive signal of several translators transmitting from the same location. The fix was an expensive output bandpass filter for the offending transmitter.”
3. There is discernible interference between an FM-HD host station’s analog signal and digital sidebands: “After turning on [HD transmissions], many stations have reported that their analog air monitors exhibit white noise in the background.”
4. “Perhaps the most serious threat to the hosting station is when dual antennas are used….For example, as reported [in an earlier issue of RWEE]…a station located in a populated area of Minneapolis turned on its [FM HD signal] using a separate antenna and was surprised to find the HD operation caused severe interference to the hosting analog station within a 2 mile area…station engineers quickly turned off [the HD signal], took down the antenna and installed a a high-level [analog/digital signal combination] system, which eliminated the interference.”
Vernier’s conclusion is pretty stark: “What we have done with the introduction of [FM-HD] is to superimpose a new transmission method over an existing allocation system, hoping it will work. In many cases it does; but there are more cases coming to light every day where there are problems….There are still those who say, ‘Don’t look a gift horse in the mouth’; the FCC gave us the use of this new spectrum, so let’s make the best of it. Being neighborly to the stations and their listeners adjacent to your FM channel seems to have taken the back seat over a more hedonistic view of ‘Let’s push on and make amends for what we have done later.'”
This is a significant piece of work. Firstly, it utterly destroys the fallacy that the HD Radio transmission protocol uses “no new spectrum” – the mantra the FCC and industry both used to ignore the problem of interference-by-design that is HD Radio. But perhaps more importantly, it suggests that, like we have seen with AM-HD adoption and proliferation, interference problems may grow on the FM dial as more stations begin hybrid analog/digital broadcasts.
The real extent of the problem may only be quantified over time and, like listeners to AM stations have discovered, we’ll suffer as it plays itself out. We may have a better glimpse at just how bad the interference potential is before the year is through, as the CPB’s study is supposed to be in final-draft stage. Most if not all of Vernier’s article is based on a more comprehensive presentation about HD-related interference he’s given to other broadcast engineers over the last year and a half.