Digital Radio and AM Signal Degradation

Digital Signals Spark Static From AM Radio [pdf] (via Slashdot

A growing number of radio listeners are encountering similar interference — hisses, whistles or static — on their favorite AM stations. The problem for WTRI began about a year ago, when Bonneville International Corp.’s WTOP, the AM station at 1500, began using a digital signal that interfered with WTRI’s analog signal in some broadcast areas. It’s one of the unexpected consequences of the radio industry’s transition to digital broadcasts.

Digital radio is touted as broadcast radio’s golden ticket, a technology that allows broadcasters to squeeze more stations into frequencies that currently hold just one. Advocates say the technology will allow radio to better compete with niche-oriented products like Internet radio and with other entertainment technologies, like iPods.

Big radio companies, such as Clear Channel Communications Inc. and CBS Corp.’s CBS Radio, have raced to embrace digital broadcasting, adding digital signals and rolling out new programming. But that has left behind many smaller AM stations that are still broadcasting only an analog signal. They are experiencing so-called side-channel interference — a phenomenon brought on in part by the fact that AM stations are packed tightly onto the dial, with only 10 kilohertz separating each one. (The problem doesn’t affect FM stations much because they reside 200 kilohertz away from each other.)

The AM stations most affected are those whose neighboring stations — nearby on the dial — add a digital signal. In most cases, including Mr. Rizer’s, the interference doesn’t stretch into a station’s core coverage area, as defined in its Federal Communications Commission license. But in fringe areas, signals can be fuzzy, or lost entirely.

[…] Critics question why Ibiquity’s technology is the only terrestrial digital-radio technology approved by the FCC.(Digital radio transmitted by satellite is a separate issue.) Ibiquity’s IBOC technology “allows…our domestic radio industry to transition to a digital radio future without requiring more spectrum,” says Peter Doyle, chief of the audio division at the FCC. That advantage more than makes up for any shortcomings, he says.

One critic is Leonard Kahn, a New York-based radio engineer and patent lawyer who has developed a hybrid digital-radio system for AM — Kahn Cam-D — that he says is better than the IBOC system, in large part because it doesn’t cause interference on neighboring stations. Several stations around the country have bought the Kahn system to boost their signals. Last month in federal court in New York, Mr. Kahn filed a lawsuit against Ibiquity, along with Clear Channel, alleging antitrust violations. Clear Channel declined to comment because it hasn’t yet seen the suit. A Ibiquity spokeswoman said “we are in the process of reviewing it.”