"...the Media Bureau (Bureau) announces that it will permit AM stations to use transmitter technologies that reduce power consumption while maintaining both audio quality and licensed coverage areas. Such technologies, known generally as Modulation Dependent Carrier Level (MDCL) control technologies or algorithms, have long been used by international broadcasters operating high-powered AM transmitters."The total number of stations using the tech is now 52, with the majority of those using a Nautel or Harris-based solution. Most of them are in Alaska, and California. This makes more sense when you look back and see the technology being pioneered by none other than Chuck Lakaytis, director of engineering with Alaska Public Broadcasting. At the NAB conference in 2011 Chuck did a presentation entitled “Reduce Power Costs of AM Transmitters using Carrier Control Techniques." Sations continue to seek waivers to reduce operating costs. You can see a list in excel here.
With experimental authorization from the FCC, Alaska Public Radio tested MDCL on two stations. their test monkeys were 720 KOTZ-AM in Kotzebue, AK and and 670 KDLG-AM in Dillingham, AK. Both are 10 Kw stations. Lakaytis estimated that over a 3-month trial that his stations had a power savings of 30% with no loss of coverage of clarity. More here.
But there is a downside. MDCL technology can cause a transmitters power to drop below the minimum threshold of 90% hence the waiver requirement. It also diminishes the signal to noise ratio in a way that can be audible... though I'll be the first to admit that on the AM band that would be nearly impossible to detect. It may also effect IBOC, but nobody really cares about that anyway.