AM licensees’ power consumption could be whacked by 40%!

AM broadcasters who have grown tired of forking over big bucks to the power company – rejoice! The Media Bureau is facilitating a cost-saving opportunity that could reduce power consumption (and thus, presumably, power bills) by 20%-40%.

Can you spell “MDCL”? That’s the Bureau’s shorthand for “Modulation Dependent Carrier Level”, which refers to various types of transmitter-control technology. It’s been in use internationally for some time, mainly by high-powered AM stations. With increases both in energy costs and in the ease of implementing MDCL algorithms, use of such technology has become more attractive in the U.S. Don’t expect a technical explanation of how it works from us – it involves algorithms, for crying out loud – but as best as we can decipher things, MDCL gear senses a station’s modulation levels on a continual basis and automatically adjusts transmitter power down (and up again) depending on modulation.

The problem is that MDCL technology can cause a station’s transmitter power to dip below the minimum level required by the FCC’s rulesi.e., 90% of the station’s nominal license power. But not to worry – the Bureau has now announced that it will routinely waive that requirement. (See below for more details on the showing that will be required.)

As with anything in life, MDCL is not perfect.

In the Bureau’s delicate phrasing, the power reduction at certain modulation levels “inevitably exacts some penalty upon audio quality”. In other words, audio distortion or decreases in the signal-to-noise ratio in the receiver may occur, along with slight erosion of coverage at the fringes of the station’s protected service area. But experience abroad and some experimental operations in Alaska indicate that such negative effects are “generally imperceptible”, according to the Bureau. Plus, MDCL-based equipment available for the U.S. market permits the broadcaster to adjust the power-reducing algorithm to minimize such negative effects.

Another caveat: while the effect of MDCL technology is apparently minimal in consumer receivers, it may have a more noticeable effect on field strength readings. Accordingly, anybody taking such readings on an AM station is cautioned to make sure that the MDCL gear has been disabled before the readings are made. (The Bureau makes clear that licensees operating with MDCL technology will be expected to cooperate by disabling their gear even when the measurements are being taken by some other licensee.)

And finally, while testing is still on-going relative to the compatibility of MDCL with hybrid AM IBOC on various types of receivers, the Bureau will nevertheless permit hybrid AM IBOC stations to use MDCL as long as (a) the hybrid signal continues to comply with spectral emissions mask requirements in Section 73.44 and (b) the relative level of the analog signal to the digital signal remains constant.

The Bureau’s announcement identifies two manufacturers with MDCL-friendly gear already available to the U.S. market: Harris Corporation and Nautel Limited. Harris offers two MDCL features (Amplitude Modulation Companding and Adaptive Carrier Control) on its new transmitters, and it can add such features to some older models. Nautel’s approach, dubbed Dynamic Carrier Control, is available on its NX series of transmitters, and can be installed on other models. Should other manufacturers develop similar technology, the Bureau will be open to waiving the rules for use their gear as well.

AM licensees intrigued by the cost-saving possibilities of MDCL can take one of two approaches, as far as the FCC is concerned.

For those cautious, toe-in-the-water-first types who aren’t inclined to go all in all at once, there’s the experimental license approach. Under Section 73.1510, you file an informal application (i.e., a letter) describing your proposed set-up, which would involve operation with MDCL for purposes of evaluating the technology. No form, no fee. Plus, even though experimental licensees are normally required to submit a report to the Commission on the results of their experimental operation, the Bureau has decided that no such reports will be required from licensees evaluating MDCL technology.

On the other hand, for those cannonball-off-the-high-board types who are comfortable diving into the deep end, the Bureau will happily modify your license on request. All you need do is send a letter requesting waiver of Section 73.1560 (that’s the minimum power level rule mentioned above) to: 

MDCL Waivers
Federal Communications Commission
Audio Division, Media Bureau
445 12th Street SW, Room 2-B450
Washington, DC 20554

The letter must specify the particular MDCL technology you’re planning to use and set out how that technology is going to be implemented at the station. Be sure to email a PDF version of the letter request to Audio Division engineering ace Ann Gallagher ( Once the request is granted, the Bureau will issue a modified license specifying that a waiver has been granted to permit use of a specific MDCL technology, resulting in the variation of transmitter power to levels below 90 percent of the station’s nominal licensed power. (Notwithstanding that waiver, the licensee will still be required to achieve its full licensed power at some audio input level, or when the MDCL is temporarily disabled. This is because the Bureau does not want such waivers to be deemed to authorize simple overall power reductions which would reduce the station’s licensed coverage area.)

The Media Bureau should be congratulated for opening this particular door. For AM licensees, it could mean significant savings. That alone is to be cheered. But the way the Bureau has done that is equally impressive. No need for a notice of proposed rulemaking, general inquiry or other time- and labor-consuming bureaucratic process. Rather, the Bureau examined the information already available, satisfied itself that any potential harm would be outweighed by the benefits to be gained, and let fly with its announcement welcoming MDCL requests. Such a streamlined approach to regulatory relaxation makes sense here. Hats off to the Bureau.