In the NFL, the quarterback and a designated defender, usually a linebacker, are supposed to be the only players on the football field who can hear their coaches calling out plays into their helmets.

The only sounds they're supposed to hear are those voices coming from the coaches' headsets. And coaches in the booth upstairs are also supposed to be able to securely communicate with the coaches on the sideline.

That's not always the case, as Steelers coach Mike Tomlin emphasized after Pittsburgh's 28-21 loss at New England in the 2015 opener. He complained about headset failures, which he charged are ''always the case when playing in New England.''

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The Steelers said they don't plan to file a formal complaint, but the Patriots are under scrutiny after the team accepted punishment for using deflated footballs and Tom Brady fought his suspension. Under Last decade, the Patriots were punished for videotaping the hand signals of opposing coaches.

NFL vice president of football communications Michael Signora said the communications equipment falls under the jurisdiction of the league, and that the headset problems were a function of a power issue at the stadium that was fixed during the game.

Players and coaches say headsets have become more reliable in recent seasons after the league switched from the outdated analog communication system to a digital network in 2012, but problems persist.

Sometimes, they hear static, crossed signals, noises that aren't supposed to be there.

''With radio frequencies, there's millions of factors that come into play, so I couldn't tell you exactly what happened there,'' said Alex Shada, director of operations for Gubser & Schnakenberg LLC, the Lincoln, Nebraska-based provider of coach-to-player communication systems for the NFL.

Since coaches and coordinators began talking to quarterbacks with radios in 1994, miscommunications and mishaps have been an occupational hazard. Hot dog vendors, musicians, and truckers have somehow ended up on team radios.

The digital system is easier to use. There's no longer a delay preceded by a beep to wait for the frequency to clear. Instead, coaches now push a button and can talk instantly and with clearer sound.

The helmets are equipped only with a speaker, not a microphone. So, the player cannot talk back to the coach.

Each team is only allowed one live helmet, designated by a small green dot on the back, on the field at a time. Once the 40-second play clock begins, coaches have 25 seconds to make a call and pass on information. The microphones for all the radio transmitters shut off automatically at the 15-second mark, so the coaches can't give live advice during a play. A league official is on site to monitor.

The NFL has said military-grade encryption codes protect the frequencies.

Obviously, signals still get crossed.

''It's rare that I've gone through an NFL game that there was not a problem with the headset,'' Browns coach Mike Pettine said.

''It's just like anything else you've got to work through on game day, like an injury or something,'' Broncos coach Gary Kubiak said. ''You've got to battle your way through it.''

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AP NFL website: www.pro32.ap.org and AP NFL Twitterfeed: www.twitter.com/AP-NFL

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Follow AP Pro Football Writer Arnie Melendrez Stapleton on Twitter: http://twitter.com/arniestapleton