Quindar tones information

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Quindar tones, most often referred to as the "beeps" that were heard during the American Apollo space missions, were a means by which remote transmitters on Earth were turned on and off so that the capsule communicator (CapCom) could communicate with the crews of the spacecraft. It was a means of in-band signaling to simulate the action of the push-to-talk and release-to-listen (often referred to as PTT) button commonly found on two-way radio systems and walkie-talkies.