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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-release to listen (often referred to as PTT) button commonly found on 2-way radio systems and walkie-talkies.
The need for Quindar tones
For Mission Control (in Houston, Texas) to stay in continuous contact with the astronauts as they traveled to and from the Moon, NASA used several tracking stations around the world, switching from one to the next as the planet turned. Dedicated telephone lines (a very expensive measure at the time) connected these stations to Houston. NASA had the option to build two separate systems for operating the transmitters - one to carry the audio from the CAPCOM and another to carry the control signal for the PTT button (out-of-band signaling) - but instead chose to combine these two systems together into a single system to reduce the operating cost of the network. The same system was used in Project Gemini and was still in use with half duplex UHF Space Shuttle communications for transmitter RF keying.
With modern digital communication systems, Quindar tones are no longer necessary because a single communication line (such as a fiber optic cable) can simultaneously carry multiple communication channels.
Implementation of Quindar tones
The "intro tone", a 250-millisecond tone at 2,525 hertz, followed by a 250 ms 2,475 Hz "outro tone".
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The Quindar system, named after its manufacturer, used two tones, both being pure sine waves that were 250ms long. The "intro tone" was generated at 2,525 Hz and signaled the "key down" keypress of the PTT button and unmuted the audio. The "outro tone" was slightly lower at 2,475 Hz and signaled the release of the PTT button and muted the audio. The two tones were generated by special equipment located at Mission Control, and they were decoded by detectors located at the various tracking stations.
Common misconceptions about Quindar tones
Two common misconceptions surround Quindar tones. The first is that one tone came from Earth and the other from the transmitters used by the astronauts while in space. This confusion exists because many ground-to-space transmissions were initiated by Mission Control and responded to by the astronauts. In this sequence, the CapCom would press the PTT, which would send the intro tone, and then speak. When finished speaking, the CapCom would release the PTT, which would send the outro tone, and the astronauts would respond to Mission Control. Therefore, those transmissions would consist of a "beep" (PTT press) followed by Houston talking, then another "beep" (PTT release) and finally the voice of the astronauts.
Another misconception about Quindar tones is that they were designed to signal the end of a transmission, similar to a courtesy tone used on many half-duplex radio repeaters. Although the astronauts may have secondarily used the Quindar outro tone to know when the CAPCOM had started/stopped speaking, no equivalent existed for Mission Control because the astronauts keyed their transmissions locally (inside the spacecraft) using either a PTT or VOX, neither of which required Quindar tones. Additionally, separate radio frequencies allowed both Houston and the astronauts to talk simultaneously if they wished and thereby made a courtesy tone as a way to minimize the possibility of both of them speaking at the same time unnecessary.
Origin of the name
Quindar tones were named for the manufacturer Quindar Electronics, Inc. Glen Swanson, historian at NASA's Johnson Space Center who edited the Mission Transcript Collection, and Steve Schindler, an engineer with voice systems engineering at NASA's Kennedy Space Center, confirmed the origin of the name. "Quindar tones, named after the manufacturer of the tone generation and detection equipment, are actually used to turn on and off, or 'key', the remote transmitters at the various tracking stations."
- "The Story Behind the Beep". The Mission Transcript Collection. January 1, 2006. Retrieved August 17, 2009.
- W. P. Varson. "Functional Description of the Unified S-Band System and Integration into the Manned Space Flight network" (PDF). Proceedings of the Apollo Unified S-Band Conference. NASA. pp. 3–12. Retrieved February 22, 2010.