The American space program, I think we’d all agree, has its own very particular set of audiovisual signatures that immediately evoke all kinds of images and feelings. There’s a certain NASA look to the objects of space travel—that gold foil stuff, lightweight aluminum frameworks, white panels with American flags on them—that is immediately recognizable. The same goes for the sounds of NASA, a certain type of distorted audio, and, perhaps most evocatively, those beeps you always hear in transmissions between astronauts and ground control. But what the hell are those beeps, anyway? What do they actually do? Relax. I’m going to tell you.


Those beeps actually have a name: they’re called Quindar Tones. And, in case you forgot what they sound like, here’s a reminder:

You could put those beeps in any voice audio feed and it would immediately feel like people talking in space. But what are the Quindar tones for?


First, let’s be specific about those beeps: there’s actually two different beeps that happen, one a sine wave tone at a frequency of 2.525 KHz that lasts for 250 milliseconds, and one that’s a sine wave tone at 2.475 KHz, for the same duration.

That first and slightly higher tone is called the intro tone and the lower one is the outro. As their names suggest, one is for the start of something, and one for the end.

What that something is related to how the CapCom—that means “capsule communicator” which was what they called the ground control team member (usually an astronaut) who was in charge of talking directly to the astronauts on the spacecraft. Having one person designated to communicate with the astronauts helps reduce any possible confusion and cross-talk.

Since the CapCom would be in the busy, noisy Mission Control room, they’d want to choose when to open their microphones to talk to the spacecraft, so NASA used a push-to-talk (PTT) system.


It’s like how a CB works, if you’re as miserably old as I am and remember that—you hold down a button while you talk, and let up when you’re done.

This is normally not a big deal to implement, but the space program had very unique requirements. In the setup that NASA developed, which used tracking stations all over the world to keep in near-constant communication with the spacecraft, the audio from CapCom to be sent into space was transmitted to the various stations across the globe via dedicated telephone lines.


These lines were just for voice audio—if NASA wanted to send control signals like transmit on and off, they’d need to run a whole parallel set of wires, which would be expensive. So, they came up with a solution: use the same lines for control signals as well!

Because the lines were optimized for human voice audio, the control tones had to be within the range of normal human speech, which is why the tones are audible.


Here’s how NASA described the setup:

“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 (Merritt Island Launch Area–now Kennedy Space Center, Bermuda, Australia, etc.) that were used to communicate with the Mercury through Apollo spacecraft and, in some cases, are still used with the Space Shuttle. A one-half second tone burst is generated when someone in a control room depresses the push-to-talk (PTT) button of their headset. The decoder at the remote transmitter site detects this tone and keys the transmitter. When the PTT button is released a different frequency tone burst is generated. When the decoder detects this second tone, it unkeys the transmitter. Because the telephone lines between the control rooms and the remote transmitters were originally designed to carry only voice frequencies, the tones had to be in the voice frequency range (“in-band signaling”) and thus audible to humans. The tone signaling could have been done on a separate phone line, but to keep costs down, signaling and audio were done on the same line.”


So, CapCom pushes down the talk button, and the intro tone is sent, and when he lets go, the outro tone is sent. That means every time you hear the slightly higher tone, Mission Control was starting to talk, and the lower tone meant they were finished.

Quindar-built equipment generated these tones and sensed their presence to control turning on or off the Mission Control audio feed.


Out in space, astronauts were often using voice-activated (VOX) microphones since their hands could be otherwise occupied, and since they didn’t need to send their audio over wires to other stations, the from-space side of the conversation did not use the tones.

a roughly period-correct Quindar tone generator rack-mount unit Photo : eBay


Also, it should be noted that neither side normally heard the intro tone when communicating, since it signals the start of broadcast. The outro tone could be heard and may have been useful for astronauts knowing when Mission Control was done transmitting.

By the way, I was reminded of these beeps while reading Mary Robinette Kowal’s excellent live-tweeting of today’s (also all-female) spacewalk:


Her feed is great for space exploration coverage, if you’re interested.

So, that’s pretty much the reason for those beeps—they’re control signals to turn the CapCom’s audio feed on and off, and using the relatively cheap Quindar hardware saved NASA a bunch of money they would have had to spend if they ran parallel control signal lines.


They’re there because it was cheaper, fundamentally, and now they’re about as iconic an evocative of American space exploration as that picture of Buzz on the moon.