In-Band Signaling: Quindar Tones

So far in this brief series on in-band signaling, we looked at two of the common methods of providing control signals along with the main content of a transmission: DTMF for Touch-Tone dialing, and coded-squelch systems for two-way radio. For this installment, we’ll look at something that far fewer people have ever used, but almost everyone has heard: Quindar tones.

What’s a Quindar?

You may never have heard what Quindar tones are, but you’ve certainly heard them if you’ve ever seen any manned spaceflight videos. Quindar tones are those short beeps you hear when NASA is communicating with astronauts, as heard in this radio network check conducted between Houston and the Honeysuckle ground station in Australia during Apollo 11:

If you listen carefully to the controller’s five-count, you’ll hear two slightly different tones. The higher pitch tone (2525 Hz) comes before a vocal transmission, while the lower pitch (2475 Hz) comes after. These iconic 250-millisecond beeps are called Quindar tones.

So what did they do? You’d be forgiven for assuming that they were some sort of “roger beep” used to automatically signal that the channel was clear and ready to be turned around for the other side of the conversation. That’s understandable, as many two-way radios today, even the cheap Family Radio Service walkie-talkies, have some kind of roger beep. For my part, I always thought those beeps were something akin to the tones overlaid onto phone conversations that are being recorded, like when you call 911. Had I given a moment’s thought to it, though, I would have realized that astronauts would have been painfully aware that everything they said was being recorded for posterity, and wouldn’t need any reminders.

Really Remote Control

Quindar tones had a much more practical purpose that makes sense when you consider the scale of a manned spaceflight mission. Keying the microphone on a radio generally closes contacts that switch the microphone into the audio circuit and trips relays that apply power to the transmitter, allowing the operator to speak on the air. That works fine when the radio is on the desk in front of you, or perhaps even when it’s down the hall. But remote transmitters present a problem, since cable runs between the operator position and the transceiver become unmanageable.

In NASA’s case, mission control in Houston had to use a worldwide network of ground stations to stay in constant contact with the missions in orbit or on the way to the moon. Like the Deep Space Network used to communicate with interplanetary missions, NASA needed multiple sites to ensure that at least one antenna could see the spacecraft as the Earth rotated. Quindar tones, named after the system’s manufacturer, Quindar Electronics, were a form of in-band signaling that allowed NASA to use the same set of leased lines to send audio to the transceiver and to switch it between receive and transmit. The high pitch tone switched on the transmitter, and the low-pitched tone turned it off.

But Not Always

Quindar tones were used just for controlling NASA’s ground stations, so there was no need for the spacecraft radios to use them. In fact, NASA took steps to prevent the astronauts from even hearing the tones. Capsule communications were full-duplex, meaning it could have been possible for a spacecraft to retransmit any Quindar tones it received, possibly interfering with ground station control. So a notch filter was inserted into the uplinked audio to filter out both tones; being so close in frequency, one filter sufficed. It wasn’t perfect, though, and you can still hear a remnant of the outro tone in this iconic clip from Apollo 13:

The instantly recognizable Quindar tones were used for in-band signaling for all of the Gemini and Apollo missions, and even saw duty during the Shuttle era. And now they’ve even inspired some interesting techno music.

29 thoughts on “In-Band Signaling: Quindar Tones

    1. Thank you for teaching the plural of ellipsis. I had no idea. What a strange language this is. (my second one, and I’ve only been learning it for a half a century, still learning new things about it)

  1. In addition to the ground-based and sea-based (yep, ships) tracking stations, NASA developed a fleet of aircraft ( to fill gaps in coverage and allow quick repositioning of the tracking asset. After Apollo, the aircraft were repurposed for other missions (ballistic reentries, chasing air-breathers, etc) in addition to on-orbit tracking.

  2. TL;DR: Quindar tones, named after the system’s manufacturer, Quindar Electronics, were a form of in-band signaling that allowed NASA to use the same set of leased lines to send audio to the transceiver and to switch it between receive and transmit. The high pitch tone switched on the transmitter, and the low-pitched tone turned it off.

  3. This method is still used in many 2-way radio networks today. It’s called TBE or tone-burst-entry and sometimes exit. It you listen to a L.E.O. transmission on a police scanner you hear a digital tone burst entry every time the police officer pushes the PTT button on his mic. I think it is identifying him by unit number etc on all units. It is also used to access radio relay repeaters. Tone burst exit is not as common though.

    Quindar Electronics is still in business today but got bought out and is also doing S.C.A.D.A. now.

    SCADA? supervisory control and data acquisition – some of you know what particular cyberworm was invented to attack SCADA back in 2005 (allegedly)?

  4. Did HaD cover the fire department signaling system used in the Los Angeles (and possibly other) area during the 70’s? If you’ve ever seen “Emergency!” you’ve heard it, but since it’s always the same station on the show it’s always the same sound. IIRC there was a systemwide alert sound first, followed by the sound(s) for the station(s) dispatch wanted to respond to a fire or other incident. It wouldn’t take them long to learn their station’s sound so after the station ID tone was done they could relax if they weren’t being dispatched.

    1. @Galane – Yes you are referring to this below. It starts off with the tone. It is called “selective call”. It activates the fire station’s klaxon horn and loudspeaker. The fire captain picks up the mic to reply to fire dispatch. A lot of American volunteer firemen have small radio receivers (looks like radio pager) that have it too. It is sequential tones. This clip is 40 minutes long… you only need to listen to first 3 seconds unless you like this American TV show?

  5. If anyone would like to play around with live QUINDAR TONES in their browser and other NASA sounds, I wrote this HTML/JavaScript file in a raw text file (so you can check my source code). Just copy and past the text into a NOTEPAD.exe (or whatever raw text processor you use) and save it in a filename on your desktop (or anywhere you want), and call it Quindar (or whatever you want) but end it in .HTM or .HTML suffix so you browser can see it. Then open it in Firefox or Chrome. Not sure about MSIE. Then play around with a live Quindar Encoder. Enjoy!

      1. I did. If you look at my source code you’ll see I used webkitAudioContext – that’s WebAudio. I also added a Blue Box feature. Just don’t get caught by Telco security playing with the ABCD tones like some Phreaker. You can use the Quindar codes to trigger the VOX circuit on your FRS/GMRS/HAM handy talkie too. The PTT button will solve the first syllable lost problem with VOX too. You’ll sound very NASA-esque :-)

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