Space Exploration/Apollo era space communications
Dan Quigley wrote at 2009-07-24 23:05:07
The above answer is completely misleading and incorrect.
The beeps you remember hearing are called Quindar tones (named for the manufacturer) and were necessary to switch NASA's ground-based communications equipment between transmit and receive for voice traffic.
The "intro tone" was 2,525 Hz, lasted for 250 milliseconds (1/4 second), and corresponded to when the Capsule Communicator (CAPCOM) pressed the Push to Talk (PTT) switch.
The "outro tone" was slightly lower in pitch 2,475 Hz, also lasted for 250 milliseconds, and corresponded to when CAPCOM released the PTT switch.
Since the earth rotates (and the spacecraft was moving) NASA had to rely on a number of remote ground stations, located in different places around the world, for voice and data communications. The Quindar tones were decoded by the radio equipment at those ground station, switching to transmit when it "heard" the intro tone, and switching back to receive on the outro tone.
Martin wrote at 2009-07-26 18:45:25
It was called a Quindar tone. This comes direct from NASA. It has nothing has nothing to do with frequency and amps. Be careful with your answers!!! See this url: and the following quote from the NASA page!!http://history.nasa.gov/alsj/quindar.html
Now the quote from the page:"These beeps are called 'Quindar-Tones'. Their purpose is to trigger the ground station transmitters when there is an outgoing transmission from Earth. The CapCom in the Mission Control Center, who is taking care of communications with the crew, uses his communication gear in a PTT mode exclusively. 'PTT' is short for Push-To-Talk, which means that the CapCom presses a button every time and as long as he wants to talk. (The crews back during Apollo - and also today - usually communicate via PTT as well, but they also have the so-called 'VOX mode' at their disposal, in which their microphones are voice-triggered by a certain adjustable threshold volume levels. VOX is used when they don't necessarily have their hands free.)
When the CapCom presses his PTT button to start a transmission, an intro tone (2.525KHz sine wave with a length of 250ms) is generated and triggers the ground station transmitters to send. And when he is finished talking and releases the button again, a slightly lower outro tone (2.475KHz, sine, 250ms) is generated to trigger the ground station transmitters to turn off. So in short, these are remote control trigger tones. ALSO SEE THE WIKI PAGE !!!:http://en.wikipedia.org/wiki/Quindar_tones
Lu Johnson wrote at 2015-04-23 00:39:55
I'm sorry to jump in, but this 'answer' is COMPLETELY incorrect. So is the writer's explanation of "HF" radio operation and terminology. It's so bad, that to anyone technical at all, the piece is hilarious. Just thought you would like to know.