The two tones were generated by special equipment located at Mission Control, and they were decoded by detectors located at the various tracking stations. The selection of the tones allowed them to travel in the same passband as a human voice, which has a range from roughly 300 Hz to 3,000 Hz. 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. 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."