Common Misconceptions About Quindar Tones
There are two common misconceptions surrounding Quindar tones. The first is that one tone originated on Earth, while the second came 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, then he would speak. When finished, 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", followed by Houston talking, then another "beep", and 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. This wasn't necessary because the Apollo Unified S-Band System was full-duplex. (This discussion does not apply to the distinct half-duplex VHF-AM voice system used during launch and landing.)
Like cellular phones, the ground and the spacecraft each transmitted continuously on separate frequencies. Both ends also continuously transmitted their FM voice subcarriers. Nothing (other than the desire to avoid confusion) kept an astronaut and a Capcom from speaking and being heard at the same time. This often happened because of the radio signals' 3 second round trip time between the earth and the moon.
The sole purpose of the Quindar tones was to mute the uplink audio to avoid annoying the astronauts when the Capcom had nothing to say. The tones were needed because the muting function had to be placed at the remote uplink transmitter to mute noise on the telephone circuit from Houston to the uplink transmitter as well as background noise in Mission Control. The astronauts didn't even hear the Quindar tones, as they were filtered out before transmission.
A modern digital replacement for the analog Apollo system would have no need for Quindar tones. A digital transmission circuit from Houston to the uplink transmitter would add no noise to a quiet channel, and background room noise in Houston could be cut off with a local switch. Even if there were a need for remote control of a transmitter function, it could be done with an inaudible out-of-band digital signal.
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