Implementation of Quindar Tones
Two tones were used in the Quindar system, named after its manufacturer, both being pure sine waves that were 250ms long. The "intro tone" was generated at 2,525 Hz and signaled the "key down" keypress of the PTT button. The "outro tone" was slightly lower at 2,475 Hz and signaled the release of the PTT button. The 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 300 Hz to 3,000 Hz.
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