When looking at moonwalk videos, we can hear bips. What are those bips? Why are there bips? Do this bips still exist in nowaday space missions?
These "beeps" are called the quindar tones (named after the manufacter Quindar Electronics, Inc).
These tones were used to simulate the action of the push to talk button. CAPCOM always had to push a button during communication. The first tone (2.525KHz / 250ms) triggers the ground station transmitters to send, the second tone (2.475KHz / 250ms) turns the transmitter off.
Quindar tones became very famous during the Apollo program but were used also before. Also the first Space Shuttle missions used them.
Today the quindar tones are not needed anymore because modern communication systems can carry multiple communication channels.