A binary-coded decimal time code began testing in 1960, and became permanent in 1961. This "NASA time code" was modulated onto a 1,000 Hz audio tone at 100 Hz, sounding somewhat like a monotonous repeated "baaga-bong". The code was also described as sounding like a "buzz-saw". On July 1, 1971, the time code's broadcast was changed to the present 100 Hz subcarrier, which is inaudible when using a normal radio (but can be heard using headphones or recorded using a chart recorder).