There are two independent time codes used for this purpose : an amplitude - modulated time code and a phase - modulated time code added in late 2012 .
An amplitude - modulated time code has been in use with minor changes since 1962 .
There are two independent time codes used for this purpose : an amplitude - modulated time code , which has been in use with minor changes since 1962 , and a phase - modulated time code added in late 2012 .