SMPTE 12M timecode
31...28 27...24 23...20 19...16 15...12 11...8 7...4 3...0 h h m m s s f f
It’s encoded like BCD.
Example )
01:28:17:18
31...28 27...24 23...20 19...16 15...12 11...8 7...4 3...0 NDF : 0 1 2 8 1 7 1 8 0000 0001 0010 1000 0001 0111 0001 1000 DF : 0 1 2 8 1 7 5 8 0000 0001 0010 1000 0001 0111 0101 1000
The high 2 bit of second nibble from the right is for DF/NDF.