FIFO sampling in the modulator

 I woul'd like to know if someone have information about FIFO in ATSC
modulator. This FIFO is located at the Data Randomizer input. The finality
of it is generate some null time to be consumed for anothers blocs (Reed
Solomon, Trellis, Sync, Field Sync, and pilot). I am looking for
information about amount of data in FIFO and clock rate. 

Thanks