The default bit rate specifies the bit rate at which the file you are attempting to play was originally recorded. If the MTX100A miscalculates that rate (as a result of jitter in the recording) you can correct the default bit rate by clicking the "Default Rate" button. The default bit rate is only used for calculations and is not used for final playout; the data bit rate is always used instead. The clock screen is covered in more detail on pages 3-8 and 3-9 of the MTX100A user manual.