Digital Clock Recovery Scheme Masks Tape Errors

Abstract

The bit error rate (BER) of the tape recorder is a primary factor in determining the overall accuracy of a space to ground PCM data link. The ultimate goal is to obtain a recorder BER signature that is dependent solely on tape quality; however, in practice, the BER may be several orders of magnitude worse than the goal. Usually this occurs when the data transfer rate is not preserved during dropout intervals - intervals where the net signal-to-noise ratio is insufficient to guarantee valid data cell boundary detection. This paper investigates tape dropout signatures and addresses the problem of maintaining time coherency between reproduced data and clock assurance throughout the dropout interval. A digital approach is presented that utilizes a dual threshold detector in conjunction with a clock recovery generator. The generator is synchronized to the tape signal by a clock, N-times the data transfer rate, coherent with the transport servo reference. Implementation of the approach is discussed for two popular PCM formats. The effectiveness of the approach as a function of cell boundary detection, system signal-to-noise ratio, and transport time base error (TBE) is considered.