9914V TAPE UNIT

2.4.3.2 NRZ DensityThe output from the pre-amplifier is taken to the first gain stage. This has a frequencyresponse which can be altered depending on the mode of operation; in NRZ mode theresponse is flat over the NRZ operating frequency range.The first stage output is taken to the gain controlled amplifier. This is used in fixed gainmode under processor control, as described in 2.4.3.1 above. The GCA output isprocessed in two parallel paths. In one, the signal is band-pass filtered and the amplitudequalified using the threshold detector detailed in 2.4.3.1. In the second, the signal isseparately band-pass filtered and then differentiated and the resultant zero crossingsdetected to extract the peak position information. The zero cross and threshold detectoroutputs are retimed to recover the NRZ data. This is then output through the data selector,giving the RDIN- signals for the DDP board.PREAMPOUTPUT8 DAC;'GAINSTAGEGAIN~ CONTROLLEDAMPLIFIER~LEVELGCV~THRESHOLD~BANDPASSFILTER-[ BANDPASS~ FILTER1THRESHOLDDETECTORI D I FFER-~I·~jTIArORi1I ZEROI:~OSS I NO:=TECTORI1D__ ~~ __ ~I~) ______ ~ ____________________ ~.: SELECTF I L TER SELECT! DATAAQP I NRZ ICONTROLROIN 0-71FIGURE 2.4.3.2(a) ADP READ BLOCK DIAGRAM - NRZ DENSITY2.4.4 E-E ModeWhen the diagnostic programs require to check the data handling circuits, withoutcorrupting data already on the tape, the E-E (electroniCS to electronics) mode of working isadopted. The lOOP Signal (from the Data Control board) allows the zero crossing andthreshold detector outputs to be replaced with STEP_elK and WDATA respectively. Thisenables data checks to be made without energising the write circuits or moving tape. Thistests the operation of the activity detect and the data retiming circuits; good or faulty datacan thereby be fed to the decoding circuits in order to check that they function correctly.2 -18 9914 V Servicing Manual 95 124766 (Draft Issue A)