Read Channels

Each read channel differential signal from the pre-amplifier (at about 100 mV peak-peak amplitude) is routed to a differentiator ba~ad on a NE592 chip on the ADP board. The read signal processing circuits are intemally r€a..configured according to the current density, resulting in two basic modes, GCRI OPEl PE and NRZ. For ease of understanding, the modes are discussed separately.

The read circuits use a mixture of proprietary MSI and LSI analogue ICs including VLSI analogue ASICs, and standard high speed CMOS logic.

2.4.3.1 PE/GCR Densities

The output from the pre-amplifier is taken to the first gain stage. This has a frequency response which can be altered depending on the mode of operation; in PE/GCR mode the response has gain increasing with frequency.

PREAMP

FIGURE 2.4.3.1 (a) ADP READ BLOCK DIAGRAM - GCR/PE DENSITIES The first gain stage output is taken to a gain controlled amplifier (GCA). This has two operating conditions, under fixed gain and under AGC.

Under AGC, the output of the amplifier is set by the LEVEL Signal, whereby the output amplitude is detected by the amplitude detector block and fed back through the AGC select block. The amplitude signal is compared with the LEVEL reference and an error signal produced which modifies the amplifier gain to stabilise the output amplitude. The presence of data at the output of the channel is sensed using the activity detect logic. This allows AGC to be selected only during data, preventing the amplifier from ^I running away ^I during the IBG or when the tape is stationary.

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Under fixed gain conditions, the amplitude input to the GCA is taken from a DAC and LEVEL is switched off. This allows the Data Control board to set the amplifier gain and hence the output level. The amplifier is used in fixed gain mode when writing (to ensure that the appropriate standards are met) and during the IBG.

The output from the GCA is band-pass filtered. The filter characteristics required are speed and density dependant and one of four filters can be selected using the filter select lines. The filters themselves are tailored to a specific combination of speeds and densities, allowing the 9914V to be process data from 1600/3200/6250 bpi at speeds from 42 to 125 ips.

The filter output is processed to provide amplitude and zero crossing information. The zero crossing detector outputs a short pulse every time the filtered waveform passes through the zero signal level in either direction. The filtered signal is qualified against a threshold level set by the THRESH signal. This is a bi-directional threshold, requiring the signal to exceed the positive threshold before setting the output and then exceed the negative threshold before resetting. The zero crossing detector output is delayed to ensure the correct phase relationship with the threshold signal for GCR data recovery.

The delayed zero crossing and threshold detector outputs are retimed to recover the GCR 1 PE data. This is then output through a data selector which can select either PE/GCR data or NRZ data, giving the RDIN* signals.for the DDP board.

t VE THRESHOLD BPF OUTPUT

- vE THRESHOLD

ZCD OUTPUT

n n n n L

DELAYED ZCD OUT

~ ^~ n ru ^rL

THRESHOLD OUT

1 I

RECOVERED DATA

I r

FIGURE 2.4.3.1 (b) GCR/PE READ WAVEFORMS

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2.4.3.2 NRZ Density

The output from the pre-amplifier is taken to the first gain stage. This has a frequency response which can be altered depending on the mode of operation; in NRZ mode the response is flat over the NRZ operating frequency range.

The first stage output is taken to the gain controlled amplifier. This is used in fixed gain mode under processor control, as described in 2.4.3.1 above. The GCA output is

processed in two parallel paths. In one, the signal is band-pass filtered and the amplitude qualified using the threshold detector detailed in 2.4.3.1. In the second, the signal is separately band-pass filtered and then differentiated and the resultant zero crossings detected to extract the peak position information. The zero cross and threshold detector outputs are retimed to recover the NRZ data. This is then output through the data selector, giving the RDIN- signals for the DDP board.

PREAMP

FIGURE 2.4.3.2(a) ADP READ BLOCK DIAGRAM - NRZ DENSITY

2.4.4 ^{E-E Mode}

When the diagnostic programs require to check the data handling circuits, without

corrupting data already on the tape, the E-E (electroniCS to electronics) mode of working is adopted. The lOOP Signal (from the Data Control board) allows the zero crossing and threshold detector outputs to be replaced with STEP_elK and WDATA respectively. This enables data checks to be made without energising the write circuits or moving tape. This tests the operation of the activity detect and the data retiming circuits; good or faulty data can 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)

tilE THRESI-'OLD

.. . t ... " . 'j~\' .... , ... 1.\ ... 11·· ... .

j. ... C··]···)···'1··~···\··r l

~

~

···u ···J···V···J.··

BPI=' OUTPUT

-VE THRESi-iOLD

ZCO OUTPUT

THRESHOLD OUT

RECOVERED DATA

FIGURE 2.4.3.2(b) NRZ READ WAVEFORMS

2.4.4.1 Calibration

The amplitude signal and GCV can be monitored using the GAIN DAC output and

comparator, this is used during calibration, which is outlined under diagnostic program 74 in Section 5. Various parameters, including pre-amplifier and ADP channel gains, write currents. and data timing are automatically calibrated, preferably using a dedicated reference tape.

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