Ferrograph 4 AH Owners Manual

Page 1

THE MANUAL

OF THE

FERRO GRAPH

Page 2

THE FERROGRAPH SERIES 4

Page 69

4.5. Noise

4.5.1. Polarized Noise

The tape shall-be thoroughly erased by bulk erasing procedure in a 50 cycle
field. It shall then be played back over the standard head and amplifier of the previous
tests, but in this operation the playback head shall be subject to a polarizing field by
having D.C. passed through its winding while playback is taking place. A current
of2 m.a. shall be passed through the bias winding of the head via a limiting resistor
of not less than 12,000 ohms. The unweighted noise signal thus produced shall not
exceed 13 microvolts on tape which passes the requirements of Test 4.2.

4.5.2. Limiting Noise

For this test the playback amplifier shall be equalized so that with the particular
tape being tested an overall record /reproduce response, level within 3'; 3 DB from
70 to 9,000 cycles, is obtained when the recording network shown in Fig. 1 is modified
so as to include a 250 pf capacitor shunted across the 100K resistor to give a fixed
amount of pre-emphasis.

The reference level against which the noise is to be measured shall be that of
the maximum 400 cycle signal output giving a distortion not exceeding 5". After
this measurement the tape shall then be thoroughly erased by a 50 cycle bulk erasing
technique to give the best possible noise level and the playback head shall also be
thoroughly demagnetized.

The noise voltage of the tape shall then be measured as the increment in output
voltage from tape stopped, to tape running, when all components over 200 c.p.s.
up to the high frequency end of the band as stated above are included. The ratio of
noise output to maximum 5% distortion signal output shall be not less than 54 DB.

AFEGUARDS

lowithstanding acceptance under all the clauses above, the tape shall not exhibit any
ar characteristics either mechanical or electrical such as would cause its performance
nplete instruments to fall outside the overall instrument performance.

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