Ampex 351 - Page 39

— lOdb 30%
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OUTPUT
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— 40db
— 50db
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— 60db
DISTORTION
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+ 20
INPUT LEVEL, db
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10
FACTORS IN DETERMINING IMPORTANT
OPERATING CHARACTERISTICS
level at
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-5 — 4 -3 -2 -I 0
BIAS CURRENT, DB oizz
Typical output (A) and distortion (B) vs. bias current.
Readings taken at 1000 cps at 15 ips.
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Reproduce Amplifier
Preliminary amplification of the signal induced in
the reproduce head is accomplished in the reproduce
(or "playback ” ) preamplifier. You will recall that the
output of a reproduce head rises directly with fre 
quency. This increasing output is at an approximate
six db per octave rate (a very technical way of saying
that the voltage output doubles each time the fre 
quency doubles) so an opposite characteristic is re 
quired to obtain a flat overall frequency response.
An integrating amplifier, which attenuates rising
frequencies at a 6 db per octave rate, is thus necessary
for the reproduce function. The NAB standard curve
incorporates this integrating amplifier modified by a
rising frequency characteristic (or “ post emphasis ” ).
This post emphasis is achieved by an r-c circuit with
a time constant dictated by tape speed and set by
standards — for example. NAB standards for 7'/2 or
15 ips calls for a 50 microsecond time constant,
which places the +3 db point at 3.180 cycles.
tape is within the gap of the record head it is sub 
jected to a high frequency alternating field that is
maximum at the center of the gap and decreases
smoothly to zero on either side, plus a signal field that
looks like an unidirectional field for that instant, we
can see the degree of similarity that exists between
the ideal magnetization method and an a-c biased
magnetic recording.
As usual, however, there is one major area of dif 
ference. In the ideal method, the unidirectional field
strength is held constant while the alternating field
decreases to zero. In magnetic recording both fields
reduce at the same rate as the point on the tape
leaves the record gap, and the remnant magnetization
on the tape will be determined by the signal strength
when the bias reduces to the critical level. As a con 
sequence, the remanent magnetization in recording,
while linear, is always less than could be achieved by
the idea] method. Another result is that the amplitude
of the bias signal becomes important, because we
find that the recorded level falls as the bias is in 
creased beyond a certain value. This is explained by
the fact that an excessive bias current can place the
critical bias field strength well beyond the trailing
edge of the gap, where the signal field strength is low.
(Remember here that the only effective signal field
is that which exists where the critical bias field is
located. )
Using a-c bias, the output of the system can be
peaked at any given frequency by the proper adjust 
ment of the bias current. A complication arises in
that the bias current necessary to achieve maximum
output at low frequencies will result in a decreased
output at high frequencies. We therefore adjust the
bias at a given wavelength of the signal on the tape
(see Record Bias Adjustment).
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Typical third harmonic distortion vs. input
400 cps, measured at 15 ips. Distortion is plotted
a db scale to obtain a logarithmic function in linear
steps.
General
The most important operating characteristic in
any sound storage device are low distortion, high
signal-to-noise, good frequency response, and low
Hutter and wow. The last was thoroughly covered in
100% Odb