Handbook for Sound Engineers

Magnetic Recording and Playback 1059

(28-8)

The previous discussion on the inductive rise of
voltage with increasing frequency in the reproduce head
also applies to these eddy components, producing a
rapid rise in eddy current power loss.

The eddy currents of the solid core of Fig. 28-20A
rise to an unacceptable level even before the upper
limits of the audio band are reached. Fortunately, this
drastic loss can be decreased by dividing the core into
many thin insulated layers or laminations, as shown in
Fig. 28-20B. For M laminations, each lamination would
generate only 1/M of the core voltage and 1/M 2 of the
loss power produced by a solid core. The core resistance
for each lamination drops only slightly since the width
of the lamination remains unchanged. The net improve-
ment for M laminations is a 1/M reduction in the eddy
current power loss. (Professional audio heads, which are
typically constructed with laminations 2 mils (50μm)
thick, will contain 20 to 120 laminations per track,
depending on the track width.) Reducing the core size
and using high-resistivity core materials such as ferrites
can achieve even further improvements.

28.3.4 Combined Characteristics

Fig. 28-21 illustrates some of the individual and
composite effects of the foregoing reproduce head char-
acteristics. The constant 6 dB/octave inductive rise of
the head has been omitted in the illustration to accen-
tuate the undesired departures from flat response.

Curves A, B, and C illustrate the gap length, tape
thickness, and spacing losses, respectively.

Curve D represents a typical resonant rise due to
head inductance and the capacitance of the head cable
and head winding. The playback amplifier
high-frequency response boost dictated by the National
Association of Broadcasters (NAB) equalization stan-
dard for 15 in/s (38 cm/s) is represented by curve E. The
combination of all of these effects in curve F yields a
response that is flat within ±1 dB. This simplified model
does not include relatively minor contributions at
mastering speeds due to eddy currents and hysteresis,
self-demagnetization effects, recording equalization,
and the effects of nonuniform distribution of recorded
flux due to coating thickness. In spite of these omis-
sions, the dominant nature of the coating thickness loss

is readily apparent. The equalization standards have

been chosen primarily to offset this thickness loss.

The composite curve F represents the overall play-

back performance from an ideal tape of finite thickness.

All the indicated response anomalies within the audio

band must be either corrected or tolerated. In some

cases, one effect can be used to offset others, such as

shaping the resonance curve to compensate for the gap

length loss. (Unlike the resonance, the gap length loss

increases with decreasing tape speed, upsetting the

compensation at lower tape speeds.)

28.3.5 Noise

The useful range of signal levels that pass through the

tape recorder is limited by the maximum signal at which

all the magnetic tape particles become completely

magnetized or saturated and also by the amount of noise

that remains when the input signal is removed. Noise in

tape recorders has many sources; the electronics, the

tape, and the heads themselves all contribute to the

residual noise.

The distortion content of the signal from a tape

recorder rises so dramatically near tape saturation that

the normal operating range must be limited to

less-than-maximum levels. For the purpose of speci-

fying and comparing tape recorders, the distortion-free

maximum operating level is typically considered to be

the output signal level at which the THD, which is

dominated by third harmonic and other odd compo-

nents, reaches 3%. The ratio of the level for 3% THD at

medium wavelength to the residual noise is defined as

the SNR of the recorder.

P V

2

R

----- -=

I

2

= R

Figure 28-21. Playback loss components at 15 in/s for

0.65 mil coating thickness.

+5

0

 5

10

15

20

25

30

Response

dB

150 Hz 1.5 kHz 15 kHz 150 kHz

1000 100 10 1.0

Frequency wavelength/gap length

E F D

B

C

A

A

A

A. Gap length loss (0.1 mil)

B. Thickness loss (0.65 mil)

C. Spacing loss (10 Min)

D. Head/cable resonance

E. NAB 50 Ms equalization

F. Overall response