Handbook for Sound Engineers

1058 Chapter 28

(76 cm/s). Beware that this level of error will be intro-
duced each time the tape is rerecorded during mixdown
and subsequent protection copying. The total error can
easily reach 5 dB or more for a typical sequence of
operations.

28.3.2.4 Crosstalk

Fringing also produces playback signal leakage or
crosstalk between adjacent tracks at long wavelengths.
The unused area or guard bands between the cores of
the head, which are nearly equal in width to the
recorded track, usually provide enough of a physical
gap to prevent flux from spilling from one track to the
next. At long wavelengths, however, the fringing flux
will jump the guard band, producing low-frequency
crosstalk.
The crosstalk component due to fringing will initially
decrease as the frequency is increased, but at midband
the decrease will eventually bottom out. The remaining
residual level of crosstalk is not due to fringing, but it is
a direct transformer-like coupling of leakage flux
between the adjacent cores in either the record or repro-
duce head. A layer of magnetic shielding material is
typically placed between the cores of the head as a
crosstalk shield to reduce this flux leakage.

28.3.3 Frequency Characteristics

28.3.3.1 Inductive Rise

Up to this point, most of the losses and response anoma-
lies have been governed by the wavelength performance
of the interface between the tape and the head. An addi-
tional set of characteristics due to the internal
frequency-dependent operation of the head must also be
considered.
The most striking characteristic in the frequency
response of a conventional coil-and-core playback head
is a continuous 6 dB/octave rise in output voltage with
rising frequency. The core and winding of the head form
an inductor in which the output voltage is proportional
to the rate of change of the flux in the core as seen in the
equation

(28-6)

where,
N is the number of turns in the winding,
'I is the change in flux,

't is the time interval.

Any ratio of the form 'x/'t is called a differential

with respect to time, and the device creating this rate of

change is called a differentiator.

If a sine-wave signal of frequency f is used for

testing the output voltage of a head, the voltage expres-

sion can be further simplified to

(28-7)

28.3.3.2 Hysteresis Loss

The constantly changing magnetic flux in the core of

the reproduce head gives rise to losses within the core

of the head. One source of these losses is the amount of

energy that is required to change the magnetization state

of the core material. Every time the flux in the core

reverses polarity, a small amount of energy is lost in

overcoming the magnetic memory or hysteresis of the

core material. The hysteresis power loss increases with

both increasing flux magnitude and frequency.

28.3.3.3 Eddy Current Loss

The changing core flux generates a voltage not only in

the winding of the head, but also within the core itself.

If the core is metallic, this voltage will cause a current

to flow within the core, as shown in Fig. 28-20. The

core currents, referred to as eddy currents because of

their similarity to swirling eddies in a stream of water,

dissipate energy that should be going to the reproduce

signal.

The amount of power (P) dissipated in the eddy

currents is given by the general power equation:

head Vout N'I

't

= ------ -

Figure 28-20. Eddy current.

head Vout= 2 SNf fluxu max

A. Solid.

B. Layered.

M layers

A. Solid