Handbook for Sound Engineers

1072 Chapter 28

dried out on the recording tape. Sticky adhesive residue
that could bond to adjacent layers, on the other hand,
must be removed.

28.3.12.4 Print Through

The energy required to activate a particle to switch
magnetic states depends on the size of the particle, with
the overall characteristics of a magnetic tape being
determined by the average size and characteristics of
many particles in the coating. A more detailed analysis
of the particles would yield a distribution of sizes, as
shown in Fig. 28-34. Although the majority of the parti-
cles cluster around the average value, a small portion of
the particles are either much smaller or much larger than
the average. The small particles give rise to spontaneous
recording as print through; the large particles produce
noise bursts.

The small particles require so little activation energy
to assume a new magnetization state that even the
thermal energy of the particles may provide enough bias
to cause the particles to be recorded by the stray
magnetic fields due to adjacent layers of recorded mate-
rial. This spontaneous recording is most evident as pre-
or post- echo at the beginning and end of a recording.
The strength of the print through image depends on both
the percentage of thermal idiots in the coating and the
ratio of remanence to coercivity of the tape. The rema-
nence measures the driving force of the signals trying to
print through. The coercivity, on the other hand, is the
stubbornness of the particles to resist this imprinting.
The effective coercivity of the small particles is dimin-
ished because the domain size is sub-optimal, rendering
the small particles more susceptible to printing.
The milling process used to provide thorough mixing
of the particles, binder, and additives prior to coating is

a rather abusive process that can create thermal idiots by

fracturing some of the desirable large particles into

smaller, low-coercivity particles. Insufficient milling,

on the other hand, provides an uneven particle disper-

sion that creates noise on the tape. The tape manufac-

turer must strike a compromise that yields both low

noise and low print through.

Print through of a signal produces both pre-echoes

and post-echoes. The pre-echoes are more troublesome

in music, however, since the pre-echoes frequently

occur in the quiet passages just before the loud note.

The post-echoes, on the other hand, are frequently

masked by the diminishing tail of the musical note and

the room reverberation.

Fortunately, the print-through process does not

produce equal amounts of pre- and post-echo, but unfor-

tunately the more undesirable pre-print echo is the

stronger. The vector magnetization components that

arise during the recording process cause the levels of

print on the outer adjacent tape layer to be several deci-

bels higher than on the inner adjacent tape layer, as

shown in Fig. 28-35. The more troublesome pre-print

echoes on musical selections can therefore be mini-

mized by storing the tape tails out to move the quiet

lead-in to the inner layer. This will also bury the louder

outer layer print through echo in the decaying signal at

the end of the music.

The use of nonmagnetic leader tape between selec-

tions is also helpful to eliminate pre-echo on selections

that begin with a rapid attack. Be aware, however, that

paper leader tape can contain a small amount of

magnetic debris that will raise the noise level as the

leader passes over the playback head.

The user can take several steps that will minimize

the amount of print through. First, the use of thicker

base films increases the spacing between layers.

Figure 28-34. Particle size distribution.

Coercive force drops

with decreasing

particle size

Coercive

peak is in

this range

Average (mean)

particle volume

Single

domain

size limit

Range of

transition

zone over

normal

temperatures

Transition zone

at temperature T

Logarithm of particle volume

Number

of

particles

at a

given

volume

Coercive force

drops with

decreasing

particle size

Figure 28-35. Pre- and post-echo print through. Courtesy

3M Co., Magnetic Audio/Video Products Div.

Signal

0

10

20

30

40

50

60

Output

dB

Tape A print level

2nd

wrap

1st

wrap

226

320 nWB/m