Handbook for Sound Engineers

1054 Chapter 28

at each wavelength that produces an odd or even
number of complete cycles in the gap.
The example recorder at 15 in/s has only 1 dB of gap
length loss at 40 kHz and 3 dB at 67 kHz, certainly not
a dominating loss. At 30 in/s these losses become even
more insignificant, with the 1dB and 3 dB frequen-
cies doubling to 80 kHz and 134 kHz.
Audio recorders are seldom designed to operate
beyond the dashed lines shown in Fig. 28-15. With this
constraint, gap length loss for professional machines
can be held below 1 dB or 2 dB by choosing an appro-
priate gap length for a given application and minimum
wavelength. Mastering recorders operating at 15 in/s

and 30 in/s and broadcast machines operating at 7.5 in/s

have playback gaps ranging from 100–200μinch,

compact cassette machines operating at 1 in/s have

gaps of 30–60μinch.

Mastering recorders may also use the record head for

playback in the sync mode. Since the record heads may

have gaps ranging from 250μinch to 1000μinch, the

sync response may suffer significant high-end loss. For

example, a 1950s vintage recorder with a 1000μinch

record gap will reach its first null at 15 kHz for a tape

speed of 15 in/s. As sync response became more impor-

tant in the mid 1960s the recorder manufacturers tight-

ened up the record gaps to 350μinch or less to improve

sync response.

If the gap length is inferred from the first measured

null, this effective gap length may be 10% to 15%

longer than the mechanical gap determined by the shim.

Various proposed explanations include magnetic degra-

dation of the inner surfaces of the pole tips due to manu-

facturing stresses and pole tip saturation. When in

doubt, add 10% to the optically measured length or shift

the response points down to 91% (1/1.1) of the theoret-

ical values. For the ATR100 example, the 1dB and

3 dB points would shift to 36 kHz and 61 kHz.

Figure 28-14. Ideal and practical magnetic heads.

Metal

Polyester

Oxide

0.5 mil

1.42 mil

20 mil

0.25 mil

Magnify

0.625 in

0.45 in

C. Magnified critical gap and adjacent tape.

A. A small gap in a ring of magnetic material.

B. Typical studio head.

Figure 28-15. Loss due to gap length and the ratio of gap

length to wavelength.

1.2

1

0.8

0.6

0.4

0.2

0

0.2

200 2K 20K

0.40.5

A. Gap length loss—linear

200K 1000K

Gap length loss

0



10



20



30



40



50



60



70



80

B. Gap length loss—dB

Gap length loss

Gap/wavelength 0.01 0.10.20.3 0.81.02.03.0

Professional

Professional

Consumer

Consumer

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