Handbook for Sound Engineers

Magnetic Recording and Playback 1051

components: fixed errors due to misadjustment and

dynamic errors due to tolerances and tape variations.

28.2.4.1 Tape Height

Height must be controlled so that the recorded tracks on

the tape will pass directly over the pickup areas of the

head. The required degree of height accuracy increases

as the tracks become narrower. Table 28-1 shows signal

loss due to height errors for several popular tape formats.

For a tape guide to position the tape accurately, the
tape must fit snugly into the guide, but the guide must
not squeeze the tape edges. The typical manufacturing
tolerances of 2 mils to 4 mils (50μm to 100μm) on
tape width and 1–3 mils (25–75μm) on tape guide
width result in a loose fit for many rolls of tape.
Sources of height error also include fixed errors in
head and guide height and core placement tolerances
within the head. A good alignment should contain no
more than 1 mil (25μm) combined error for the head
and guides, but this degree of accuracy requires the use
of optical measurement devices that are not commonly
available in a recording studio. Typical maintenance
shop practices will yield errors in the range of 2–3 mils
(50–75μm). When this alignment error is added to a
typical core placement error of 1 mil (25μm) and a tape
guide clearance error of 2 mils (50μm), the signal loss
or variation can easily exceed 1 dB on a 24 track
recorder.
A relatively simple method of reducing the sensitivity
to height errors is to use different widths for the record
and reproduce head core widths. Using either a wide
playback head on a narrow recording or a narrow play-
back head on a wide recording will reduce or eliminate
the losses due to height variation. Differing track widths,
however, give rise to a common operator error. Setting
the normal and sync reproduce levels from a full-track

alignment tape, which has signal recorded across the

entire width of the tape, will produce a level error on the

wider of the two heads. The amount of error, which

depends on the ratio of the core widths of the two heads,

must be subtracted from the actual meter reading of the

wider core to determine the true flux level. For example,

a recorder with 37 mil (0.93 mm) record cores and

43 mil (1.08 mm) reproduce cores would be set to read

0 VU in sync playback and +1.3 VU in normal playback

from a full-track alignment tape.

28.2.4.2 Head Azimuth

Not only must the tape passing across the head be at the

correct height, but also the recorded signal on the tape

must be parallel to the pickup gap in the reproduce

head. Any angular error is referred to as azimuth error.

Table 28-2 gives the amount of signal loss due to

azimuth error for a 15 kHz signal at 15 in/s (38 cm/s), a

1 mil (25μm) wavelength O.

For a typical professional recorder with guides

spaced 6 inches (15.2 cm) apart, the worst case combi-

nation of guide and tape sizes could produce a

maximum dynamic guiding error of 5 mils (125μm)

at each guide, yielding an azimuth error of 0.1or

6 min. This error would generate a signal fluctuation

of 3 dB for a 250 mil (6.35 mm) track width as indi-

cated in Table 28-3. Overlapping heads or tracks offer

no azimuth loss improvement.

After many years of decreasing track widths to fit

more tracks on a tape, there is a resurgence of wider

track formats such as 2 tracks on ½ inch or 1 inch tape

and 8 tracks on 2 inch tape. Although these formats

Table 28-1. Loss Due to Height Error

Loss Height Errors in Mils for Various Track

Widths*

dB loss % loss 84 mil 70 mil 43 mil 37 mil 21 mil

0.1 1.14 0.96 0.80 0.49 0.42 0.24

0.3 3.39 2.85 2.38 1.46 1.26 0.71

0.5 5.59 4.70 3.92 2.41 2.07 1.17

1.0 10.87 9.13 7.61 4.68 4.02 2.28

*84 mil—some 2 track ¼ inch stereo

70 mil—4 track ½ inch, 8 track 1 inch, 16 track 2 inch

37 mil—4 track ¼ inch, 24 track 2 inch

43 mil—24 track 2 inch (on some systems)

21 mil—stereo compact cassette, 8 track ¼ inch

Table 28-2. Azimuth Loss at 15 kHz, 15 in/s (1 mil

wavelength)

Azimuth Error (minutes)

dB loss 250 mil 70 mil 37 mil 21 mil

0.5 2.55 9.1 17.3 30.4

1.0 3.60 12.9 24.3 42.8

3.0 6.10 21.7 41.1 72.4

6.0 8.30 27.3 56.0 98.6

where,

W is the track width,

D is the azimuth error,

O is the wavelength.

loss 20

sin SWutanD

---------------------------------------O -

SWutanD

O

---------------------------

= log----------------------------------------