Handbook for Sound Engineers

1052 Chapter 28

yield superior SNR and reductions in amplitude modu-
lation noise, the wide tracks can create level stability
problems. These wider tracks will have large signal
variations at high frequencies if the tape guides permit
even small dynamic guiding errors. As a result, when a
transport is fitted with wide-track heads, the guiding is
also usually modified to hold the tape more accurately.

For multitrack recorders, the time and phase rela-
tionship between audio channels that are recorded on
separate tracks may be more critical than the level of
short-wavelength signals. Azimuth errors contribute to
differential timing errors between tracks, since the
azimuth tilting causes one track to be reproduced
slightly later than the other. As the distance between
tracks becomes large, such as for 1 inch and 2 inch
(2.5 cm and 5 cm) formats, the timing error becomes
critical. A typical method to measure this timing error is
to record the same high-frequency signal on two tracks,
and then measure the phase difference between tracks.
Table 28-4 shows the amount of worst-case phase
difference and timing difference at a 1 mil (25μm)
wavelength introduced by a 0.5 dB head azimuth error
for the outer pair of tracks.

The magnitude of both the height loss and the
azimuth loss could be greatly reduced if the widths of
the tape guides and tape matched perfectly. One method
to achieve this objective is to use adapting guides with
spring-loaded movable flanges so that the guide adjusts
itself to the tape width. Some digital audio recorders
with numerous very narrow tracks utilize spring-loaded
guides to maintain close repeatability of the tape path.

A similar effect can be achieved with fixed- flange
guides if a curvature is deliberately introduced into the
tape path. Fig. 28-12 illustrates two possible methods to
achieve this curvature. Typically, an offset of less than
5 mils (125μm) is adequate to overcome the worst-case
combination of clearance between the tape and guides
and the maximum amount of natural bowing in the tape
due to slitting and subsequent handling distortions.

Although the dynamic guiding variations are greatly
reduced by forcing the tape to maintain a distorted tape
path, the increased force applied to the edges of the tape

produces new problems. Not only do both the guides

and the edges of the tape experience higher wear rates,

but scrape flutter is also increased dramatically. The

edges of a tape are very rough due to the shearing action

used in the tape-slitting process. When these rough

edges slide firmly against the distorting guide flange,

tape vibrations are excited, producing scrape flutter.

28.2.4.3 Tape Guides

Tape guides come in many shapes, sizes, and basic

types, as shown in Fig. 28-13. Each guide contains

flanges that press against the edges of the tape to steer

it. In all cases except the edge-only guide, the tape

wraps around the guide to generate stiffness so that the

steering force exerted by the flange can move the entire

width of the tape and not just buckle the edge. Typically,

at least 10qof wrap is required for adequate stiffness.

Rotating guides are generally less effective than

stationary guides. Since the tape is in firm contact with

the spinning surface of a rotating guide, rather than in

sliding contact as with the stationary guide, the force

Table 28-3. Errors Due to 0.5 dB Azimuth Error (1 mil
wavelength)

Format Phase Error Timing Error

3 inch stereo 151 q 0.28 ms

1 inch 8 track 867 q (2.4 rotations) 0.16 ms

2 inch 24 track 3500 q (9.7 rotations) 0.65 ms

Figure 28-12. Deliberate tape curvature to reduce guiding

errors.

Figure 28-13. Five styles of tape guides.

A. Edge only. B. One-piece stationary.

C. Three-piece stationary. D. Fixed-flange rotating.

E. Rotating-flange rotating.