Handbook for Sound Engineers

1056 Chapter 28

(28-5)

where,

x is 2 S× thickness/wavelength.

Although this expression yields a drop of 6 dB per
octave, as shown in Fig. 28-18, this curve is not the
same shape as the response of a low-pass filter made
from a resistor and capacitor. The response in Fig.
28-18 is down 4 dB at the intersection of the asymptotes
rather than the typical 3 dB for a single pole RC filter.
This difference in shapes means that a simple RC boost
circuit will not properly correct for the thickness loss.
Depending on the choice of RC boost frequencies, the
difference in shape will produce an error of 0.5–1.0 dB
in the midband response.

28.3.2.1 Equalization Boosts

This thickness loss of Fig. 28-22, must be corrected by

applying compensating boosts in either the record or

reproduce circuitry. Although this loss is a playback

deficiency, the choice of whether to correct the loss

during record or playback is somewhat arbitrary. The

amount of record boost is limited by the magnetic satu-

ration characteristics of the tape; playback boost is

limited by the high-frequency noise characteristics of

the tape and the reproduce head and associated circuitry.

The minimum amount of boosting required to

achieve flat response can be considered to be a neces-

sary equalization. The industry has developed a set of

internationally recognized standards to promote

compatibility of tapes. Each standard deals with the

necessary and discretionary equalizations to define the

exact characteristics of the recorded tape. Using the tape

flux characteristics as a standard implicitly specifies the

partitioning of equalizations between the recording and

reproducing functions. Table 28-4 lists the commonly

encountered standards.

Unlike the absolute nature of the reproduce charac-

teristics, the record characteristics of the recorder must

have enough flexibility to accommodate a number of

different tape sensitivities and frequency characteristics.

Once the reproduce section has been calibrated to the

standard with a standard alignment tape, all further

adjustments are to produce a recorded tape on the

machine that accurately matches the standard tape.

The amount of thickness loss can always be reduced

by utilizing thinner coatings, but any decrease in thick-

ness also causes an equal drop in low- and midfrequency

output and SNR. To preserve the existing standards, the

tendency has been to adjust the coating thickness of new

tapes to emulate the high-frequency losses of the older

tape types while trying to achieve maximum low- fre-

quency output. This somewhat self-defeating strategy

has been overcome in recent thin-coat high-energy tapes

Figure 28-18. Loss due to ratio of coating thickness to
wavelength.

Coating thickness lossdB 20 x

1 – e–x

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

0

10

20

30

.01 0.1 1.0 10

T/L—Thickness/wavelength

Output in dB

A. 15 kHz 30 in/s T = 0.65 mil ( 7.4 dB)

B. 15 kHz 15 in/s T = 0.65 mil ( 12.4 dB)

C. 15 kHz 1^7 / 8 in/s T = 0.2 mil ( 20.0 dB)

D. 15 kHz 1^7 / 8 in/s T = 0.65 mil ( 30.3 dB)

E. Digital 30 kbit/in T = 0.2 mil ( 31.5 dB)

A

B

C

D

E

20log^2 P×T/L    

1 e^2 P4/L

Table 28-4. Common Tape Record-Playback Equipment Equalization Standards.

Transition Frequencies and Time Constants

Standard Type Tape 1 ips 3 ¾ ips 7½ ips 15 ips 30 ips

IEC Fe 2 O 3 100/1326 Hz 50/1768 Hz 0/2274 Hz 0/4547 Hz

1590/120 μs 3180/90 μs /70 μs /35 μs

Metal 100/2274 Hz

1590/70 μs

NAB 50/1768 Hz 50/1768 Hz 50/3180 Hz 50/3180 Hz

3180/90 μs 3180/90 μs 3180/50 μs 3180/50 μs

AES 0/9095 Hz /17.5 μs

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