Handbook for Sound Engineers

Analog Disc Playback 1017

duced in recording. From this point on, the equalization
used for cutting was called preemphasis, and equaliza-
tion used in playback equipment, postemphasis.
The 1 kHz signal was chosen as the reference point
because it was a convenient halfway point between the
low and high frequencies. As time went by and further
improvements were made, the equalization was
extended to the higher frequencies as well. What
emerged from the long and at times controversial
subject of equalization are the RIAA and NAB equal-
ization curves. The first curve was used by the Record
Industry Association of America (RIAA) and the
second, which is almost identical to the first curve, by
the National Association of Broadcasters (NAB).
People still debate about two versions of the
recording equalization. The DIN (Deutsche Industrie
Norm) standard used in European countries calls for
additional equalization at the extreme low end during
playback to improve the SNR and stability of the
system due to mechanical disturbances—i.e., turntable
rumble—which can affect the overall performance of
the system.
The NAB (RIAA) curve used presently in the play-
back equipment is shown in Fig. 27-2. The numerical
values for the characteristic are shown in Table 27-1.
For recording, the inverse curve is used. It means that if
the playback signal is boosted +19.3 dB, the same
signal should be recorded at the level of –19.3 dB so
that the overall result will be 0 dB deviation from the
ideal flat response curve.

Equalization is used to record the sound at the most

advantageous levels for the best results as far as distor-

tion and noise are concerned and to reproduce it so that

the original balance between the frequencies can be

restored. The RIAA curve is used for phonograph discs.

Tape recorders record signals on tape, and tape recording

has limitations that differ from the limitations found in

mechanical recording and, therefore, require different

preemphasis and postemphasis for best results.

The RIAA curve covers the range from

20 Hz–20 kHz. The DIN curve, as shown in Fig. 27-3,

extends the control over playback down to 2 Hz where

the equalization returns back to 0 dB. As can be seen

from the graphs, the curves have complex shapes;

equalizer circuits use capacitors and resistors, and their

values determine the amount of signal equalization that

can be expressed as a function of a time constant in

microseconds as derived from the equation

(27-1)

where,

T is a time constant,

C is capacitance in farads,

R is the total effective resistance of the supply network

in ohms.

This is part of the equation to determine the attenua-

tion at various frequencies:

(27-2)

where,

Zis 2Sf,

T is CR of Eq. 27-1.

The RIAA curve consists of three time constants;

75 μs to roll off the high frequencies, 318μs to produce

the slope below 1 kHz with a knee at 500 Hz, and a

Figure 27-2. NAB (RIAA) standard reproducing characteristic.

+20

+15

+10

+5

0



5



10



15



20

20 50 100 200 500 1K 2K 5K 10K 20K

Frequency—Hz

Relative output—dB

318 MS 75 MS

3180 MS

Table 27-1. Preferred Frequencies and Calculated

Recording Characteristics

Frequency

(Hz)

Recording

Characteristics

(dB)

Frequency

(Hz)

Recording

Characteristics

(dB)

20.0 –19.3 800.0 –0.8

25.0 –19.0 1000.0 0.0

31.5 –18.5 1250.0 +0.7

40.0 –17.8 1600.0 +1.6

50.0 –16.9 2000.0 +2.6

63.0 –15.8 2500.0 +3.7

80.0 –14.5 3150.0 +5.0

100.0 –13.1 4000.0 +6.6

125.0 –11.6 5000.0 +8.2

160.0 –9.8 6300.0 +10.0

200.0 –8.2 8000.0 +11.9

250.0 –6.7 10,000.0 +13.7

315.0 –5.2 12,500.0 +15.6

400.0 –3.8 16,000.0 +17.7

500.0 –2.6 20,000.0 +19.6

630.0 –1.6

TCR=

attenuationdB= 10 log 1 +Z^2 T^2