LMC1982 Datasheet, PDF (10/14 Pages) National Semiconductor (TI) – Digitally-Controlled Stereo Tone and Volume Circuit with Two Selectable Stereo Inputs

Application Information (Continued)
The bass turn-over frequency is nominally
when maximum boost is chosen. The inflection points (the
frequencies where the boost or cut is within 3 dB of the final
value) are for treble and bass
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FIGURE 3. System Block Diagram Utilizing the External Processing Loop (One Channel Shown)
8.3 kHz. If the tone control capacitors size is decreased
these frequencies will increase. With C2 = C3 = 0.0068 µF
the 2 dB steps take place at 130 Hz and 11.2 kHz.
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FIGURE 4. The Tone Control Amplifier
Increasing the values of C2 and C3 decreases the turnover
and inflection frequencies: i.e., the Tone Control Response
Curves shown in Typical Performance Curves will shift left
when C2 and C3 are increased and shift right when C2 and
C3 are decreased. With C2 = C3 = 0.0082 µF, 2 dB steps
are achieved at 100 Hz and 10 kHz. Changing C2 and C3 to
0.01 µF shifts the 2 dB per step frequency to 72 Hz and
LOUDNESS
The human ear has less sensitivity to high and low frequen-
cies relative to its sensitivity to mid-range frequencies be-
tween 2 kHz and 6 kHz for any given acoustic level. The low
and high frequency sensitivity decreases faster than the sen-
sitivity to the mid-range frequencies as the acoustic level
drops. The LMC1982’s loudness function can be used to
help compensate for the decreased sensitivity by boosting
the gain at low and high frequencies as the volume control
attenuation increases (see the curve labeled “Gain vs Fre-
quency with Loudness Active”).
The LMC1982’s loudness function uses external compo-
nents R1, R2, C4 and C5, as shown in Figure 5, to select the
frequencies where bass and treble boost begin. The amount
of boost is dependent on the volume attenuator’s setting.
The loudness characteristic, with the volume attenuator set
at 40 dB, has a transfer function of
The external components R1 and C4 can be eliminated and
pin 10(19) left open if bass boost is the only desired loud-
ness characteristic.
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