LMC1983 Datasheet, PDF (11/14 Pages) National Semiconductor (TI) – Digitally-Controlled Stereo Tone and Volume Circuit with Three Selectable Stereo Inputs

Application Information (Continued)
The LMC1983’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 11(18) left open if bass boost is the only desired loudness
characteristic.
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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, 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
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.
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 LMC1983’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”).
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FIGURE 5. Loudness Control Circuit
SERIAL DATA COMMUNICATION
The LMC1983 uses the INTERMETAL serial bus (IM Bus)
standard. Serial data information is sent to the LMC1983
over a three wire IM Bus consisting of Clock (CLK), Data
(DATA), and Identity (ID). The DATA line is bidirectional and
the CLK and ID lines are unidirectional from the micropro-
cessor or micontroller to the LMC1983. The LMC1983’s bidi-
rectional capability is accomplished by using an open drain
output on the DATA line and an external 1 kΩ pull-up resistor.
The LMC1983 responds to address values from 01000000
(40_H) through 01000111 (47_H). The addresses select one of
the eight available functions (see Table 1). The IM Bus’ lines
have a logic high standby state when using TTL logic levels.
As shown in Figure 6, data transmission is initiated by low
levels on CLK and ID. Next, eight address bits are sent. This
address information includes the code to select one of the
LMC1983’s desired functions. Each address bit is clocked in
on the rising edge of CLK. The ID line is taken high after the
eight bits of address data are received by the LMC1983.
FIGURE 6. LMC1983’s INTERMETAL Serial Bus Timing
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The controlling system continues toggling the CLK line eight
more times. Data that determines the selected function’s op-
erating point is written into, or single bit information on DIGI-
TAL INPUT 1 or DIGITAL INPUT 2 is read from, the
LMC1983. Finally, the end of transmission is signaled by
pulsing the ID line low for a minimum of 3 µs. The transmit-
ted function data is latched and the function changes to its
new setting.
Table 1 also details the serial data structure, range, and bit
assignments that sets each function’s operating point. The
volume and tone controls’ function control data binarily incre-
ments from zero to maximum as the function’s operating
point changes from 80 dB attenuation to 0 dB attenuation
(volume) or −12 dB to +12 dB (tone controls). Note that not
all data bits are needed by each function. The extra bits
shown as “X”s (“don’t cares”) are position holders and have
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