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”).

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.

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 k

Ω pull-up resistor.

The LMC1983 responds to address values from 01000000

(40

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.

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

DS011279-6

FIGURE 4. The Tone Control Amplifier

DS011279-7

FIGURE 5. Loudness Control Circuit

DS011279-8

FIGURE 6. LMC1983’s INTERMETAL Serial Bus Timing

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