LMC1983 Datasheet, PDF (10/14 Pages) National Semiconductor (TI) – Digitally-Controlled Stereo Tone and Volume Circuit with Three Selectable Stereo Inputs

Application Information (Continued)
EXTERNAL SIGNAL PROCESSING
The SELECT OUT pins (7 and 22) enable greater system
design flexibility by providing a means to implement an ex-
ternal processing loop. This loop can be used for noise re-
duction circuits such as DNR (LM1894) or multi-band
graphic equalizers (LMC835). If both are used, it is important
to ensure that the noise reduction circuitry precede the
equalization circuits. Failure to do so results in improper op-
eration of the noise reduction circuits. The system shown in
Figure 3 utilizes the external loop to include DNR and a
multi-band equalizer.
TONE CONTROL RESPONSE
Bass and treble tone controls are included in the LMC1983.
The tone controls use just two external capacitors for each
stereo channel. Each has a corner frequency determined by
the value of C2 and C3 (see Figure 4 ) and internal resistors
in the feedback loop of the internal tone amplifier. The
maximum-boost or cut is determined by the data sent to the
LMC1983 (see Table 1).
The typical tone control response shown in Typical Perfor-
mance Curves were generated with C2 = C3 = 0.0082 µF
and show the response for each step. When modifying the
tone control response it is important to note that the ratio of
C3 and C2 sets the mid-frequency gain. Symmetrical tone
response is achieved when C2 = C3. However, with
C2 = 2(C3) and the tone controls set to “flat”, the frequency
response will be flat at 20 Hz and 20 kHz, and +6 dB at
1 kHz.
The frequency where a tone control begins to deviate from a
flat response is referred to as the turn-over frequency. With
C = C2 = C3, the LMC1983’s treble turn-over frequency is
nominally
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
DS011279-5
FIGURE 3. System Block Diagram Utilizing the External Processing Loop (One Channel Shown)
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