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)

www.national.com

10