TLH10789 – 19

FIGURE 3 System Block Diagram Showing Inclusion of DNR

Noise

Reduction (LM1894) and Equalizer (LMC835) (One Channel OnlyLMC1992)

The SELECT OUT pins (8 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 mulit-band graph-

ic equalizers (LMC835) It is important to ensure that if both

are used the noise reduction circuitry precede the equaliza-

tion circuits Failure to do so will result in improper operation

of the noise reduction circuits The system shown in

Figure

3 utilizes the external loop to include DNR and a multi-band

equalizer

AUDIO MUTE

A mute function with attenuation of 100 dB is possible with

code set to 01000XX0000 (open circuit)

TONE CONTROL RESPONSE

Base and treble tone controls are included in the LMC1992

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

(Figure 4) and internal resistors in

the feedback loop of the internal tone amplifier The maxi-

mum amplitude boost or cut is determined by the data sent

to the LMC1992 (see Table I)

The typical tone control response shown in the Typical Per-

00082 mF 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

‘‘flat’’ the frequency response will be flat at 20 Hz and 20

The frequency where a tone control begins to deviate from

a flat response will be referred to as the turn-over frequen-

frequency is nominally

1

The base turn-over frequency is nominally

1

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

1

1

8