MAS9191A datasheet(19/36 Pages) MAS

19 / 36 page

DA9191A.000

July 31, 1997

FUNCTION

◆ TX Audio

The TX audio block is in the power down mode after

a reset. The block is set to the operation mode with

the AUDIOP bit of register 07HEX. The same bit also

controls the RX audio block. The TX audio block

starts with switch M2, which is controlled by bits 4

and 5 in register 00HEX. Switch M2 connects one of

the following blocks as an input source: 1) After a

reset the input source is connected to signal ground.

2) In the second position the input is connected to

the microphone amplifier A6. The gain of amplifier A6

is determined with external components. The

maximum gain is 30 dB. 3) In the third position the

input is connected to EXTMIC, which is for external

accessories. 4) The fourth position selects the DTMF

generator. The DTMF generator is controlled with

registers 05HEX and 06HEX. Register 05HEX controls

the low frequencies and register 06HEX controls the

high frequencies. A detailed description of how to

use these registers is in the REGISTERS section.

The DTMF generator is in the power down mode

after a reset and can be set up with the DTMFTP bit

of register 07HEX.

The signal is then filtered with anti-aliasing filter F8.

The gain at 1kHz is 0dB and the cutoff frequency is

15kHz. After the filter the signal is amplified with

GC3. The gain is

-3dB...+3dB with 16 steps

according to bits 0..3 in register 00HEX. The amplified

signal is then fed to lowpass filter F9LP and highpass

filter F9HP, which are 4th order SC filters. During

normal operation the output of filter F9HP is

connected to output MICOUT. With switch SYS0-

SYS1 the highpass filter can be transferred to the

output of GC5 after the signal has been compressed

and pre-emphasized. Bit 6 in register 12HEX and bit 6

in register 14HEX control switch SYS0-SYS1. Connect

MICOUT

through

22nF

capacitor

the

compressor input COMPIN. The MICOUT can also

be used as a source for the side tone amplifier and

for detecting the audio level with the uncommitted Op

Amp (See APPLICATIONS section). The compressor

is an SC audio type with a 2:1 compressing ratio.

The detailed values are found in the ELECTRICAL

CHARACTERISTICS section under Compressor.

The compressor can be bypassed internally with

switch S12 or externally with S13. The bypass gain is

0dB (100 mVrms). The compressor requires one

external 22nF capacitor between pins CAMP20 and

CWCIN. Another 22nF capacitor is needed between

pins CAMP2IN and COUT. This capacitor also

serves as an external DC blocking capacitor between

the compressor output and gain control GC4 input.

2:1

DTMF

Generator

Audio Transmit Path

MIC

MICSGND

MICFB

EXTMIC

GC3

F9LP

SYS0

SYS1

SYS0

SYS1

F9HP

COMPRESSOR

S13

S12

GC4

F10

S14

PRE-EMPHASIS

GC5

SYS0

SYS1

LIMITER

S15

F11

S16

GC7

GC6

TXTST

S19

F13

GC9

F14

TX data, ST

S20

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