AM79C30AVC datasheet(25/101 Pages) AMD

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Am79C30A/32A Data Sheet

Programmable Analog Preamplifier

A programmable analog preamplifier GA is included in

front of the A/D converter and is adjustable in 6-dB in-

crements from 0 dB to +24 dB. The existing GX gain

stage in the transmit path may be used for finer adjust-

ment of transmit gain. This preamplifier eliminates the

need for an external operational amplifier when inter-

facing electret-type handsets to the DSC circuit.

Analog Sidetone

Analog sidetone takes the analog input to the transmit-

ter ADC and sums it into the single-ended input of the

EAR output buffer. The summing point is after the out-

put selection switch. The analog sidetone path has pro-

grammable attenuation between –6 and –27 dB, plus

infinity (off). Default is infinity. Programming is via four

bits in the Extended FIFO Control Register, EFCR.6–3.

The programming values are given in Table 20.

Signal Processing

Transmitter

The transmitter performs a series of operations as de-

scribed below.

1. An ADC converts the incoming analog signal at a

sampling rate of 512 kHz.

2. The Band Pass filter and a series of decimators re-

ject DC and 50- to 60-Hz line frequencies while re-

ducing the sampling rate to 8 kHz.

3. The X filter is an 8-tap user-programmable filter for

tuning the microphone. The default is flat with unity

gain.

4. The GX filter is a programmable gain filter that al-

lows the user to program a gain of 0 to +12 dB in

0.5-dB steps. The default value is 0 dB.

5. The

µ-law or A-law digital compression algorithm

converts the linear output of the GX filter to

µ- or

A-law code. The default algorithm is

µ-law code.

The MSB (sign bit) is transferred first to (or from) the

MUX.

Receiver

The receiver performs a series of operations described

as follows:

1. An expander converts the input A- or

µ-law data to

digital linear data. The most significant bit is trans-

ferred from the MUX first. The default value is µ-law.

2. The GR filter is a programmable gain filter that al-

lows the user to program a gain of –12 to 0 dB in

0.5-dB steps. The default value of GR is 0 dB.

3. The GER and Sidetone Gain (STG) are program-

mable constant multipliers which allow the user to

program a gain of –10 to +18 dB in 0.5-dB steps

(default value 0 dB) and –18 to 0 dB in 0.5-dB steps

(default value –18 dB) respectively. The GER pro-

vides volume control (for the hearing impaired) and

should be programmed to 0 dB for normal opera-

tion. The sidetone gain path provides feedback from

the transmitter.

4. The R filter is provided to correct for speaker atten-

uation distortion and is a user-programmable filter

similar to the X filter in the transmitter.

5. A series of interpolators increases the sampling

frequency.

6. A DAC converts the digital signal to the analog

audio output signal.

PEAK Hold Registers

Logic in the form of two microprocessor accessible

peak hold registers will be provided to allow for support

of a software based speaker phone solution. These

registers, one in the transmit path (PEAKX) and one in

the receive path (PEAKR), will provide the compressed

maximum (peak) absolute value of the data in the path

since the register was last read. With appropriate soft-

ware, this can be used to implement a hands-free func-

tion. Refer to the MAP block diagram for the location of

these registers in the processing path.

The following assumptions are made:

1. The GX and GR blocks are used as gain/attenua-

tors, without modification to their range or resolu-

tion.

2. The data is presented in compressed A-law format,

without the alternate bit inversion. The sign bit is not

presented.

3. The data extraction point for the transmit path is

after the X filter.

4. The data extraction point for the receive path is im-

mediately following the expander.

5. The compressed data from the transmit and receive

paths is presented using the same compression

algorithm.

Table 20.

Analog Sidetone

0000 =

∞

0100 = –22.5 dB

0001 = –27.0 dB

0101 = –21.0 dB

0010 = –25.5 dB

0110 = –19.5 dB

0011 = –24.0 dB

0111 = –18.0 dB

1000 = –16.5 dB

1100 = –10.5 dB

1001 = –15.0 dB

1101 = –9.0 dB

1010 = –13.5 dB

1110 = –7.5 dB

1011 = –12.0 dB

1111 = –6.0 dB

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