am79c30a Advanced Micro Devices, am79c30a Datasheet - Page 25

• Manufacturer Part Number: am79c30a
• Description: Digital Subscriber Controller ?dsc ?circuit
• Manufacturer: Advanced Micro Devices

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
2. The Band Pass filter and a series of decimators re-
3. The X filter is an 8-tap user-programmable filter for
4. The GX filter is a programmable gain filter that al-
5. The
0000 =
0001 = –27.0 dB
0010 = –25.5 dB
0011 = –24.0 dB
1000 = –16.5 dB
1001 = –15.0 dB
1010 = –13.5 dB
1011 = –12.0 dB
sampling rate of 512 kHz.
ject DC and 50- to 60-Hz line frequencies while re-
ducing the sampling rate to 8 kHz.
tuning the microphone. The default is flat with unity
gain.
lows the user to program a gain of 0 to +12 dB in
0.5-dB steps. The default value is 0 dB.
converts the linear output of the GX filter to - or
A-law code. The default algorithm is -law code.
The MSB (sign bit) is transferred first to (or from) the
MUX.
-law or A-law digital compression algorithm
Table 20. Analog Sidetone
0100 = –22.5 dB
0101 = –21.0 dB
0110 = –19.5 dB
0111 = –18.0 dB
1100 = –10.5 dB
1101 = –9.0 dB
1110 = –7.5 dB
1111 = –6.0 dB
Am79C30A/32A Data Sheet
Receiver
The receiver performs a series of operations described
as follows:
1. An expander converts the input A- or -law data to
2. The GR filter is a programmable gain filter that al-
3. The GER and Sidetone Gain (STG) are program-
4. The R filter is provided to correct for speaker atten-
5. A series of interpolators increases the sampling
6. A DAC converts the digital signal to the analog
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-
2. The data is presented in compressed A-law format,
3. The data extraction point for the transmit path is
4. The data extraction point for the receive path is im-
5. The compressed data from the transmit and receive
digital linear data. The most significant bit is trans-
ferred from the MUX first. The default value is µ-law.
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.
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.
uation distortion and is a user-programmable filter
similar to the X filter in the transmitter.
frequency.
audio output signal.
tors, without modification to their range or resolu-
tion.
without the alternate bit inversion. The sign bit is not
presented.
after the X filter.
mediately following the expander.
paths is presented using the same compression
algorithm.
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