am79c30a Advanced Micro Devices, am79c30a Datasheet - Page 30

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

Note:
The coefficient 9008 provides an attenuation of infinity when
GR, GX, and/or STG are enabled.
30
Gain (dB)
–18.0
–17.5
–17.0
–16.5
–16.0
–15.5
–15.0
–14.5
–14.0
–13.5
–13.0
–12.5
–12.0
–11.5
–11.0
–10.5
–10.0
–9.5
–9.0
–8.5
–8.0
–7.5
–7.0
–6.5
–6.0
–5.5
–5.0
–4.5
–4.0
–3.5
–3.0
–2.5
–2.0
–1.5
–1.0
–0.5
0.0
Table 27. STG Gain Coefficients
MSB
AA
BB
CB
8B
8B
8B
8B
8B
8B
9F
9C
9B
9A
9A
A2
A2
A6
B2
91
91
91
91
91
91
91
91
91
92
91
92
92
92
92
93
93
94
08
Hex Code
Am79C30A/32A Data Sheet
LSB
AC
7C
2E
2A
3B
4B
C5
B6
A4
FB
AA
B3
B3
EA
4A
8D
44
35
2A
24
22
23
32
F9
12
22
32
27
91
F9
22
A2
A3
42
52
B2
08
Overflow/Underflow Precautions When Using
Programmable Gains
Care must be taken so that at any point in the signal
processing path, the combination of gains and filters
and/or tones does not result in a signal that is larger
than full scale. Full scale is defined as the digital repre-
sentation of the maximum analog signal that is allowed
into the transmitter or out of the receiver with all filters
and gain stages at their default (0 dB) settings (e.g., in
A-Law, the transmitter full scale is ±1.25 V
ceiver full scale is ±2.5 V
the peak signal be kept as close to full scale as possible
at any point in the signal processing path in order to
minimize digital truncation effects in the A/D, D/A, and
MAP DSP.
Consider the following example: STG is programmed
for infinite attenuation, GR is programmed to –6 dB
while GER is programmed to +12 dB, and the R filter is
programmed to exhibit a net gain of –6 dB. Assume the
analog full scale out of the receiver is ± 2.5 V
full scale PCM code is possible from the MUX. After
GR, the equivalent analog signal will be 2.5 / 2 = ±1.25
V
5 V
dB, the signal will be clipped after GER and distorted
for PCM codes between full scale and 6 dB below full
scale due to the intermediate result at the output of
GER.
Be very careful when programming the tone ring-
ers/generators. For example, if one of the DTMF tones
is programmed to 0 dB, a tone is generated that is
equivalent to a ± full scale signal in the transmit path.
This means no headroom is left for the other DTMF
tone. Therefore, the DTMF generator should never be
programmed to exceed full scale if signal quality is to
be maintained. In the receive path, similar caution
should be exercised in order to prevent the combination
of Tone Generator, Sidetone, GR, and GER from clip-
ping the signal.
Extended Programming Ranges
Some applications of the DSC will require greater flex-
ibility in the programming of the MAP’s internal gain
and attenuation blocks. For example, applications such
as software-based hands-free utilizing the PEAKX and
PEAKR registers may need attenuation as well as gain
within the MAP transmit path. The preceding gain ta-
bles do not specifically detail this capability, but due to
the DSP implementation of these gain and filter blocks,
the DSC is capable of performance beyond these rec-
ommended ranges. (GA and ASTG are not imple-
mented in DSP and are limited to their stated range and
step size.) Table 28 lists guaranteed ranges, while
Table 29 shows the limits by design.
P
. However, after GER the signal will be 1.25
P
. Even though the R filter will have a net gain of –6
P
). Likewise, it is desirable that
P
and the re-
P
, and a
4, or +