AM79C30AVC datasheet(30/101 Pages) AMD

30 / 101 page

Am79C30A/32A Data Sheet

Note:

The coefficient 9008 provides an attenuation of infinity when

GR, GX, and/or STG are enabled.

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 VP and the re-

ceiver full scale is ±2.5 VP). Likewise, it is desirable that

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 VP, and a

full scale PCM code is possible from the MUX. After

GR, the equivalent analog signal will be 2.5 / 2 = ±1.25

VP. However, after GER the signal will be 1.25 × 4, or +

5 VP. Even though the R filter will have a net gain of –6

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.

Table 27.

STG Gain Coefficients

Gain (dB)

Hex Code

MSB

LSB

–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

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