STW5098 datasheet(25/85 Pages) STMICROELECTRONICS

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STw5098

Functional description

25/85

Filter, with DC offset removal and FIR image filtering; and a standard mono voice-channel

filter (uses left channel input and feeds both channel output). The AD path includes a digital

gain control (ADCLG, ADCRG in CR12 and CR13 respectively) in the range -57 to +8 dB.

The maximum gain from Mic Preamplifier to AD interface is then 47 dB. When Audio filter is

selected in both AD and DA paths then DA audio data can be summed to AD data and sent

to the AD Audio Interface (see DA2ADG in CR15). Left and right channels can be

independently switched on and off to save power, if needed (bits ENADCL and ENADCR in

CR1)

4.13

DA paths

In each entity the DA path converts digital data from the digital audio interface to analog

domain and feeds it to the analog mixer. Incoming audio data is processed with a DSP

where different filters are selectable (bit DAVOICE in CR29): Audio filter, stereo, with FIR

image filtering, bass and treble controls (bits BASS and TREBLE in CR14), de-emphasis

filter; and a standard voice channel filter, mono (uses left channel input and feeds both

channel output). A dynamic compression function is available for both audio and voice filters

(bit DYNC in CR14). The DA path includes a digital gain control (DACLG, DACRG in CR10

and CR11 respectively) in the range -65 to 0 dB. AD to DA mixing (sidetone) can be

enabled: see CR16 for details. Left and right channel can be independently switched on and

off to save power, if needed (bits ENDACL and ENDACR in CR1).

4.14

Analog-only operations

Each entity from the STw5098 can operate without AMCK master clock if analog-only

functions are used. It is possible to mix Microphone and Line In preamplifiers signals and

listen through headphones, loudspeaker or send them to line-out. The analog-only operation

is enabled with bit ENOSC in CR0. When ENOSC=1 the AD and DA paths cannot be used.

In Analog Mode, each of the two entities can handle two different stereo audio signals, so it

can be used as a front end for an external voice codec that does not include microphone

preamplifiers and power drivers: mic signal is sent through Microphone preamplifiers directly

to line out drivers (Transmit path), while Receive signal is sent through Line In amplifiers to

the selected power drivers.

4.15

Automatic Gain Control (AGC)

STw5098 provides a digital Automatic Gain Control in AD path for each entity. The circuit

can control the input gain at MIC preamplifier, Line In amplifier or both (bits ENAGCMIC and

ENAGCLIN in CR35). When one input is selected, the center gain value used for the input is

fixed with bits MICLG, MICRG, LINLG and LINRG in CR3 to CR6 (like in normal operation),

then the AGC circuit adds to all the gains a value in the range -10.5 dB to +10.5 dB (or,

extended with bit AGCRANGE in CR35, -21 dB to 21 dB), in order to obtain an average level

at the digital interface output in the range -6 dB to -30 dB (selected with bits AGCLEV in

CR35). The AGC added gain acts directly in the input gain, to avoid input saturation and

improve S/N ratio, so it cannot exceed the input gain range. When MIC and Line-In inputs

are selected simultaneously the control is performed on the sum of the two, preserving the

balance fixed with input gains. Different values for Attack and Decay constants can be

selected, depending on the kind of signal the AGC has to control (i.e. voice, music). The

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