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Technical Description
Expandor Gain
The gain in the expandor is a function of the input signal level with the following
relationship for the expandor:
2(X0 - X1 ) = Y0 - Y1
where:
The relationship states that a change in the input from X0 to X1 will produce a
change in the output from Y0 to Y1 that equals 1:2.
The gain, A, is then:
A = Y1 - X1 = Y0 - 2X0 +X1
RX HF Attenuation
Following the expander is a 0 to -49 dB attenuator for full duplex handsfree opera-
tion. This stage is programmable in steps of 7 dB. When the external audio signal on
the system connector is low, the software implements a handsfree algorithm that
attenuates the unused audio path to prevent feedback from external speakers to the
microphone.
Under normal modes of operation this stage is set to 0 dB. However, during the gen-
eration of DTMF tones to the earpiece, this stage is used to attenuate the signal to
prevent clipping.
RX Volume Control
The RX audio level is controlled by a 0 to -24 dB gain stage in steps of 3 dB.
EARSENS Programmable Gain Stage
A programmable gain stage is provided to trim the level to the Earpiece to provide
84 dBspl with a 1 kHz tone at 2.3 kHz deviation and max. volume setting (0 dB).
This stage is programmable in 0.4 dB steps from -2.8 to 3.2 dBs.
Test Mode 28 'Earpiece' is used to trim this stage.
Externally Programmable Gain Stage
An inverting amplifier follows the EARSENSE stage. The input and feedback ele-
ments are external to allow for external programming of gain and filtering.
This stage is bypassed when the earpiece drivers are put in external mode.
28
X0 = reference input (unaffected level)
Y0 = reference output
X1 = input
Y1 = output
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