U4082B Datasheet, PDF (27/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

U4082B
Equation 13 is independent of the volume control setting. Conversely, the acoustic coupling of a
designed system helps determine the minimum slope of that line. Using the component values
of Figure 1-2 on page 2 in equation 13 yields a G_AC(MAX) of –37 dB. Experience has shown, how-
ever, that an acoustic coupling loss of 40 dB is desirable.
• The M_T line helps define the maximum side tone coupling (G_ST) allowed in the system. G_ST
can be found using the following equation:
G_ST
=
---------------R-----⁴---------------
2 × R₂ × G_FO
(14)
Using the component values of Figure 1-2 on page 2 in equation 14 yields a maximum side tone
of 0 dB. Experience has shown, however, that a minimum of 6.0 dB loss is preferable.
The above equations can be used to determine the resistor values for the level detector inputs.
Equation 6 can be used to determine the R₁-R₃ ratio, and equation 10 can be used to determine
the R₁-R₂ ratio. In Figure 8-3 on page 23, R₁-R₄ each represent the combined impedance of the
resistor and coupling capacitor at each level detector input. The magnitude of each RC's imped-
ance should be kept within the range of 2.0 kΩ to 15 kΩ in the voice band (due to the typical
signal levels present) to obtain the best performance from the level detectors. The specific R
and C at each location will determine the frequency response of that level detector.
9. Application Information
9.1 Dial Tone Detector
The threshold for the dial tone detector is internally set at 15 mV (10 mVrms) below V_B (see Fig-
ure 3-3 on page 7). That threshold can be reduced by connecting a resistor from RI to ground.
The resistor value is calculated from:
R = 10 k -V----^B-- – 1
∆V
where V_B is the voltage at pin 15, and ∆V is the amount of threshold reduction. By connecting a
resistor from V_S to RI, the threshold can be increased. The resistor value is calculated from:
R = 10 k
-V----^S----^–-----V----^B--
∆V
–
1
where ∆V is the amount of the threshold increase.
9.2 Background Noise Monitors
For testing or circuit analysis purposes, the transmit or receive attenuators can be set to the “on”
position by disabling the background noise monitors and applying a signal so as to activate the
level detectors. Grounding the CPR pin will disable the receive background noise monitor,
thereby indicating the “presence of speech” to the attenuator control block. Grounding CPT does
the same for the transmit path.
Additionally, the receive background noise monitor is automatically disabled by the dial tone
detector whenever the receive signal exceeds the detector's threshold.
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