U4082B Datasheet, PDF (26/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

This equation defines the line voltage at Tip/Ring necessary to switch comparator C2 in the
presence of a microphone voltage. The highest V_L occurs when the circuit is in transmit mode
(G_T = +6.0 dB). Using the typical numbers for equation 10 yields:
V_L = 840 × V_M (or V_M = 0.0019 × V_L) (11)
At idle, where the gain of the two attenuators is –20 dB (0.1V/V), equations 6 and 10 yield the
same result:
V_M = 0.024 × V_L
(12)
Equations 7, 11, and 12 define the thresholds for switching, and are represented in Figure 8-4
The "M" terms are the slopes of the lines (0.52, 0.024, and 0.0019) which are the coefficients of
the three equations. The M_R line represents the receive to transmit threshold, in that it defines
the microphone signal level necessary to switch to transmit in the presence of a given receive
signal level. The M_T line represents the transmit to receive threshold. The M_I line represents the
idle condition, and defines the threshold level on one side (transmit or receive) necessary to
overcome noise on the other.
Figure 8-4. Switching Thresholds
V_M
M_R
M_I
M_T
V_L
Some comments on the graph (see Figure 8-4):
• Acoustic coupling and side tone coupling were not included in equations 7 and 12. Those
couplings will affect the actual performance of the final speakerphone due to their interaction
with speech at the microphone and the receive signal coming in at Tip/Ring. The effects of
those couplings are difficult to predict due to their associated phase shifts and frequency
response. In some cases the coupling signal will add, and other times subtract from the
incoming signal. The physical design of the speakerphone enclosure, as well as the specific
phone line to which it is connected, will affect the acoustic and side tone couplings,
respectively.
• The M_R line helps define the maximum acoustic coupling allowed in a system, which can be
found from the following equation:
G_AC(MAX)
=
---------------R-----¹----------------
2 × R₃ × G_MA
(13)
26 U4082B
4743D–CORD–03/06