TS34118CS28C8 TSC [Taiwan Semiconductor Company, Ltd], TS34118CS28C8 Datasheet - Page 17

• Manufacturer Part Number: TS34118CS28C8
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: TSC [Taiwan Semiconductor Company, Ltd]

Design Equations - Continue
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 MRX 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 MTX line represents the transmit to receive
threshold, The MI line represents the idle condition, and defines the threshold level on one side (transmit or receive)
necessary to overcome noise on the other. Some comments on the above graph:
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 in Equation 13 yields a GAC-MAX of –
37dB. Experience has shown, however, that an acoustic coupling loss of >40dB is desirable.
Using the component values in Equation 14 yields a maximum side tone of 0dB. Experience has shown, however, that
a minimum of 6.0dB 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 R4/R2 ratio. In Figure 10, R1-R4 each
represent the combined impedance of the resistor and coupling capacitor at each level detector input. The magnitude
of each RC’s impedance should be kept within the range of 2.0K-15KΩ 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.
- Acoustic coupling and side tone coupling were not included in Equations 7 and 12. Those couplings will affect the
- The MRX line helps define the maximum acoustic coupling allowed in a system, which can be found from the
- The MTX line helps define the maximum side tone coupling (GST) allowed in the system, which can be found from
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 following equation:
following equation:
G
ST
G
=
AC-MAX
2
×
=
R2
R4
2
×
×
G
R3
FO
R1
×
G
MA
Figure 11. Switching Threshold
(Equation 13)
(Equation 14)
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Voice Switched Speakerphone Circuit
TS34118
Version: A08