ts34118 Taiwan Semiconductor Company, Ltd. (TSC), ts34118 Datasheet - Page 14

• Manufacturer Part Number: ts34118
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: Taiwan Semiconductor Company, Ltd. (TSC)

TS34118
Switching Time
If the circuit is switching to “slow idle”, the time constant is determined by the C
(see Figure 6). With C
(for 95% change). The switching period to slow idle begins when both speakers have stopped talking. The switching
time back to the original mode will depend on how soon that speaker begins speaking again. The sooner the speaking
time starts during the 1.8 seconds period, the quicker the switching time since a smaller voltage excursion is required.
That switching time is determined by the internal current sources as described above.
The above switching times occur, however, after the level detectors have detected the appropriate signal levels, since
their outputs operate the Attenuator Control Block. Referring to Figure 4, the rise time of the level detector’s outputs to
new speech is quick by comparison (§1.0ms), determined by the internal 350 resistor and the external capacitor
(typically 2.0µF). The output’s decay time is determined by the external capacitor, and an internal 4.0µF current source
giving a decay rate of §60ms for 120mV excursion at RLO or TLO. However, the overall response time of the circuit is
not a constant since it depends on the relative strength of the signals at the different level detectors, as well as the
timing of the signals with respect to each other. The capacitors at the four outputs (RLO1, RLO2, TLO1, TLO2) must be
equal value (±10%) to prevent problems in timing and level response.
The rise time of the level detector’s outputs is not significant since it is so short. The decay time, however, provides a
significant part of the “hold time” necessary to hold the circuit during the normal pauses in speech.
The components at the inputs of the level detectors (RLI1, RLI2, TLI1,TLI2) do not affect the switching time, but rather
affect the relative signal levels required to switch the circuit, as well as the frequency response of the detectors.
Design Equations
Referring to Figure 10(the coupling capacitors have been omitted for simplicity), the following definitions will be used (all
measurements are at 1.0KHz)
I) Transmit Gain
The transmit gain, from the microphone output (V
microphone, and the desired transmit level. For example, a typical electret microphone will produce §0.35mVrms under
normal speech conditions. To achieve 100mVrms at Tip / Ring, an overall gain of 285V/V is necessary. The gain of the
transmit attenuator is fixed at 2.0 (+6.0dB), and the gain through the hybrid normally 0.4 (-8.0dB). Therefore a gain of
357V/V is required of the microphone and hybrid amplifiers. It is desirable to have the majority f that gain in the
microphone amplifier for three reasons:
1) The low level signals from the microphone should be amplifier as soon as possible to minimize signal/noise
-G
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-G
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-G
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20.1dB);
0.4V/V, or –8.0dB);
or 26.8dB);
problems.
TX
HR
FO
RX
MA
HA
HT
ST
SA
AC
is the gain of the transmit attenuator, measured from TXI to TXO;
is the side tone gain, measured from HTO-/HTO+ to the filter input (measured at 0.18V/V , or –15dB);
is the gain of the receive attenuator measured from RXI to RXO;
is the gain from Tip/Ring to the filter input for receive signals (measured at 0.833V/V or –1.6dB);
is the gain of the filter stage, measured from the input of the filter to RXI, typically 0dB at 1.0KHz;
is the acoustic coupling, measured from the speaker differential voltage to the microphone output voltage.
is the gain of the speaker amplifier, measured from RXO to the differential output of the TS34119 (typical 22V/V
is the gain of the microphone amplifier measured from the microphone output to TXI (typically 35V/V, or 31dB);
is the gain of hybrid amplifiers, measured from TXO to the HTO-/HTO+ differential output (typically 10.2V/V, or
is the gain from HTO-/HTO+ to Tip/Ring for transmit signals, and includes the balance network (measured at
T
= 5.0µF, and R
T
= 120K , the time constant is §600ms, giving a switching time of § 1.8 seconds
14-20
M
) to tip and Ring, is determined by the output characteristics of the
T
capacitor and R
2004/09 rev. B
T
, the external resistor