TS34118

14-20

2004/09 rev. B

Switching Time

(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 350Ωresistor 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)

20.1dB);

0.4V/V, or –8.0dB);

or 26.8dB);

I) Transmit Gain

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

problems.