U4091BM-N Datasheet, PDF (20/38 Pages) ATMEL Corporation – PROGRAMMABLE TELEPHONE AUDIO PROCESSOR

Term Definitions
1. Transmit means the transmit attenuator is fully on, and the receive attenuator is
at maximum attenuation.
2. Receive means the receive attenuator is fully on, and the transmit attenuator is at
maximum attenuation.
3. In Idle mode, the transmit and receive attenuator are at half of their maximum
attenuation.
– Change mode means both the transmit and receive speech are present in
approximately equal levels. The attenuators are quickly switched (30 ms) to
the opposite mode until one speech level dominates the other.
– Idle means speech has ceased in both transmit and receive paths. The
attenuators are then slowly switched (1.5 s) to idle mode.
4. Switching to full transmit or receive modes from the idle mode is done at a fast
rate (30 ms).
Summary of Truth Table
1. The circuit will switch to transmit mode if:
– Both transmit level detectors sense higher signal levels than the respective
receive level detectors and
– The transmit background-noise monitor indicates the presence of speech
2. The circuit will switch to receive mode if:
– Both receive level detectors sense higher signal levels than the respective
transmit level detectors, and
– The receive background-noise monitor indicates the presence of speech
3. The circuit will switch to the reverse mode if:
– the level detectors disagree on the relative strengths of the signal levels, and
at least one of the background-noise monitors indicates speech.
4. The circuit will switch to idle mode when:
– Both speakers are quiet (no speech present), or
– When one speaker speech level is continuously overridden by noise at the
other speaker's location
The time required to switch the circuit between transmit, receive and idle is determined
by internal current sources and the capacitor at Pin CT. A diagram of the CT circuitry is
shown in Figure 14 on page 21. It operates as follows:
• CCT is typically 4.7 µF.
• To switch to transmit mode, ITX is turned on (IRX is off), charging the external
capacitor to -240 mV below VM. (An internal clamp prevents further charging of the
capacitor.)
• To switch to receive mode, IRX is turned on (ITX is off), increasing the voltage on
the capacitor to +240 mV with respect to VM.
• To switch to reverse mode, the current sources ITX, IRX are turned off, and the
current source IFI is switched on, discharging the capacitor to VM.
• To switch to idle mode, the current sources ITX, IRX, IFI are turned off, and the
current source ISI is charging the capacitor to VM.
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