The telephone network uses DC current to determine the hook state of the telephone, so line current must be
present during a call. The silicon DAA provides a DC holding circuit to keep the line current constant during a
fax call.
The silicon DAA converts the analog signal to a digital signal for DSP processing, and also converts the digital
signal to an analog signal for transmitting data through a telephone line.
The magnetically coupled signals that cross the isolation barrier go either through a transformer or a relay.
The DSP in the fax card communicates with the ASIC in the formatter using the high-speed serial interface.
Hook state
Another magnetically coupled signal is the control signal that disconnects the downstream telephone
devices (such as a phone or answering machine). A control signal originating on the DSP can change the relay
state, causing the auxiliary jack (downstream jack) to be disconnected from the telephone circuit.
The product takes control of calls that it recognizes as fax calls. If the product does not directly pick up the
call, it monitors incoming calls for the fax tone or for the user to direct it to receive a fax. This idle mode is
also called eavesdropping. This mode is active when the product is on-hook but current exists in the
downstream phone line because another device is off-hook. During eavesdropping, the receive circuit is
enabled but has a different gain from the current that is generated during normal fax transmissions.
The product does not take control of the line unless it detects a fax tone or the user causes it to connect
manually. This feature allows the user to make voice calls from a phone that is connected to the product
without being cut off if a fax is not being received.
Downstream current detection
The line voltage monitoring module of the silicon DAA can detect the line state as well as the downstream
device. It tells DSP via DIB that an active device (telephone, modem, or answering machine) is connected to
the auxiliary port on the product (the right side of the RJ-11 jack). The DSP uses the signal to ensure that the
product does not go off-hook (and disconnect a downstream call) until it has been authorized to do so (by a
manual fax start or the detection of the appropriate tones).
Hook switch control
In the silicon DAA the CODEC controls the hook switch directly. The CODEC is activated when it receives
commands from the DSP. When the circuit is drawing DC current from the central office it is considered off-
hook. When no DC current flows the state is considered on-hook.
Ring detect
Ring detect is performed by the line voltage monitoring module of the silicon DAA, and is a combination of
voltage levels and cadence (time on and time off). Both must be present to detect a valid ring. The CODEC
works with DSP as well as the firmware to determine if an incoming signal is an answerable ring.
Line current control
The DC current from the CO needs to have a path to flow from TIP to RING. The DC impedance emulation line
modulator and DC terminations modules in the silicon DAA act as a DC holding circuit, and works with the
firmware to achieve the voltage-current characteristic between TIP and RING. The impedance (the current-
voltage characteristic) changes corresponding to certain special events, such as pulse dialing or when the
product goes on-hook.
36 Chapter 1 Theory of operation ENWW
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