A dynamically adjustable digital gyrator device for a telephone line interface
(e.g., a DAA) utilizing extended feedback and a dynamically adjustable filter to
achieve stable convergence in the control of DC line current on a telephone line
a short period of time. The low pass filter of the digital gyrator includes at
least two (2) different fast filter settings and at least two (2) different slow
filter settings based on possible load terminations (e.g., PBX, TAS, etc.), which
can be determined based on the dynamic detection of an oscillation on the line.
For instance, in a disclosed example, a voice band modem system is allowed to go
off-hook with a 1 Hz cutoff low pass filter, and then the output of the low pass
filter is checked for an oscillation. If an oscillation is detected, then the poles
and/or zeroes of the low pass filter cutoff are changed to a slower convergence
rate cutoff frequency (e.g., to 0.1 Hz). As a result, the exemplary voice band
modem will converge stably and quickly (e.g., in less than 1 second) before dialing
begins. By controlling the DC line current digitally, software can be used to set
the DC line current in accordance with predefined characteristics. Predefined characteristics
can be set to accommodate varying country specifications instead of using switches
to control resistors and capacitors.
