Low pass active filter apparatus

For use in a signal transmission system which propagates pulse amplitude modulated (PAM) signals comprising pulses having a predetermined periodicity and in which it is desired to pass signals in a predetermined frequency band below about 4 KHz, low-pass active filter apparatus comprising:

• a. first means which receives said PAM signal and produces at the output thereof the amplitude of each pulse of the PAM signals for the full period of the PAM signal rate,
• b. a four-pole, low-pass, RC active filter network connected to the output of said first means, said active filter network comprising one and another series connected second-order active filter sections, said one and another active filter sections comprising a low frequency pole pair section and a high frequency pole pair section, respectively,
• c. said one low frequency pole pair, second-order, active filter section, per se, having predetermined low-pass filter operation characteristics representable in an s-plane graphical depiction of active circuit operation as a predetermined pattern of location of a complex pole pair relative to the s-plane coordinates,
• d. said first means, per se, having a frequency response which causes undesired frequency roll-off in the desired pass band below 4 KHz, and
• e. second means operatively associated with said one low frequency pole pair, second-order, active filter section for causing the complex pole pair to be moved away from said predetermined pattern of location such that the undesired frequency roll-off characteristic due to the first means is compensated to provide desired in-band frequency characteristics throughout said predetermined frequency band.