Telephone subscriber line unit with sigma-delta digital to analog converter

A telephone subscriber line circuit for coupling a subscriber to a digital transmission path in a telephone switching system comprising in combination:

• two-to-four wire converter means coupled to a near-end subscriber line to provide a transmit path and a receive path for said line,
• analog-to-digital converter means for converting near-end analog signals coupled thereto from said two-to-four wire converter in said transmit path, said converter means operative to provide a digital signal comprising a plurality of digital words, each having the same number of bits with each word manifesting a weighted value of said near-end analog signal, with at least one bit in each word indicative of a sign bit indicating whether said word and therefore said weighted value of said analog signal is positive or negative,
• means for applying said digital signal to said digital transmission path,
• means coupled to said digital transmission path adapted to receive a far end digital signal of the same format as said applied digital signal,
• interpolator means responsive to said digital signal as received for providing a second digital signal having a lesser number of bits than said far digital signal and an increased word rate,
• digital to analog converter means in said receive path of said line circuit and coupled to said interpolating means to provide an analog signal, said converter means comprising:
• adding means for receiving said second digital signal and adding to each word of said signal an error signal, to provide at an output, a modified word having a given number of bits including a sign bit manifesting the sum of a word in said second digital signal as modified by said error signal,
• sign detecting means responsive to said modified word for providing an output pulse when the sign of said word is indicative of one value as positive or negative, said output pulse indicative of the value of said analog signal as being positive or negative,
• feedback control means responsive to the sign of said modified word as detected for inverting said sign and adding the same to said modified word to provide said error signal for application to said adding means to cause each successive digital word as applied to said adding means to be modified by said error signal as based on the sign of said previous modified word, to cause said sign detecting means to provide said output pulse for each word in said second digital signal having the same sign bit,
• low pass filter means coupled to said sign detecting means for providing a replica of said analog signal as derived from said far end digital signal,
• means for coupling said far end analog signal to said two or four wire converter means to enable a subscriber to receive and therefore respond to said far end analog signals.