Interpolator apparatus for increasing the word rate of a digital signal of the type employed in digital telephone systems

An interpolator apparatus for increasing the word rate of an input digital signal, said input digital signal consisting of a plurality of digital words, each having the same number of given bits, with said word rate of said digital signal being of a predetermined value, comprising:

• interface means responsive to said digital signal at said increased word rate, wherein each word is presented in a pattern in said output signal indicative of said input word followed by a given number of zero words, each having the same number of bits as said input word, with said last zero bit word again followed by said input word followed by said given number of zero bit words, with said pattern repeating a given number of times to provide said output signal at said increased rate, whereby for a given input word X in said digital signal, said output signal is of the form:
• X.sub.o,0.sub.o . . . 0.sub.no,X.sub.1 0.sub.1 . . . 0.sub.n1 . . . X.sub.L 0.sub.L1 . . . 0.sub.nL
• where:
• X.sub.o =X.sub.1 =X.sub.L =said input word X of said given number of bits
• 0.sub.o =0.sub.l =0.sub.L1 =first zero bit word of said given number of bits after a respective input word X.sub.N
• .sub. no =0.sub.nl =0.sub.nL =last zero bit word of said given number of bits after a respective input word X.sub.N
• where:
• 0.sub.o =0.sub.no =0.sub.l =0.sub.Ll =0.sub.nl =0.sub.nL,
• low pass filter digital recursive filter means responsive to said output digital signal to provide at an output, a filtered digital signal manifesting a plurality of digital words at said increased word rate, each of a lesser number of bits than said words of said output signal, with each word manifesting a value providing a smooth transition between successive words at said increased word rate.

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,
• 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, said interpolating means comprising:
• interface means responsive to said received digital signal for repeating each of said words a given number of times at said increased word rate, with a given separation region between said words as repeated, said interface means including means for inserting a fixed number of zero bit words between said words as repeated, wherein said fixed number of zero bit words occupy said given region, with each of said zero bit words having the same number of bits as each digital word, to thereby provide an output digital signal of a predetermined pattern,
• low pass digital recursive filter means responsive to said output digital signal from said interface means to provide at an output of said filter, a filtered digital signal manifesting a plurality of digital words, each of said lesser number of bits, with each word manifesting a value providing a smooth transition between successive words at said increased rate,
• digital to analog converter means in said receive path of said line circuit and coupled to said recursive filter and responsive to said output to provide an analog signal, manifesting said far end analog signal,
• means for coupling said far end analog signal to said two to four wire converter means to enable a subscriber to receive and therefore respond to said far end analog signals.