Transmission of data signals over telephone lines

Analog signals representing individual digital values (+/-1, +/-3) pass through a telephone line to a receiver. These signals may be first provided in a pseudo random sequence. A linear echo canceller and a first adder eliminate, to an extent, echo signals resulting from second analog signals transmitted on the same telephone line by the receiver. A non-linear echo canceller and a second adder further reduce the echo signals and specifically reduce non-linear components in the echo signals. Adjustable signal delays achieve optimal performance of the linear and non-linear echo cancellers. An equalizer containing four (4) different modules then compensates for signal distortions introduced by the telephone line and minimizes the effect of noise in the telephone line. The equalizer modules are a digital gain control element, a feed forward digital filter and two (2) feedback digital filters. A detector module produces in one of several different ways at the receiver an estimate of the digital data (+/-1, +/-3) transmitted at the other end of the telephone line. The detector either extracts the digital information based on peaks in the received (non-equalized) signal or by adding the equalized signals with preset threshold values. A scrambler-descrambler module locally generates a replica of the digital symbols transmitted in analog form at the other end of the telephone line, based on a limited number (e.g. 23) of correctly detected digital values. The scrambler-descrambler module may also operate as a descrambler to recover data scrambled by the transmitter at the other end.