High speed modem suitable for operating with a switched network

A low baud rate, high bit rate modem adapted for efficient information transfer through a communication medium with good tolerance to noise bursts and effective to time the demodulation process to compensate for phase distortion, frequency offset and amplitude distortion comprising a modulator and demodulator, said modulator comprising:

• a sub-carrier generator for generating representations of a plurality of information and timing sub-carriers,
• first means responsive to digital information signals for periodically modulating each of said information sub-carriers once per baud time,
• means for accumulating a representation of each of said sub-carriers and transmitting a sum of said representation a fixed plurality of times within each said baud time,
• said demodulator comprising:
• a timing unit,
• A/D converting means responsive to a received signal for periodically producing a digital representation thereof when initiated by said timing unit,
• a second sub-carrier generator for generating representations of a second plurality of information and timing sub-carriers, each nominally frequency coherent with a different sub-carrier of said first plurality,
• correlating means for correlating said first and second plurality of sub-carriers,
• decision means responsive to said correlating means with respect to said information sub-carriers for recreating digital signals representative of said modulating digital information signals and for compensating for phase and amplitude distortion and responsive to said correlating means with respect to said timing sub-carriers for compensating for frequency offset and for inhibiting said timing unit to enable said A/D converting means and said correlating means only for a period T.sub.O, less than said baud time, wherein said period T.sub.O is a period of a fundamental f.sub.0 of each of said sub-carriers.

A method of data transmission suitable for the transmission of digital data over a switched network and for reducing the effects of distortion, comprising the transmitting steps of:

• (a) generating within a selected time interval T.sub.b at least plural samples of each of a plurality of sub-carriers, each of said sub-carriers being harmonics of a fundamental f.sub.0,
• (b) modifying samples of different ones of said sub-carriers in accordance with intelligence to be transmitted at a rate no higher than once within said selected time interval T.sub.b, wherein said fundamental period T.sub.O is less than T.sub.b,
• (c) summing a sample from each of said sub-carriers to produce a sum,
• (d) transmitting a signal representative of said sum,
• (e) repeating steps (c) and (d) a number of times within said selected time interval T.sub.b,
• and the receiving steps of:
• (f) sampling a received signal,
• (g) correlating samples of said received signal with samples of a plurality of locally generated sub-carriers throughout the interval T.sub.O less than T.sub.b,
• (h) detecting said intelligence by the result of said correlation, and
• (i) adjusting frequency and absolute phase of said locally generated sub-carriers to achieve coherency between transmitter sub-carriers and receiver sub-carriers in response to said correlation.