Title: Synchronizing circuit for modems in a data communications network

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Claims: What we claim as new and desire to secure by Letters Patent of the United States is:

1. A modem for connection between a digital unit and a transmission link, said modem comprising:

A. a transmitter for converting received digital signals from the digital unit into a modulated signal including

i. means for periodically transmitting a framing signal, thereby to establish a transmitted time reference,

ii. means for converting the digital signals to coordinate signals having a first set of values and a second set of values,

iii. control means for causing said converting means normally to transmit coordinate signals with the first set of values and, in response to a framing signal, to transmit coordinate signals with the second set of values, and

iv. modulator means responsive to the coordinate signals from said converting means for transmitting modulated signals onto the transmission link, and

B. receiver means including

i. means responsive to received modulated signals on the transmission link for producing received coordinate signals corresponding to the transmitted coordinate signals,

ii. means for comparing the received coordinate signals and predetermined signal levels corresponding to boundaries of first and second sets of decision regions, each of the first regions corresponding to a digital value and having one of the first set of values and each of the second regions corresponding to the second set of coordinate values and having a digital value which duplicates the digital value in one of the first decision regions, said comparison means transmitting a digital value corresponding to the selected decision region, and, when a second of the decision regions is selected, a received framing signal,

iii. synchronization detector means responsive to each received framing signal for establishing a received time reference corresponding to the transmitted time reference, thereby to detect synchronism between the transmitter and receiver operations during a transfer of data, and

iv. means for transmitting to the digital unit digital signals in response to the digital value from said comparison means.

2. A modem as recited in claim 1 wherein the number of coordinate signal states in the second set is less than the number in the first set, said control means in said transmitter including means responsive to the periodic framing signal and to the values of the first set of coordinate signals that have corresponding values in the second set of coordinate signals for effecting the transmission of the coordinate signals with the second set of values.

3. A modem as recited in claim 2 wherein the difference between the first and second sets of values is an amplitude difference and said control means includes means for causing said converting means to transmit signals with a second set of amplitude values.

4. A modem as recited in claim 2 wherein said transmitter includes a pseudo-random number generator with a recurrent counter for scrambling incoming digital signals for transmission to said conversion means, said framing signal transmitter including means for detecting a predetermined state of said pseudo-random number generator to transmit the framing signal.

5. A modem as recited in claim 4 wherein

A. said decoder includes

i. a pseudo-random number generator for receiving the digital values, said pseudo-random number generator including a first cyclic counter, and

B. said framing signal responsive means in said receiver includes:

i. a second cyclic counter with an interval corresponding to the interval between successive transmitted framing signals,

ii. means responsive to the received framing signal for monitoring said second cyclic counter,

iii. means responsive to a first state of said second cyclic counter during a monitoring operation for setting said first counter in said pseudo-random number generator to a predetermined state, and

iv. means responsive to other states of said second cyclic counter during a monitoring operation for resetting said second cyclic counter to a predetermined state.

6. A modem as recited in claim 2 wherein said conversion means in said transmitter includes digital-to-analog conversion means including means responsive to the periodic framing signal, and said comparison means in said receiver includes analog-to-digital conversion means and a gating array for producing the digital values in response to digital signals from said analog-to-digital converter.

7. A modem as recited in claim 6 additionally comprising equalization means between said analog-to-digital converter and said array in said transmitter comparison means for compensating the digital signals from said converter means.

8. A modem as recited in claim 6 wherein said modulator means includes means for transmitting a quadrature carrier amplitude modulated signal and said receiver means decodes a quadrature carrier amplitude modulated signal.

9. A modem as recited in claim 6 wherein said transmitter conversion means produced coordinate signals which define a predetermined plurality of transmission points, each of said transmission points and decision region boundaries being defined in a Cartesian coordinate system, said boundaries additionally being rectilinear to define rectangular and nonrectangular decision regions.

10. A modem as recited in claim 9 wherein said digital-to-analog conversion means in said transmitter produces analog coordinate signals with relative unit magnitudes of 0, 1, 2.707, 3 and 5 and the predetermined signal levels corresponding to boundaries in said receiver comparison means have relative unit magnitudes of 0, 1.25, 2, 3 and 4.

Other info:

Inventors: Lyon, David L. (Cambridge, MA, US)
Holsinger, Jerry L. (Burlington, MA, US)
Williams, David E. (Andover, MA, US)

Application Number: 515877
Filing Date: 1974-10-18
Publication_date: 1976-05-04
Assignee: Intertel, Inc. (Burlington, MA)
Primary Class(es): 375/261 375/222, 375/269, 375/362
Other Classes:
US Patent Ref:

3303284	Feb, 1967	Lender	178/67.
3619503	Nov, 1971	Ragsdale	178/67.
3736507	May, 1973	Wolejsza et al.	325/60.

Other Refs:
Primary Examiner: Libman, George H.
Assistant Examiner:
Attorney: Cesari and McKenna