Method and apparatus for minimizing error propagation in correlative digital and communication system

• US 5,287,351 A
• Filed: 03/16/1992
• Issued: 02/15/1994
• Est. Priority Date: 11/27/1990
• Status: Expired due to Term

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Scientific-Atlanta Incorporated (Cisco Systems, Inc.)
• Inventors: Wall, William E. Jr.
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): HOM, SHICK C
• Application Number:
  US07/851,588
• Time in Patent Office: 701 Days
• Field of Search: 370/77, 370/20, 370/73, 375/39, 375/57, 455/60, 381/15
• US Class Current: 370/206
• CPC Class Codes: H04B 7/18523 Satellite systems for providing broadcast service to terrestrial stations, i.e. broadcast satellite service arrangements specially adapted for satellite broadcast receiving H04H40/90; picture transmission via satellite H04N1/00103; television transmission via satellite H04N7/20
• CPC Class Codes: H04H 20/28 Arrangements for simultaneous broadcast of plural pieces of information
• CPC Class Codes: H04H 20/79 using downlink of the CATV systems, e.g. audio broadcast via CATV network
• CPC Class Codes: H04N 19/89 involving methods or arrangements for detection of transmission errors at the decoder
• CPC Class Codes: H04N 21/2365 Multiplexing of several video streams
• CPC Class Codes: H04N 21/2383 Channel coding or modulation of digital bit-stream, e.g. QPSK modulation arrangements for detecting or preventing errors in the information received by adapting the channel coding H04L1/0009; analogue front ends or means for connecting modulators, demodulators or transceivers to a transmission line H04L27/0002
• CPC Class Codes: H04N 21/426 Internal components of the client ; Characteristics thereof H04N5/44 takes precedence
• CPC Class Codes: H04N 21/4347 Demultiplexing of several video streams
• CPC Class Codes: H04N 21/4382 Demodulation or channel decoding, e.g. QPSK demodulation analog front ends or means for connecting modulators, demodulators or transceivers to a transmission line H04L27/0002
• CPC Class Codes: H04N 5/765 Interface circuits between an apparatus for recording and another apparatus
• CPC Class Codes: H04N 7/10 Adaptations for transmission by electrical cable H04N7/12 takes precedence
• CPC Class Codes: H04N 7/16 Analogue secrecy systems; Analogue subscription systems
• CPC Class Codes: H04N 7/22 Adaptations for optical transmission

Abstract

A digital audio communication system includes a digital modulation and demodulation scheme which efficiently uses the bandwidth and channel spacing of a cable television system. A 30 channel digital bit stream is demultiplexed into six groups of five channels. Each group of channels thereafter modulates a carrier by a quadrature partial response (QPR) process. The QPR signal, an amplitude modulated, double sideband, carrier suppressed (AM DSBSC) signal, is then transmitted over the cable system to a multiplicity of subscribers, each of which has a QPR demodulator. The demodulators are of the decision feedback type having a modified Costas loop carrier recovery circuit. The grouping of an odd number (five) of digital audio channels per QPR modulator minimizes error propagation due to the correlative QPR demodulation process. The decision feedback decoding is implemented in simple current nodes where a bilevel output from the decoded data is subtracted from a tertiary level output of a mixer. The resulting bilevel current is converted to a voltage, filtered, and limited to produce a logic output level. The modified Costas loop provides an error voltage based upon the difference of the received data states versus the nominal data states and the quadrant the data occupies. The error voltage is generated by differencing the outputs of two analog switches which have as their inputs analog signals representative of the amplitude level of the data and its inverse of one phase. The sign of the data bit of the opposite phase is used to control the switch and thus select which signal is used.

Claims

1. A method for communicating the information contained in a plurality of serialized digital bit streams over a communication link comprising the steps of:

grouping said plurality of serialized digital bit streams into at least one group having an odd number of bit streams per group;

time division multiplexing the bits of each group together to form a single multiplexed bit stream from each group;

demultiplexing said multiplexed bit streams into a first bit stream and a second bit stream of alternating bits from said multiplexed bit stream;

modulating a first carrier with said first bit stream;

modulating a second carrier of the same frequency which is 90°

out of phase with said first carrier with said second bit stream; and

transmitting said first and second carriers over a communication link.

2. A method for communicating digital data as set forth in claim 1 wherein steps of modulating said first and second carriers include:

encoding the information as partial response data.

3. A method for communicating digital data as set forth in claim 1 wherein said step of grouping includes:

grouping said plurality of serialized digital bit streams into at least one group having five bit streams per group.

4. An apparatus for minimizing error propagation in a quadrature partial response communication system having a plurality of channels, said apparatus comprising:

a demultiplexer which sequence bits from a plurality of channels having an odd number into pairs of quadrature phases I, Q;

a modulator for modulating the phases I, Q on the quadrature phases of a carrier to produce independent I, Q data streams;

a communication channel;

a quadrature partial response demodulator for receiving said I, Q data streams from said communication channel and demodulating said I, Q data streams; and

multiplexer for multiplexing said I, Q data streams into the original bit sequence.

5. An apparatus for minimizing error propagation in a quadrature partial response communication system having a plurality of channels, said apparatus comprising:

a demultiplexer which sequence bits from a plurality of channels having an even number into pairs of quadrature phases I, Q;

means for inserting a spacing bit at the end of each sequence for the plurality of channels;

a modulator for modulating the phases I, Q on the quadrature phases of a carrier to produce independent I, Q data streams;

a communication channel;

a quadrature partial response demodulator for receiving said I, Q data streams from said communication channel and demodulating said I, Q data streams; and

multiplexer for multiplexing said I, Q data streams into the original bit sequence.