Method and apparatus for training linear equalizers in a PCM modem

Abstract

A pulse code modulation (PCM) modem system employs a relatively white training signal to optimize the adaptive filter coefficients in the receiver equalizers. During the training mode, any line coding or equivalent spectral shaping is disabled to provide a training signal sequence having a substantially even spectral content. The presence of DC within the training signal reduces the likelihood that the error function of the equalizers will settle at a local minimum. Following the training interval, the encoder enables the line coder to condition the digital input sequences, introduce DC nulls, and reduce the detrimental effects of baseline wander.

1. A method for transmitting data in a pulse code modulation (PCM) modem system, said method comprising the steps of:
 initializing a training mode characterized by a training interval;  
 adjusting spectral shaping in a transmit modem during at least a portion of said training interval to thereby reduce color in the spectrum of transmit signals, relative to signals transmitted by said transmit modem during a normal data mode, wherein said adjusting step comprises the step of decreasing the number of redundancy bits associated with said  a line coder;  
 transmitting a predetermined training sequence from said transmit modem during said training interval;  
 readjusting spectral shaping in said transmit modem to thereby increase color in the spectrum of signals transmitted after said training interval, relative to signals transmitted by said transmit modem during said training interval, wherein said readjusting step comprises the step of increasing the number of redundancy bits associated with said line coder; and  
 transmitting data symbols in accordance with a PCM protocol.  
 
     
     
       2. A method according to  claim 1 , further comprising the step of adaptively adjusting at least one equalizer located in a receive modem in response to said training sequence. 
     
     
       3. A method according to  claim 1 , wherein said training sequence is characterized by a substantially white spectrum over a predetermined bandwidth. 
     
     
       4. A training method for use with a pulse code modulation (PCM) modem system, said method comprising the steps of:
 initializing a training mode for a PCM transmitter modem configured to transmit colored transmit signals during a data mode and a PCM receiver modem configured to receive and process said colored transmit signals during said data mode;  
 transmitting a predetermined training sequence from said PCM transmitter modem to said PCM receiver modem during said training mode, said training sequence having a less-colored spectrum, relative to said colored transmit signals, over a limited bandwidth, wherein the spectrum of said training sequence is characterized by a reduction in DC suppression relative to said transmit signal;  
 adaptively adjusting the transfer function of at least one equalizer resident at said PCM receiver modem during said training mode; and  
 switching said PCM transmit  transmitter modem from said training mode to said data mode following said training mode.  
 
     
     
       5. A pulse code modulation (PCM) modem comprising:
 means for shaping the spectrum of a PCM transmit signal, said means for shaping reducing spectral components near DC in said PCM transmit signal;  
 a logic circuit element configured to generate a predetermined training sequence having at least a first portion characterized by a substantially white spectrum over a predetermined bandwidth, wherein said predetermined training signal includes a second portion characterized by a colored spectrum over said predetermined bandwidth, relative to the substantially white spectrum of said first portion;  
 means for selecting an output from one of said means for shaping and said logic circuit element; wherein  
 said means for selecting selects output associated with said logic circuit element during a training mode; and  
 said means for selecting selects output associated with said means for shaping during a data mode.  
 
     
     
       6. A PCM modem according to  claim 5 , wherein said means for shaping comprises a line coder. 
     
     
       7. A PCM modem according to  claim 5 , wherein said transmit signal exhibits spectral nulls near DC caused by said means for shaping. 
     
     
       8. A PCM modem according to  claim 5 , wherein said second portion is configured such that the spectrum of said second portion includes suppressed spectral components near DC. 
     
     
       9. A PCM modem according to  claim 5 , wherein said second portion is configured such that the spectrum of said second portion emulates the spectrum of an output signal from said means for shaping. 
     
     
       10. A training method for use with a pulse code modulation (PCM) modem system, said method comprising the steps of:
 initializing a training mode for a PCM transmitter modem configured to transmit colored transmit signals during a data mode and a PCM receiver modem configured to receive and process said colored transmit signals during said data mode;  
 transmitting a predetermined training sequence from said PCM transmitter modem to said PCM receiver modem during said training mode, said training sequence having a less-colored spectrum, relative to said colored transmit signals, over a limited bandwidth, wherein said transmitting step comprises the steps of transmitting a first portion of said predetermined training sequence having relatively white spectral characteristics within said limited bandwidth and transmitting a second portion of said predetermined training sequence having relatively colored spectral characteristics within said limited bandwidth, said second portion being transmitted after said first portion;  
 adaptively adjusting the transfer function of at least one equalizer resident at said PCM receiver modem during said training mode; and  
 switching said PCM transmit  transmitter modem from said training mode to said data mode following said training mode.  
 
     
     
       11. A method according to  claim 10 , wherein said second portion of said predetermined training sequence is configured to emulate the spectrum of said colored transmit signals. 
     
     
       12. A training method for use with a pulse code modulation ( PCM )  modem system, the method comprising:      initializing a training mode for a PCM transmitter modem configured to transmit colored transmit signals during a data mode;        transmitting a predetermined training sequence from the PCM transmitter modem during the training mode, the training sequence having a less - colored spectrum, relative to the colored transmit signals, over a limited bandwidth, wherein the spectrum of the training sequence is characterized by a reduction in DC suppression relative to the colored transmit signals; and        switching the PCM transmitter modem from the training mode to the data mode following the training mode.     
     
     
       13. The method of  claim 12 , wherein the training sequence is characterized by a substantially white spectrum over a predetermined bandwidth. 
     
     
       14. A training method for use with a pulse code modulation ( PCM )  modem system, comprising:      initializing a training mode for a PCM receiver modem configured to receive and process a colored receive signals during a data mode;        receiving a predetermined training sequence during the training mode, the training sequence having a less - colored spectrum, relative to the colored receive signals, over a limited bandwidth, wherein the spectrum of the training sequence is characterized by a reduction in DC suppression relative to the colored receive signals; and        adaptively adjusting a transfer function of at least one equalizer resident at the PCM receiver modem during the training mode; and        switching the PCM receiver modem from the training mode to the data mode following the training mode.     
     
     
       15. The method of  claim 14 , wherein the training sequence is characterized by a substantially white spectrum over a predetermined bandwidth. 
     
     
       16. A training method for use with a pulse code modulation ( PCM )  modem system, said method comprising:      initializing a training mode for a PCM transmitter modem configured to transmit colored transmit signals during a data mode;        transmitting a predetermined training sequence from the PCM transmitter modem during the training mode, the training sequence having a less - colored spectrum, relative to the colored transmit signals, over a limited bandwidth, wherein the transmitting step comprises transmitting a first portion of the predetermined training sequence having a relatively white spectral characteristics within the limited bandwidth and transmitting a second portion of the predetermined training sequence having a relatively colored spectral characteristics within the limited bandwidth, the second portion being transmitted after the first portion; and        switching the PCM transmitter modem from the training mode to the data mode following the training mode.     
     
     
       17. The method of  claim 16 , wherein the training sequence is characterized by a substantially white spectrum over a predetermined bandwidth. 
     
     
       18. A training method for use with a pulse code modulation ( PCM )  modem system, said method comprising:      initializing a training mode for a PCM receiver modem configured to receive colored transmit signals during a data mode;        receiving a predetermined training sequence by the PCM receiver modem during the training mode, the training sequence having a less - colored spectrum, relative to the colored transmit signals, over a limited bandwidth, wherein the receiving step comprises receiving a first portion of the predetermined training sequence having a relatively white spectral characteristics within the limited bandwidth and receiving a second portion of the predetermined training sequence having a relatively colored spectral characteristics within the limited bandwidth, the second portion being received after the first portion; and        switching the PCM receiver modem from the training mode to the data mode following the training mode.     
     
     
       19. The method of  claim 18 , wherein the training sequence is characterized by a substantially white spectrum over a predetermined bandwidth. 
     
     
       20. A training method for use with a pulse code modulation ( PCM )  modem system, the method comprising:      initializing a training mode for a PCM modem configured to transmit a colored transmit signal during a data mode;        transmitting a predetermined training sequence from the PCM modem during the training mode, the predetermined training sequence having a less - colored spectrum, relative to the colored transmit signal, over a limited bandwidth; and        switching the PCM modem from the training mode to the data mode following said training mode.     
     
     
       21. The method of  claim 20 , further comprising:
   disabling a spectral shaping function at the PCM modem during the training mode, the spectral shaping function providing a DC null in the spectrum of signals transmitted by the PCM modem; and        enabling the spectral shaping function at the PCM modem after the training mode to thereby facilitate transmission of colored signals by the PCM modem.     
     
     
       22. The method of  claim 21 , wherein the spectral shaping function comprises line coding. 
     
     
       23. A training method for use with a pulse code modulation ( PCM )  modem system, the method comprising:      receiving a colored transmit signal at a PCM receiver modem having a data mode and a training mode, the PCM receiver modem configurable to process the colored transmit signal during the data mode;        processing a predetermined training sequence during the training mode, the training sequence having a less - colored spectrum, relative to the colored transmit signal, over a limited bandwidth;        adaptively adjusting a transfer function of at least one equalizer resident at the PCM receiver modem during the training mode; and        switching the PCM receiver modem from the training mode to the data mode following the training mode.     
     
     
       24. A pulse code modulation ( PCM )  modem comprising:      a line coder, the line coder reducing spectral components near DC in a PCM transmit signal;        a logic circuit element configured to generate a predetermined training sequence having at least a first portion characterized by a substantially white spectrum over a predetermined bandwidth, wherein the predetermined training signal includes a second portion characterized by a colored spectrum over the predetermined bandwidth, relative to the spectrum of the first portion;        a switch element, the switch element selecting an output from one of the line coder and the logic circuit element; wherein      the switch element selects output associated with the logic circuit element during a training mode; and        the switch element selects output associated with the line coder for shaping during a data mode.       
     
     
       25. The PCM modem of  claim 24 , wherein the transmit signal exhibits spectral nulls near DC caused by the line coder. 
     
     
       26. The PCM modem of  claim 24 , wherein the second portion is configured such that the spectrum of the second portion includes suppressed spectral components near DC. 
     
     
       27. The PCM modem of  claim 24 , wherein the second portion is configured such that the spectrum of the second portion emulates the spectrum of an output signal from the line coder. 
     
     
       28. A modem comprising:
   a line coder, the line coder reducing spectral components near DC in a PCM transmit signal;        a logic circuit configured to generate a training sequence having at least a first portion characterized by a substantially white spectrum over a predetermined bandwidth and a second portion characterized by a colored spectrum over the predetermined bandwidth relative to the spectrum of the first portion;        a switch element, the switch element selecting an output from one of the line coder and the logic circuit element; wherein      the switch element selects output associated with the logic circuit element during a training mode; and        the switch element selects output associated with the line coder for shaping during a data mode.       
     
     
       29. The modem of  claim 28 , wherein the transmit signal exhibits spectral nulls near DC caused by the line coder.