High-speed modem synchronized to a remote codec

Abstract

A modem that operates reliably at a symbol rate that corresponds to twice its bandwidth even when it is coupled to a receiving A/D converter that operates under control of a clock is realized by synchronizing the modem's operation to the A/D's clock. The superior operation of this modem advantageously extends to A/D clock frequencies beyond the frequency of twice the modem's bandwidth. To minimize quantization noise, the modem's output is conditioned to minimize intersymbol interference by adjusting the modem's output to the A/D converter's sampling times and slicing levels. When the A/D's clock is higher than twice the bandwidth of the modem's output signal, some intersymbol interference cannot be avoided. In accordance with this invention, the position and value of this interference is computed at the receiver and subtracted from the received signal.

We claim:  
     
       1. Apparatus comprising: 
       first means for developing a clock signal of period T seconds; and  
       second means, responsive to the first means, for developing an output signal of the apparatus, which output signal is band-limited to frequencies below f c  and contains information-bearing analog pulses, at least some of which are spaced T seconds apart;  
       wherein T is less than ½ f c ,  
       wherein each of the information-bearing analog pulses represents a magnitude that corresponds to an applied symbol from a constellation of symbols,  
       wherein in each set of M consecutive clock periods T there is a grouping of N of said analog samples, where M and N are integers and N is less than M, and  
       wherein said second means comprises N analog pulse generation means with their outputs combined, and each one of the analog samples in the grouping of N of said analog samples is generated by a different one of the N analog pulse generation means.  
     
     
       2. The apparatus of  claim 1  wherein at least one of the analog pulse generation means is a filter. 
     
     
       3. The apparatus of  claim 1  wherein said second means comprises 
       N analog pulse generation means, responsive to applied symbols, that contribute to the output signal of the apparatus, and  
       switch means for applying symbols to the N analog pulse generation means to cause each one of the analog samples in the grouping of N of said analog samples to be generated by a different one of the analog pulse generation means, resulting thereby in each one of the analog pulse generation means to be associated with a particular clock period in the set of M consecutive clock periods T.  
     
     
       4. The apparatus of  claim 3  wherein the symbols are applied to the second means at intervals mT, where m is an integer greater than 0. 
     
     
       5. The apparatus of  claim 3  further comprising a buffer for receiving symbols at rate 2 f c  and outputting symbols to the second means at intervals mT, where m is an integer greater than 0. 
     
     
       6. The apparatus of  claim 3  wherein each of the analog pulse generation means is characterized by a transfer function that develops essentially no output when the other of the N analog samples develop a peak output out of their respective analog pulse generation means. 
     
     
       7. The apparatus of  claim 3  wherein each of the analog pulse generation means is characterized by a transfer function            y   p          (   t   )       =       K   p        sinc                     t   -     t   p         8        T   s                           ∏       q   =   1       q   ≠   p       7          sin                     π        (     t   -     t   q       )         8        T   s                               
       where p is an index integer, K p  is a constant (and there are p such constants), t p  is the p th  sampling interval, T s  is the sampling period, t q  is the q th  sampling interval where q is another integer index ranging from one to seven, and y p  is the p th  signal sample. 
     
     
       8. The apparatus of  claim 1 , wherein the first means further comprising control means responsive to signals from a remote system to develop said clock signal, which control means includes delay means for changing the phase of the developed clock in response to a training signal applied to said remote system. 
     
     
       9. The apparatus  claim 8  wherein said control means affects magnitudes of the symbols applied to the second means. 
     
     
       10. The apparatus  claim 8  wherein said control means affects magnitudes of the information-bearing analog pulses developed by the second means. 
     
     
       11. Apparatus comprising: 
       an D/A converter operating under control of a first clock, for communicating signals to a A/D converter having a second clock,  
       control means for receiving information from the A/D converter,  
       means, responsive to said control means, for synchronizing the first clock to the second clock, and  
       means, responsive to said control means, for adjusting levels of the output signal of the D/A converter.  
     
     
       12. Apparatus comprising: 
       an A/D converter operating under control of a first clock, for receiving signals from a D/A converter having a second clock,  
       control means for deriving information from the signal received by the A/D converter,  
       means, responsive to said control means, for synchronizing the first clock to the second clock, and  
       means, responsive to said control means, for adjusting the slicing levels of the A/D converter.  
     
     
       13. An arrangement for communicating digital information comprising a digital-to-analog conversion means, a analog-to-digital conversion means operating with a clock, and an analog medium coupling the output of the digital-to-analog conversion means with the input of the analog-to-digital conversion means, CHARACTERIZED IN THAT: 
       the digital-to-analog conversion means develops an analog output in synchronism with said clock at levels that are cognizant of slicing levels of the analog-to-digital conversion means, and of said analog medium.  
     
     
       14. The arrangement of  claim 13  said analog output is of such a magnitude that when the analog output signal is communicated to the analog-to-digital conversion means, the analog-to-digital conversion means develops a digital output that is the same as the applied digital signal. 
     
     
       15. An modem including a transmitting section for developing an analog signal in response to an applied digital stream, which analog signal is destined to a remote clocked conversion means, a receiving section for developing a digital string in response to an analog signal from the remote clocked conversion means, means for developing a clock in synchronism with the clock in the remote conversion means, and means for employing the developed clock in the transmitting section, the receiving section comprising: 
       a switched filter,  
       a transmission-medium-emulating filter responsive the output of the switched filter,  
       a codec connected to the output of the transmission-medium-emulating filter,  
       a combiner for subtracting the output of the codec from the output of the transmission-medium-emulating filter,  
       a band limiting filter responsive to an output signal of the combiner, and  
       an input subtracter, for subtracting the output of the band limiting filter from incoming signals to form a symbol signal, the symbol signal being applied to the switched filter and to A/D means for developing said digital string.  
     
     
       16. A method for modifying operational parameters of a modem connected to a digital channel bank via analog lines, where the modem has a receiving portion and a transmitting portion, the method comprising the steps of: 
       connecting the modem, through the channel bank, to a predefined destination, which connecting is effected over digital facilities following the channel bank, having communication and signal processing capabilities,  
       the modem receiving a known signal, through the digital facilities though the channel bank, from the predefined destination,  
       the modem processing the received signal to develop first control signals for the receiving portion and second control signals for the transmitting portion,  
       the modem equalizing the receiving portion in response to the first control signals developed by the step of processing, and  
       the modem equalizing the transmitting portion in response to the second control signals developed by the step of processing.  
     
     
       17. An adaptation resource for connection to a digital network and which assists in adapting a modem receiver having an A/D converter that receives a voiceband analog signal from said network via a subscriber line; wherein said adaptation resource transmits a predetermined digital signal to a network D/A converter that converts the digital signal to a voiceband analog signal for transmission via said subscriber line to said modem receiver. 
     
     
       18. The resource of  claim 17  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem all of the levels that the receiver is responsive to, whereby the slicing levels of said modem receiver may be adjusted. 
     
     
       19. The resource of  claim 17  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem voiceband analog signals that adapt an equalizer in said modem receiver. 
     
     
       20. The resource of  claim 17  wherein said predetermined digital signal is transmitted during a training period. 
     
     
       21. The resource of  claim 17  wherein said predetermined digital signal is a PCM signal. 
     
     
       22. The resource of  claim 21  wherein said PCM signal comprises  8  bits per sample at an  8  kHz sampling rate to result in a  64  kb/s rate. 
     
     
       23. The adaption resource of  claim 17  wherein the analog output of said D/A converter is of such magnitude that when the analog output signal is communicated to the A/D converter, the A/D converter develops a digital output that is the same as the applied digital signal. 
     
     
       24. The adaptation resource of  claim 23  wherein said applied digital signal is a PCM signal. 
     
     
       25. The resource of  claim 17  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem voiceband analog signals that synchronize a modem receiver clock to network clock that controls said network D/A converter. 
     
     
       26. A modem comprising a receiver having an A/D converter capable of receiving a voiceband analog signal via a subscriber line from a network D/A converter coupled to an adaptation resource that transmits a predetermined digital signal to said network D/A converter during a training period, and with said modem capable of being adapted by the voiceband analog signal resulting from said predetermined digital signal. 
     
     
       27. The modem of  claim 26  wherein said modem is capable of being adapted during said training period by adjusting the slicing levels of said receiver in response to a voiceband analog signal that includes all of the levels that the receiver is responsive to. 
     
     
       28. The modem of  claim 26  wherein said modem is capable of being adapted during said training period by training a receive equalizer in said modem receiver in response to a voiceband and analog signal. 
     
     
       29. The modem of  claim 26  wherein said predetermined digital signal is a PCM signal. 
     
     
       30. The modem of  claim 29  wherein said PCM signal comprises  8  bits per sample at an  8  kHz sampling rate to result in a  64  kb/s rate. 
     
     
       31. The adaptation resource of  claim 26  wherein the analog output of said D/A converter is of such magnitude that when the analog output signal is communicated to the A/D converter, the A/D converter develops a digital output that is the same as the applied digital signal. 
     
     
       32. The modem of  claim 31  wherein said applied digital signal is a PCM signal. 
     
     
       33. The modem of  claim 26  wherein said modem is capable of being adapted during said training period by synchronizing the modem receiver clock to a network clock that controls said network D/A converter. 
     
     
       34. A method of adapting a modem comprising a receiver having an A/D converter that receives an analog signal from a network via a subscriber line, steps comprising: 
       
         transmitting a predetermined digital signal from an adaptation resource to a network D/A converter for conversion to an analog signal, and transmitting said analog signal via said subscriber line to said modem during a training period. 
       
     
     
       35. The method of  claim 34  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem all of the levels that the modem receiver is responsive to, whereby the slicing levels of said modem receiver may be adjusted. 
     
     
       36. The method of  claim 34  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem analog signals that train a receive equalizer in said modem receiver. 
     
     
       37. The method of  claim 34  wherein said predetermined digital signal is a PCM signal. 
     
     
       38. The method of  claim 37  wherein said PCM signal comprises  8  bits per sample at an  8  kHz sampling rate to result in a  64  kb/s rate. 
     
     
       39. The method of  claim 34  wherein more than one modem is adapted by said adaptation resource. 
     
     
       40. The method of  claim 34  wherein the analog output of said D/A converter is of such magnitude that when the analog output signal is communicated to the A/D converter, the A/D converter develops a digital output that is the same as the applied digital signal. 
     
     
       41. The method of  claim 40  wherein said applied digital signal is a PCM signal. 
     
     
       42. The method of  claim 34  wherein said predetermined digital signal causes said network D/A converter to transmit to the modem an analog signal that trains a timing recovery means that synchronizes a modem receiver clock to a network clock that controls said network D/A converter.