Abstract:
Systems and methods for digital upconversion of digital signals are provided. In one embodiment, the system includes a digital frequency adjustment system and a digital to analog conversion system. In a feature of the embodiment, the digital frequency adjustment system consists of set of digital upconversion and upsample elements that shift upwards the frequency of baseband signals. In a further feature of the embodiment, a tree structure of sets of upsample and upconversion elements is used. In another embodiment, the system includes digital and analog frequency adjustment systems in which the frequencies of the input signals are partially upshifted within both the digital and analog domains. Methods for digital upconversion of digital signals are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/826,281, filed Apr. 19, 2004, which in turn is a continuation of U.S. application Ser. No. 10/452,221, filed Jun. 3, 2003, which issued as U.S. Pat. No. 6,724,335 on Apr. 20, 2004. U.S. application Ser. Nos. 10/826,281 and 10/452,221 are hereby incorporated by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to digital communications signals, and more particularly, to upconversion of digital signals. 
         [0004]    2. Background of the Invention 
         [0005]    Cable television systems generally require a conversion system for frequency converting the transmitted channels from baseband frequencies to their designated RF frequencies for transmission over the cable medium. This system is typically part of a cable television (CATV) headend system, where the composite, multi-channel CATV signal is generated and amplified for distribution to customers. 
         [0006]    Within existing headend systems, each frequency converters typically use multiple analog mixing stages, with one or more analog phase-locked loops (PLL) to generate the local oscillators. One converter is required for each channel, and there can be more than one hundred channels in a typical CATV system. These systems are often costly and require significant amounts of hardware. Additionally within existing systems, control of signal amplitude for each channel can be complex. 
         [0007]    What is needed is a cost-effective system and method for frequency converting baseband television signals and creating composite, multi-channel CATV signals within a CATV headend system. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is directed to systems and methods for digital upconversion of baseband television signals and other types of signals, such as those associated with cable modems, that can be used in cable television headend systems. In one embodiment, the digital headend upconversion system includes a demultiplexer, a digital frequency adjustment system and a digital to analog (DAC) conversion system. In one embodiment the digital frequency adjustment system includes a set of upsample and upconversion elements that shift upwards the frequency of baseband signals. In another embodiment, a tree structure of sets of upsample and upconversion elements is used. The digital to analog conversion system includes a single digital to analog converter or a set of converters. 
         [0009]    An alternative embodiment of the digital headend upconversion system is a digital hybrid headend upconversion system that includes a demultiplexer, a digital frequency adjustment system and an analog frequency adjustment system. In this embodiment, the frequencies of baseband signals that are input to the upconversion system are partially upshifted within the digital domain and partially upshifted within the analog domain. The digital frequency adjustment system is as described above, except that the frequencies of the baseband signals are partially adjusted rather than upshifted to final desired frequencies for distribution. The analog frequency adjustment system includes a set of digital to analog converters followed by a set of band pass filters, followed by a set of mixers, followed by another set of band pass filters, followed by another set of mixers, and finally followed by a set of low pass filters. The outputs of each of the low pass filters are summed together to form the desired frequency upconverted composite signal for distribution throughout a cable network. In a further feature, within a digital or digital hybrid upconversion system, an individual channel gain adjustment system can be included to allow precise gain adjustment controls for individual channels. 
         [0010]    Methods for digital upconversion of television signals are also provided. In one embodiment, the method includes receiving digital baseband television signals, demuxing those signals, upsampling and upconverting the demuxed signals, then recombining the signals and performing a digital to analog conversion. In one embodiment, upsampling and upconverting the demuxed signals occurs in a two steps. In alternative embodiments, a tree structure of upsampling and upconversion elements in used, such that upsampling and upconverting occurs in multiple two-step phases. 
         [0011]    In another embodiment, the frequencies of digital input signals are partially upshifted within the digital domain and partially upshifted within the analog domain. 
         [0012]    Use of the invention provides two principal benefits. First, use of the invention reduces the cost and complexity of hardware needed for a cable television headend system. Second, use of the invention simplifies digital control of channel amplitude for the television signals. 
         [0013]    Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention are described in detail below with reference to accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    The invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawing in which an element first appears is indicated by the left-most digit in the corresponding reference number. 
           [0015]      FIG. 1  is a diagram of a digital headend upconversion system, according to an embodiment of the invention. 
           [0016]      FIG. 2  is a diagram of a digital frequency adjustment system, according to an embodiment of the invention. 
           [0017]      FIG. 3  is a diagram of a digital frequency adjustment system that includes cascading upsample and upconversion elements, according to an embodiment of the invention. 
           [0018]      FIG. 4A  is a diagram of a digital to analog converter system, according to an embodiment of the invention. 
           [0019]      FIG. 4B  is a diagram of a digital to analog converter system that includes a series of digital to analog converters, according to an embodiment of the invention. 
           [0020]      FIG. 5A  is a diagram of a digital hybrid headend upconversion system, according an embodiment of the invention. 
           [0021]      FIG. 5B  is a diagram of an analog frequency adjustment system, according to an embodiment of the invention. 
           [0022]      FIG. 6  is a diagram of an upconversion element, according to an embodiment of the invention. 
           [0023]      FIG. 7  is a method for digital upconversion of baseband television signals, according to an embodiment of the invention. 
           [0024]      FIG. 8  is a method for digital hybrid upconversion of baseband television signals, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility. 
         [0026]      FIG. 1  illustrates digital headend upconversion system  100 , according to an embodiment of the invention. Digital headend upconversion system  100  includes demultiplexer  110 , digital frequency adjustment system  120  and digital to analog converter (DAC) system  130 . An input signal is provided to digital headend upconversion system  100  over connection  135 , and an output signal is transmitted from digital headend upconversion system over connection  145 . Digital headend upconversion system  100  can be used within a cable television headend system. When used within a cable television headend system, inputs into demultiplexer  110  are multiple baseband television channels in a digital format. The outputs of demultiplexer  110  are coupled to the input of digital frequency adjustment system  120 , and the outputs of digital frequency adjustment system  120  are coupled to the input of DAC system  130 , which transmits its output over connection  145 . 
         [0027]    In an alternate embodiment of digital headend upconversion system  100 , demultiplexed signals are provided to the system, so that demultiplexer  110  is not required. In another alternate embodiment of digital headend upconversion system  100 , a individual channel gain adjustment system can be coupled to either the output of demultiplexer  110  or to the output of digital frequency adjustment system  120 . Use of an individual channel gain adjustment system allows individual channel gains to be scaled digitally which is more precise and less prone to drift than current analog approaches. 
         [0028]    Digital headend upconversion system  100  converts digital baseband television signals to an analog signal in which the digital baseband television signals have been upconverted in frequency to the desired radio frequency (RF) frequencies to create a multi-channel RF spectrum. This output, or multi-channel RF spectrum, can then be distributed over a cable television distribution system to individual cable subscribers. 
         [0029]    Digital frequency adjustment system  120  can be implemented in a number of alternative embodiments.  FIG. 2  is a diagram of digital frequency adjustment system  120 , according to one embodiment of the invention. In this embodiment, digital frequency adjustment system  120  includes upsample elements  205 A, B, C, and n; upconversion elements  210 A, B, C, and n; and a summing device  220 . 
         [0030]    As discussed with respect to  FIG. 1 , digital baseband television signals can be input to demultiplexer  110 . Demultiplexer  110  provides a set of output signals to an array of upsample elements  205 A,  205 B,  205 C through  205   n , such that each of the baseband signals outputted from demultiplexer  110  is transmitted to an upsample element that will upsample the baseband signal. The number of upsample elements  205  used will be a function of the number of baseband channels to be upconverted. The upsample elements interpolate intermediate data points between signal points, and add those to the signal to facilitate less complex digital to analog conversion. 
         [0031]    Outputs from the upsample elements  205 A, B, C and n are coupled to the inputs of upconversion elements  210 A, B, C or n. As discussed below with respect to  FIG. 6 , the design of each of upconversion elements  210  is the same, except for their operating frequencies. Each upconversion element  210  will be coupled to one upsample element  205 . For example, upsample element  205 A is coupled to upconversion element  210 A, upsample element  205 B is coupled to upconversion element  210 B, upsample element  205 C is coupled to upconversion element  210 C and so forth, such that upsample element  205   n  is coupled to upconversion element  210   n.  The outputs from all of the upconversion elements  210  are coupled to summing device  220 . Summing device  220  combines these signals to output an upconverted digital signal. The output of summing device  220  is then coupled to the input of DAC system  130 . As can be observed in  FIG. 1 , the signals traversing digital frequency adjustment system  120 , remain in digital form and therefore enable significant digital control of channel amplitudes. 
         [0032]      FIG. 3  provides an alternative embodiment of digital frequency adjustment system  120 , according to an embodiment of the invention. In this embodiment, the upsampling and upconverting is carried out in a tree structure or series of upsampling and upconverting steps to minimize component complexity. In this case, digital frequency adjustment system  120  includes a first set of upsample elements  305 A,  305 D,  305 X and  305   n;  a first set of upconversion elements  310 A,  310 D,  310 X, and  310   n;  a first set of summing devices  315 A and  315 B; a second set of upsample elements  320 A and  320 B; and a second set of upconversion elements  330 A and  330 B; and a summing device  325 . As in the embodiment depicted in  FIG. 2 , the number of upsample elements within the first set of upsample elements will be a factor of the number of baseband channels, and the number of steps within the tree structure. 
         [0033]    As in the previous case, demultiplexer  110  provides a set of output signals to an array of upsample elements  305 A,  305 D,  305 X through  305   n , such that each of the baseband signals output from demultiplexer  110  is transmitted to an upsample element. Outputs from upsample elements  305 A, D, X and n are coupled to the inputs of upconversion elements  310 A, D, X and n that will upconvert the baseband signal to a desired RF frequency. Each upconversion element  310  will be coupled to one upsample element  305 . 
         [0034]    Up until this point, the embodiment described with respect to  FIG. 3  appears the same as the embodiment described with respect to  FIG. 2 . At this point, however, the embodiments differ. Rather than having the outputs from the upconversion elements coupled to a single summing device, as was the case with respect to the embodiment depicted in  FIG. 2 , the outputs from the upsample elements are coupled to two summing devices. More precisely, the outputs of upconversion element  310 A through  310 D are coupled to summing device  315 A and the outputs of upconversion elements  310 X through  310   n  are coupled to summing device  315 B. The output from summing device  315 A is then coupled to the input of upsample element  320 A, while the output from summing device  315 B is coupled to upsample element  320 B. The outputs of the second set of upsample elements—upsample elements  320 A and  320 B—are then coupled to the inputs of upconversion elements  330 A and  330 B. The outputs from upconversion elements  330 A and  330 B are coupled to summing device  325 . Summing device  325  combines these signals to output an upconverted digital signal. The output of summing device  325  is then coupled to the input of DAC system  130 . 
         [0035]    The embodiment depicted in  FIG. 3  provides a tree structure of upsample and upconversion elements in which two sets of upsample and upconversion elements are used. The description of this embodiment is illustrative, and not intended to limit the invention to a tree structure having only two sets of upsample and upconversion elements. Rather, any number of sets of upsample and upconversion elements within the tree structure can be used. The number of sets to be used will be a tradeoff between reducing the complexity of individual upsample and upconversion elements by having a greater number of upsample and upconversion elements, and the complexity of having an increasing number of upsample and upconversion elements, and summing devices. The number of baseband signals being converted will factor into the number of sets within a tree structure to be used. Based on the teachings herein, individuals skilled in the art can select the appropriate number of sets of upsample and upconversion elements based on their particular application. 
         [0036]    Alternative embodiments of DAC system  130  can also be used within the invention. In one embodiment, a single digital to analog converter can be used within DAC system  120 . Alternatively, a series of digital to analog converters can be used.  FIG. 4A  illustrates the embodiment in which a single digital to analog converter  405  is used. In this embodiment, the output from digital frequency adjustment system  120  is coupled to the input of digital to analog converter  405 . The output of digital to analog converter  405  is then provided for distribution through a cable television network. 
         [0037]      FIG. 4B  illustrates an embodiment in which multiple digital to analog converters are used. In this case, DACs  415 ,  420 ,  425  and  430  are used. In the embodiment depicted in  FIG. 4B , DAC  415  processes signal band  1 , DAC  420  processes signal band  2 , DAC  425  processes signal band  3 , and DAC  425  processes signal band  4 . The outputs of each of DACs  415 ,  420 ,  425  and  430  are then coupled to the inputs of filters  432 ,  434 ,  436  and  438 . Filters  432 ,  434 ,  436  and  438  will be a combination of lowpass, bandpass, and high-pass filters depending on the particular frequency to be processed. The use of the filters reduces the complexity of the digital to analog converters. In other embodiments, filters may not be used. The filter outputs are combined by summing device  440  to generate an output signal. By using parallel DACs, the resolution requirement of an individual DAC is reduced. Specifically, for each factor of four increase in the number of DACs, one less bit of resolution is necessary. Thus, while additional hardware is needed, the complexity of that hardware is reduced. The number of parallel DACs may range from 2 to the number of bands within the baseband television signal. 
         [0038]      FIG. 5A  is a diagram of digital hybrid headend upconversion system  500 , according to an embodiment of the invention. As in the case of digital headend upconversion system  100 , digital hybrid headend upconversion system  500  converts digital baseband television signals to an analog signal in which the digital baseband television signals have been upconverted in frequency to the desired RF frequencies to create a multi-channel RF spectrum. This output, or multi-channel RF spectrum, can then be distributed over a cable television distribution system to individual cable subscribers. 
         [0039]    Digital hybrid headend upconversion system  500  includes demultiplexer  505 , digital frequency adjustment system  515  and analog frequency adjustment system  510 . Digital hybrid headend upconversion system  500  represents a hybrid system in which a portion of the frequency adjustment occurs within the digital domain and a portion occurs within the analog domain. 
         [0040]    An input signal is provided to digital hybrid headend upconversion system  500  over connection  502 , and an output signal is transmitted from digital headend upconversion system over connection  504 . Digital hybrid headend upconversion system  500  can be used within a cable television headend system. When used within a cable television headend system, inputs into demultiplexer  110  are multiple baseband television channels in a digital format. The outputs of demultiplexer  110  are coupled to the input of digital frequency adjustment system  515 . The outputs of digital frequency adjustment system  515  are coupled to the inputs of analog frequency adjustment system  510 , and the outputs of analog frequency adjustment system  510  are transmitted over connection  504 . In an alternate embodiment of digital hybrid headend upconversion system  500 , demultiplexed signals are provided to the system and demultiplexer  505  is not required. 
         [0041]    Digital frequency adjustment system  515  operates under the same principles as described with respect to frequency adjustment system  120  with either a single set of upsample and upconversion elements or multiple sets of upsample and upconversion elements in a tree structure. The differences between digital frequency adjustment system  515  and digital frequency adjustment system  120  are that (1) digital frequency adjustment system  515  will not adjust the channel frequencies to the final desired channel frequencies and (2) digital frequency adjustment system  515  can provide multiple outputs. 
         [0042]    Thus, for example, digital frequency adjustment system  515  can be the same as digital frequency adjustment system  120  as depicted in  FIG. 3 , except that summing device  325  would not be used, and the outputs from upconversion elements  330 A and  330 B would be coupled to the inputs of analog frequency adjustment system  510 . Digital frequency adjustment system  515  can have one set of upsample and upconversion elements, or multiple sets. Additionally, whereas the upsample and upconversion elements in  FIG. 3  were selected to upconvert the channel frequencies to the desired level for distribution within a cable network, the upsample and upconversion elements used within digital frequency adjustment system  515  can be selected to upconvert the signals to two-thirds (or some other fraction) of the final desired frequencies. The decision on how much frequency upconverting will be done by each system is a design decision based on the particular application, and a cost-benefit analysis of using upsample and upconversion elements versus using digital to analog converters within analog frequency adjustment system  510 . 
         [0043]      FIG. 5B  is a diagram of an analog frequency adjustment system  510 , according to an embodiment of the invention. Analog frequency adjustment system  510  includes DACs  520  and  525 ; band pass filters (BPF)  530 ,  535 ,  550  and  555 ; mixers  540 ,  545 ,  560  and  565 ; low pass filters (LPF)  570  and  575 ; and summing device  580 . In this embodiment, two upconversion processing paths are formed for a band  1  and a band  2  of the input signal. Band  1  and band  2  represent non-overlapping spectrum bands (e.g., band  1  could be one half of the cable television channels and band  2  could be the other half) of the input signal. The band  1  upconversion processing path includes DAC  520 , BPF  530 , mixer  540 , BPF  550 , mixer  560  and LPF  570 . Similarly, band  2  upconversion processing path includes DAC  525 , BPF  535 , mixer  545 , BPF  555 , mixer  565  and LPF  575 . The processing of these two bands is the same, except for the center frequency to which each of band  1  and band  2  will be upconverted. 
         [0044]    Along the band  1  upconversion processing path, the signal from which band  1  is to be upconverted is input into DAC  520 . The output of DAC  520  is coupled to the input of BPF  530 . The output of BPF  530  is band  1  upconverted to a center frequency of f 0  to provide a set of first intermediate signals. The output of BPF  530  is coupled to the input of mixer  540 , which has a frequency of f 1  to provide a set of second intermediate signals. The output of mixer  540  is coupled to the input of BPF  550 . The output of BPF  550  is the band  1  signal upconverted to a center frequency of f 1 +f 0  to produce a set of third intermediate signals. The output of BPF  550  is coupled to the input of mixer  560 , which has a frequency of f 3 . The output of mixer  560  is coupled to LPF  570 . The output of LPF  570  is the band  1  signal converted to a frequency of f 1 +f 0 −f 3 =f a . 
         [0045]    Similarly, along the band  2  upconversion processing path, the signal from which band  2  is to be upconverted is input into DAC  525 . The output of DAC  525  is coupled to the input of BPF  535 . The output of BPF  535  is band  2  upconverted to a center frequency of f 0 . The output of BPF  535  is coupled to the input of mixer  545 , which has a frequency of f 2 . The output of mixer  545  is coupled to the input of BPF  555 . The output of BPF  555  is the band  2  signal upconverted to a center frequency of f 2 +f 0 . The output of BPF  555  is coupled to the input of mixer  565 , which has a frequency of f 3 . The output of mixer  565  is coupled to LPF  575 . The output of LPF  575  is the band  2  signal converted to a frequency of f 2 +f 0 −f 3 =f b . 
         [0046]    The outputs of the band  1  upconversion processing path and band  2  upconversion processing path are coupled to the input of summing device  580 . Summing device  580  combines the signals from band  1  and band  2  upconversion processing path to produce an output signal that consists of the combination of the band  1  signal with a center frequency of f a  and the band  2  signal with a center frequency of f b . 
         [0047]      FIG. 6  is a diagram of an upconversion element  210 , according to an embodiment of the invention. Upconversion element  210  consists of a complex mixer  610  and a digital synthesizer  620 . Digital synthesizer  620  is coupled to complex mixer  610 , such that when an input signal is received by complex mixer  610  the frequency can be upconverted using the frequency provided by digital synthesizer  620 . The upconverted signal is then output from complex mixer  610 . In some cases, upconversion element  210  can have a transfer function of 1, that is, the frequency of the signal output is the same as the frequency of the signal input. 
         [0048]      FIG. 7  is a method  700  for digital upconversion of baseband television signals, according to an embodiment of the invention. Method  700  begins in step  710 . In step  710 , a digital baseband television signal is received. In step  720 , the received digital baseband television signal is demultiplexed into multiple channels or bands. In step  730 , the demuxed signals are upsampled in frequency. In step  740 , the demuxed signals that have been upsampled are then upconverted. In step  750 , the signals produced in step  740  are summed together to create a single upconverted digital signal. In step  760 , the digital upconverted signal is converted to an analog signal that can be transmitted over a cable distribution network to individual subscribers. In step  770 , method  700  ends. 
         [0049]    In an alternative embodiment, steps  730 ,  740 , and  750  can serially be repeated multiple times. When they are repeated the frequency will be adjusted only a portion of the desired adjustment on each repeated cycle of these three steps. If these steps are repeated, in step  750 , the upsampled and upconverted signals are combined together to produce two or more composite signals until these series of steps are repeated for the last time. The last time the steps are repeated, step  750  should produce a single combined single. In step  760  this signal would then be converted to an analog signal. 
         [0050]      FIG. 8  is a method  800  for digital upconversion of baseband television signals, according to an embodiment of the invention. Method  800  begins in step  810 . In step  810 , a digital baseband television signal is received. In step  815 , the received digital baseband television signal is demultiplexed. In step  820 , the demuxed signals are upsampled in frequency. In step  825 , the demuxed signals that have been upsampled are then upconverted. In step  830 , the signals produced in step  825  are summed together to create at least two bands containing upconverted digital signals. In step  835 , the bands containing upconverted digital signals are converted to analog signals. In step  840 , the analog signals are upconverted in frequency within the analog domain. In step  845 , the upconverted analog signals are filtered to extract the desired frequency bands. Steps  840  and  845  can be repeated to upconvert the frequency in multiple steps, instead of using a single upconversion. In step  850 , the extracted frequency bands are combined to create an analog signal for transmission within a cable television system. In step  870 , method  800  ends. 
         [0051]    In an alternative embodiment, steps  820 ,  825  and  830  can serially be repeated multiple times. When they are repeated the frequency will be adjusted only a portion of the desired adjustment on each repeated cycle of these three steps. If these steps are repeated, in step  830 , the upsampled and upconverted signals are combined together to produce two or more composite signals. In step  840  outputs produced in step  830  would be converted to analog signals. 
         [0052]    Exemplary embodiments of digital headend conversion systems and methods that can be used to upconvert the frequency of a received digital television baseband signal to produce an RF multi-channel television spectrum for distribution. The present invention is not limited to these examples. These examples are presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the present invention.