Abstract:
A modulator generates a combined signal consisting of audio and video signals and converts the combined signal to one of a plurality of frequencies in dependence upon a desired output frequency and broadcast standard. The modulator includes a summing amplifier, a first frequency synthesizer and a second frequency synthesizer. The summing amplifier has a first input for receiving a video signal, a second input for receiving a first audio signal, a third input for receiving a second audio signal, and an output for outputting a modulated summed signal. The first frequency synthesizer generates a first frequency for mixing with the modulated summed signal to generate a high intermediate frequency (HI-IF) signal. The second frequency synthesizer generates a second frequency for mixing with said HI-IF signal to generate a desired RF output signal.

Description:
BACKGROUND 
   The present invention generally relates to cable television (CATV) and consumer video communication systems. More particularly, the invention relates to a dual-conversion universal modulator having programmable synthesized phase-locked loop oscillators driving their respective mixers which select a specific HI-IF frequency depending upon what output frequencies or standards are desired. Such standards include NTSC, PAL, NICAM, DIN, SECAM and any other known standard. 
   To allow reception of more than the 12 VHF channels on an older television receiver, most CATV systems require a settop terminal at a subscriber&#39;s location. Today, settop terminals not only provide a means for accepting a plurality of channels broadcast with varying bandwidths and guardbands for forward and reverse frequencies, but they also secure pay television services from unauthorized viewing. Other functions include decoding digital video and audio, interactive services, creating personalized viewer channels and the like. 
   In addition to the conversion from a cable transmission to a standard output frequency, a variety of descrambling techniques are employed depending upon the techniques used at a system headend. CATV equipment manufacturers are developing more sophisticated scrambling techniques using complicated encryption methods and digital processing to thwart pirating. 
   Most settop terminals are tunable. A block diagram for a prior art settop terminal is shown in  FIG. 1 . Incoming signals from a CATV transmission network are coupled to an input bandpass amplifier and up-converted to a high intermediate frequency (HI-IF). The up-conversion requires a tunable local oscillator which selects a desired channel and an associated mixer. The mixer is coupled to a bandpass filter and down-converted to an IF channel using a fixed-frequency local oscillator and mixer. The output channel is filtered and forwarded to a subscriber&#39;s television receiver. Prior art settop terminals use one down-converter mixer with an oscillator having slight frequency agility to provide an output at one or two preselected channel frequencies. The output frequencies and bandwidths depend upon the transmission standard used. 
   In the United States, the NTSC (National Television System Committee) is the standard for color television. Other countries have chosen different systems. SECAM (sequentiel couleur avec mémoire) is used by France and Russia. PAL A and PAL B (phase alternation line) are used by many European countries such as Germany and the United Kingdom. Accordingly, television receivers are typically manufactured for a specific transmission standard. For worldwide use, a settop terminal must be adapted to the established broadcast standards. 
   U.S. Pat. No. 5,640,697 teaches the use of two predetermined frequencies for each local oscillator, whereby the second oscillator frequency can be adjusted independently of the first oscillator frequency. Adjustment between the two frequencies is used to adapt to the different output frequencies, while eliminating noise caused by the local oscillators. Similar to U.S. Pat. No. 5,640,497, German Patent No. Application 4,306,578 adjusts the oscillator frequencies by a predetermined amount in order to eliminate noise. PCT International Patent Application No. 84/04637 employs two local oscillators that generate predetermined frequencies, in which the second oscillator is selected between one of two frequencies to eliminate this noise. 
   Accordingly, there exists a need for an inexpensive method to adapt the output of a settop terminal to a variety of television broadcast standards. 
   SUMMARY 
   The present invention is a universal modulator that accepts baseband audio and video inputs and modulated audio or data and converts the combined signal to one of a plurality of frequencies in dependence upon a desired output frequency and broadcast standard. The universal modulator is located between baseband video and audio outputs of a settop terminal demodulator/decoder and an antenna input of a television receiver or other audio/video component (such as a VCR). The universal modulator includes a dual conversion architecture using an up-converter mixer and a down-converter mixer. Each mixer receives an oscillator input from a corresponding addressable, programmable, PLL (phase-locked loop) frequency synthesizer. Each PLL frequency is controlled by firmware in the settop terminal. Configuration is performed via manual input using settop terminal controls, or interrogation directly by the CATV headend or by programmed settings. A communication bus coupled to the firmware distributes addressable instructions to selectively control each PLL frequency and obviate oscillator difference beat frequencies (ODBFs) that may be manifested. 
   Accordingly, it is an object of the present invention to provide a universal modulator within a settop terminal which is able to couple a CATV transmission network to a customer&#39;s television receiver notwithstanding the broadcast standard used to transmit the television programs. 
   Other objects and advantages will become apparent to those skilled in this art after reading the detailed description of the preferred embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art CATV settop terminal. 
       FIG. 2  is a block diagram of a settop terminal incorporating the present invention. 
       FIG. 3  is a block diagram of the preferred embodiment of the universal modulator of the present invention for use in a settop terminal. 
       FIG. 4  is a block diagram of an addressable, programmable, phase-locked loop. 
       FIG. 5  is a flow chart of the universal modulator configuring process. 
       FIG. 6  is a flow chart of the ODBF translation process. 
       FIG. 7  is a block diagram of a prior art headend. 
       FIG. 8  is a block diagram of a headend made in accordance with the present invention. 
       FIGS. 9A and 9B  are graphs of oscillator difference beat frequencies. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiment will be described with reference to the drawing figures where like numerals represent like elements throughout. 
     FIG. 2  is a block diagram of a settop terminal  17  with a universal modulator  19  shown coupled to the outputs  23 ,  25 ,  27  of a demodulator/decoder  21 . The demodulator/decoder  21  supplies a customer&#39;s channel selection to the universal modulator  19  as a baseband audio signal via the output  23  and as a baseband video signal via the output  25 . An alternate (second) audio signal, such as a NICAM carrier or modulated audio signal which differs from the baseband audio signal, may also be supplied to the universal modulator  19  via the output  27  of the demodulator/decoder  21 . A reference clock signal  37  originating from a master oscillator (not shown) and a common communication bus  39  are also coupled to the universal modulator  19 . The functional description of the demodulator/decoder  21  is beyond the scope of the present invention and shall not be described in further detail. 
   The higher quality baseband audio and video signals provided by outputs  23  and  25  of the demodulator/decoder  21  are made available as settop terminal outputs  31 ,  33 , respectively, and may be coupled to television receivers that have baseband inputs. The alternate audio signal provided by output  27  may be made available as settop terminal output  29 . For television receivers that lack these features, the universal modulator  19  provides an up-conversion output  35  compatible with the television broadcast standard used, from baseband to VHF or UHF for coupling to an antenna input. 
   The universal modulator  19  is shown in more detail in  FIG. 3 . The common communication bus  39  shown is an I 2 C interface from Phillips® Electronics. Other bus communication protocols may alternatively be used. The configuration for a settop terminal  17  may be downloaded from the CATV system headend via a dedicated channel, or inband on the VBI of a channel. One skilled in this art would appreciate that an advanced cable system can address and interrogate a specific settop terminal and alter its functionality. If the settop terminal has all configurations stored in firmware, the CATV system headend may simply instruct the settop terminal  17  of the standard being used. In this fashion, the settop terminal  17  does not require a technician to configure the unit but can auto-configure upon initial energization. 
   The communication bus protocol permits configuring component parameters to a particular broadcast standard using a unique addressing system within the settop terminal  17 . As shown in  FIG. 3 , the I 2 C bus  39  communicates with: an addressable programmable PLL frequency synthesizer  41  for a baseband audio mixer  69 , a solid state switch  43 , adjustable amplifiers for the baseband video input  45  and baseband audio input  47 , an addressable programmable PLL frequency synthesizer  49  for an up-conversion mixer  91  and an addressable programmable PLL frequency synthesizer  51  for a down-conversion mixer  101 . Although the addressable programmable PLL frequency synthesizer  51  has been described as being coupled to a “down-conversion” mixer  101 , the down-conversion mixer  101  may in fact further up-convert a HI-IF signal  93  to a higher frequency signal. It should be noted that each PLL frequency synthesizer  41 ,  49 ,  51  has an associated oscillator driver L01, L02, L03 respectively (not shown). Each respective component has its own address to permit firmware contained parameters to be loaded for a given broadcast standard configuration. 
   An alternate (second) audio carrier  53 , provided by the output  27  of the demodulator/decoder  21 , is coupled to the solid state switch  43 . The output of the switch  43  is coupled to a first input  55  of a summing amplifier  57 . The baseband video input  59  is coupled to a clamp  61  which limits signal amplitude. The output from the clamp  61  is coupled to the video adjustable amplifier  45  where signal gain is increased or attenuated depending upon the broadcast standard. The output from the adjustable amplifier  45  is coupled to a hard limiter  63  which clips signal peaks. The output from the limiter  63  is coupled to a second input  65  of the summing amplifier  57 . The baseband audio input  67  is coupled to a baseband audio mixer  69  via an adjustable amplifier  68 . The baseband audio mixer  69  modulates the baseband to the broadcast standard. The baseband audio mixer  69  may be selectively activated or deactivated by the I 2 C bus as required to support the standard in use. The output from the baseband audio mixer  69  is coupled to a lowpass filter  71  to remove RF. A second input to the audio lowpass filter  71  is provided as a modulated audio input  72 . The audio lowpass filter  71  is coupled to an audio adjustable amplifier  47  where signal gain is increased or attenuated. The audio adjustable amplifier  47  output is coupled to a third input  73  of the summing amplifier  57 . 
   Each of the mixers mix a signal input with the outputs of the three addressable, programmable PLL frequency synthesizers  41 ,  49 ,  51 . The PLL output frequencies vary depending on the broadcast standard and the RF output frequency  105  desired. An addressable, programmable PLL frequency synthesizer  41 ,  49 ,  51  is shown in  FIG. 4 . 
   The PLL  41 ,  49 ,  51  includes a phase detector  75 , a voltage-controlled oscillator (VCO)  77  and a loop filter  79 . The programmable PLL uses digital and analog techniques for frequency synthesis. The phase detector  75  compares two input frequencies  81   a ,  81   b  and generates an output  83  that is a measure of their phase difference. If both inputs  81   a ,  81   b  differ in frequency, the output is periodic at the difference frequency. If the frequency input does not equal the frequency output of the VCO  77 , the phase-error signal, after being filtered, causes the VCO frequency to deviate in the direction of the input frequency. When the frequencies match, the VCO  77  locks to the input frequency maintaining a fixed phase relationship with the input signal. The filtered output of the phase detector  75  is a dc signal. A modulo-n counter  87  is coupled between the VCO  77  output and the second input  81   a  to the phase detector  75  to generate a multiple of the input reference frequency providing frequency synthesis. 
   Each PLL synthesizer  41 ,  49 ,  51  employed in the present invention  19  is addressable such that the input frequency can be adjusted by using an input modulo-n counter  89  or divide-by-n to adjust output frequency. Both the input frequency divide-by-n  89  and loop frequency divide-by-n  87  are addressable components. Each of the PLLs  41 ,  49 ,  51  are addressed and controlled in accordance with a predetermined settop terminal  17  configuration. The configuration determines both the output frequency and operating bandwidth of the settop terminal  17  and adjusts the up- and down-converter PLLs  49 ,  51  accordingly. 
   Referring back to  FIG. 3 , the summer amplifier  57  output is modulated with the frequency output from the second programmable PLL  49  to drive the up-conversion mixer  91  and translate the summed output to a high intermediate frequency (HI-IF)  93 . The HI-IF  93  is higher than the highest expected frequency in the summed amplifier  57  output bandwidth. In the present invention  19 , the input to the up-conversion mixer  91  is not bandwidth limited. 
   The summing amplifier  57  output frequencies are translated to a new bandwidth, starting at a low frequency of the second PLL  49  minus the highest input band frequency, and ending at a high frequency of the third PLL  51  minus the lowest input band frequency. The second PLL  49  frequency is selected to translate the summing amplifier  57  output to correspond to the passband of an intermediate lowpass filter  95 . The output from the lowpass filter  95  is coupled to a buffer amplifier  97  to restore gain losses. The output from the buffer amplifier  97  is input to a final lowpass filter  99 . The buffer amplifier  97  maintains the system noise figure by overcoming the losses in the up-conversion mixer  91  and first HI-IF filter  95 . The signal is filtered by a HI-IF filter  99 , with the output coupled to a down-conversion mixer  101 . The third PLL synthesizer  51  is coupled to the down-conversion mixer  101 . The difference between the HI-IF  93  and the third PLL  51  frequency is the desired output channel in the IF band. It should, however, be noted that the down-conversion mixer  101  may accept the HI-IF  93  and further up-convert the signal to a higher frequency RF signal. The output is then filtered via a low pass filter  103 , (or other appropriate filter if up-converted), and forwarded as an RF output frequency  105  for reception by a television receiver. 
   As discussed above, the second  49  and third  51  programmable PLLs are controlled by the common communication bus  39 . The bus  39  is coupled to a processor in the settop terminal demodulator/decoder  21  which receives instructions from the system headend or from the settop terminal&#39;s  17  keypad. The configuration takes place transparently upon initial energization of the unit  17  if the system headend is equipped to send broadcast configuration instructions to the settop terminal  17 . If the system headend does not have this capability, the settop terminal  17  is configured via the keypad and function display (not shown). The configuration request, whether from the headend or at a consumer location, outputs the predetermined parameters onto the I 2 C bus  39  for each of the addressable components. The predetermined parameters are related to the standard that is being employed by the CATV system on which the settop terminal  17  is located. These parameters will include the determination of whether a second audio carrier  53  exists, whether the baseband audio input  67  or the modulated audio input  72  are to be used and the frequency at which the RF output frequency  105  is desired. These parameters may also include any other configurable parameters which are employed by any of the addressable components coupled to the communication bus  39 . It should also be recognized that since many of the components are addressable by the communication bus  39 , a user may manually input and address a particular component and selectively configure that component if desired. 
   An undesirable artifact of dual conversion is the generation of harmonics based on the fundamental oscillator frequencies. The harmonics of the second and third PLL frequency synthesizers  49 ,  51  mix with each other, thereby creating ODBFs. To obviate the intrusive effects of these PLL harmonics, the system and method of the present invention  119  eliminate this type of interference by translating the significant ODBFs out of the desired output channel. 
   A flowchart of the preferred method of the present invention  19  is shown in  FIG. 5 . Upon making the necessary connections to the CATV cable  15  and subscriber&#39;s television receiver, the settop terminal  17  is energized (step  201 ) establishing communication with the system headend. If the cable system headend has forward communication ability (step  205 ), the settop terminal is instructed how to configure itself for the applicable broadcast standard by downloading the parameters for the regional standards being used and the channel broadcast maps (step  207 ). The predetermined PLL frequencies derived from the channel and broadcast maps in memory are converted into corresponding “divide-by” numbers for the PLL modulo-n, converters  87 ,  89  and output to the second  49  and third  51  PLL frequency synthesizers. The settop terminal  17  acknowledges when configuration is complete. If the cable system does not have forward communication capability, the user will be prompted to enter the applicable information via a display and keypad, thereby manually loading the applicable broadcast configuration (step  209 ). 
   The settop terminal  19  reviews the loaded channel and broadcast maps. The predetermined frequencies are examined for potential ODBFs (step  211 ). If it is determined that ODBF&#39;s are likely (step  213 ), an ODBF translation is performed (step  215 ) as shown in  FIG. 6  (which will be explained in greater detail hereafter). Otherwise, the original frequencies are maintained (step  217 ) ( FIG. 5 ) The frequencies are addressed to their respective PLL synthesizers as words over the I 2 C communication bus (step  219 ). 
   Referring to the flow diagram of  FIG. 6 , the elimination of ODBFs is achieved by selectively adjusting the frequencies of the second  49  and third PLLs  51  to obtain the desired RF output frequency. For a typical NTSC signal, the up-converter mixer  91  modulates the input video  59  and audio signals  67  with the output  93  of the second PLL  49  to up-convert the input RF signal of the selected channel to the HI-IF  93  (step  301 ).
 
L01=audio carrier frequency  (Equation 1)
 
L02=HI-IF  (Equation 2)
 
   The down-converter mixer mixes  101  the HI-IF  93  with the output of the third PLL synthesizer  51  (step  303 ) to down-convert, (or further up-convert if desired), to obtain the desired RF output frequency  105 .
 
 L 03=( HI - IF )+ RF  output  (Equation 3)
 
   Multiples of the second and third PLL synthesizer  49 ,  51  fundamental frequencies define the even and odd harmonics,
 
m(L02) and m(L03), for m=1, 2, 3, 4, . . . ∞,  (Equation 4)
 
which represent all possible harmonics (step  305 ). However, due to the high system frequencies involved, examination of frequencies beyond the 10th harmonic is unnecessary.
 
   The existence of an interfering ODBF is determined by serially calculating the differences between two harmonics of the second  49  and  51  third PLL synthesizers that are separated by at least one degree until the absolute value of an ODBFm,n is within a given bandwidth or a predetermined number of ODBFm,n values are calculated. When an ODBFm,n absolute value is found within the RF channel bandwidth, it is designated as an interfering oscillator difference beat frequency (ODBF). The general equation for calculating ODBFs is:
 
 OBDF   m,n =( m+n ) ( L 02)−( m )( L 03), for m=1, 2, 3, 4, . . . 10,  (Equation 5)
 
with n=1 for a first series, n=2 for a second series, n=3 for a third series, and so on up to n=8 for all previously calculated harmonics (step  305 ). The ODBFm,n calculated from the differing degrees of the second  49  and third PLL  51  harmonics are then examined (step  307 ). For example, if the ODBF lies outside of the desired RF output channel bandwidth, no adjustment of the second  49  and third  51  PLL frequency synthesizers is required.
 
   For an ODBF which falls inband, the following equations can be used to determine which direction the second  49  and third  51  PLL frequencies should be adjusted to translate the ODBF out of band. In these equations, CLB is the channel low-band; CMB is the channel mid-band; and CHB is the channel high-band.
 
If −CHB≦ODBF&lt;−CMB; then HI-IF is moved downward.  (Equation 6A)
         (If the result of Equation 5 is negative and the magnitude is greater than the mid-band of the desired RF output channel (step  309 ), the HI-IF is moved downward (step  311 )).
 
If −CMB≦ODBF&lt;−CLB; then HI-IF is moved upward.  (Equation 6B)
   (If the result of Equation 5 is negative and the magnitude is less than or equal to the mid-band of the desired RF output channel (step  313 ), the HI-IF is moved upward (step  315 )).
 
If CLB≦ODBF≦CMB; then HI-IF is moved downward.  (Equation 6C)
   (If the result of Equation 5 is positive and the magnitude is less or equal to than the mid-band of the desired RF output channel (step  317 ), the HI-IF is moved downward (step  319 )).
 
If CMB&lt;ODBF≦CHB; then HI-IF is moved upward.  (Equation 6D)
   (If the result of Equation 5 is positive and the magnitude is greater than the mid-band of the desired RF output channel (step  321 ), the HI-IF is moved upward (step  323 )).       

   The second  49  and third  51  PLLs are then adjusted (step  327 ) in accordance with the following: To translate the oscillator difference beats below or above the desired RF output channel, the following equation is used to determine the A in frequency for the second  49  and third  51  PLL frequency synthesizers. 
   
     
       
         
           
             
               
                 Δ 
                 = 
                 
                   
                     CMB 
                     - 
                     
                       [ 
                       
                         
                           
                             ( 
                             
                               m 
                               + 
                               n 
                             
                             ) 
                           
                           ⁢ 
                           
                             ( 
                             LO2 
                             ) 
                           
                         
                         - 
                         
                           m 
                           ⁡ 
                           
                             ( 
                             LO3 
                             ) 
                           
                         
                       
                       ] 
                     
                   
                   
                     
                       ( 
                       
                         m 
                         + 
                         n 
                       
                       ) 
                     
                     - 
                     m 
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 ) 
               
             
           
         
       
     
   
   The new second  49  and third  51  PLL frequencies (L02′ and L03′ respectively) are derived as shown in L02′ is calculated as shown in  FIG. 6 . 
   The new PLL frequencies L02′ and L03′ translate the ODBFs above or below the desired RF output channel. The new PLL frequency values are used to program the second  49  and third  51  PLL frequency synthesizers (step  327 ). 
   In an alternative embodiment, a fixed value for Δ can be used to simplify the calculations and the operation of the system. For example, a value of 4 MHz for Δ will suffice for NTSC and PAL systems. 
   The present invention will now be explained with reference to several examples. In the first example, if the HI-IF is 960 MHz and the desired RF output channel has a picture carrier frequency of 319.25 MHz, we have the following:
 
L02=HI-IF=960 MHz; and  (from Equation 2)
 
   
     
       
         
           
             
               
                 
                   
                     
                       LO3 
                       = 
                       
                         HI 
                         - 
                         IF 
                         + 
                         
                           RF 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           output 
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           960 
                           + 
                           319.25 
                         
                         = 
                         
                           1279.25 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             MHz 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   from 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ) 
               
             
           
         
       
     
   
   The graph for ODBFs versus the RF output frequencies for m=2 and n=1 is shown in  FIG. 9A . If m=2 and n=1, then: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ODBF 
                           
                             2 
                             , 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           960 
                           ) 
                         
                       
                       = 
                         
                       ⁢ 
                       
                         
                           
                             ( 
                             
                               m 
                               + 
                               n 
                             
                             ) 
                           
                           ⁢ 
                           
                             ( 
                             LO2 
                             ) 
                           
                         
                         - 
                         
                           m 
                           ⁡ 
                           
                             ( 
                             LO3 
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           3 
                           ⁢ 
                           
                             ( 
                             960 
                             ) 
                           
                         
                         - 
                         
                           2 
                           ⁢ 
                           
                             ( 
                             1279.25 
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           2880 
                           - 
                           2558.5 
                         
                         = 
                         
                           321.5 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           MHz 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   from 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
         
       
     
   
   Since the desired RF output channel has a picture carrier frequency of 319.25 MHz (and assuming the bandwidth is 6 MHz for an NTSC channel), the ODBF is in-band for the desired RF output channel. From Equation 6D, since the ODBF is above the mid-band of the desired RF output channel, the HI-IF is moved upward. Assuming that Δ will be a fixed value of 4 MHz, L02′ will be 964 MHz and L03′ will be 1283.25 MHz. 
   Accordingly, from Equation 5: 
   
     
       
         
           
             
               
                 
                   
                     ODBF 
                     
                       2 
                       , 
                       1 
                     
                   
                   ⁡ 
                   
                     ( 
                     964 
                     ) 
                   
                 
                 = 
                   
                 ⁢ 
                 
                   
                     3 
                     ⁢ 
                     
                       ( 
                       964 
                       ) 
                     
                   
                   - 
                   
                     2 
                     ⁢ 
                     
                       ( 
                       
                         964 
                         + 
                         319.25 
                       
                       ) 
                     
                   
                 
               
             
           
           
             
               
                 = 
                   
                 ⁢ 
                 
                   
                     2892 
                     - 
                     2566.5 
                   
                   = 
                   
                     325.5 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     MHz 
                   
                 
               
             
           
         
       
     
   
   The ODBF is now out of band. 
   In the second example, if the HI-IF is 960 MHz and the desired RF output channel has a picture carrier frequency of 481.25 MHz, we then have the following:
 
L02=HI-IF=960 MHz; and (from Equation 2)
 
   
     
       
         
           
             
               
                 
                   
                     
                       LO3 
                       = 
                       
                         HI 
                         - 
                         IF 
                         + 
                         
                           RF 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           output 
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           960 
                           + 
                           481.25 
                         
                         = 
                         
                           1441.25 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             MHz 
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   from 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ) 
               
             
           
         
       
     
   
   The graph for ODBFs versus the RF output frequencies for m=3 and n=1 is shown in  FIG. 9B . If m=3 and n=1, the ODBF can be calculated as: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ODBF 
                           
                             3 
                             , 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           960 
                           ) 
                         
                       
                       = 
                         
                       ⁢ 
                       
                         
                           4 
                           ⁢ 
                           
                             ( 
                             960 
                             ) 
                           
                         
                         - 
                         
                           3 
                           ⁢ 
                           
                             ( 
                             1441.25 
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         3840 
                         - 
                         4323.75 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           - 
                           483.75 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           MHz 
                           . 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   from 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
         
       
     
   
   Since the selected channel is 481.25 MHz, (and assuming an NTSC channel), the ODBF is in-band and the HI-IF must be relocated. The result of Equation 5 for this example is negative and the magnitude is greater than the mid-band of the desired RF output channel (481.25 MHz). Accordingly, from Equation 6A, the HI-IF is moved lower. Assuming that A will be a fixed value of 4 MHz, L02′ will be 956 MHz and L03′ will be 1437.25 MHz. Recalculating the ODBF provides: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ODBF 
                           
                             3 
                             , 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           956 
                           ) 
                         
                       
                       = 
                         
                       ⁢ 
                       
                         
                           4 
                           ⁢ 
                           
                             ( 
                             956 
                             ) 
                           
                         
                         - 
                         
                           3 
                           ⁢ 
                           
                             ( 
                             
                               956 
                               + 
                               481.25 
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         3824 
                         - 
                         4311.75 
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         
                           - 
                           487.75 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           MHz 
                           . 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   from 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
         
       
     
   
   The ODBF is now out of band. 
   Due to the simple design of the present invention and since there are no shielding requirements to avoid ODBFs, the universal modulator may be incorporated onto a single integrated circuit. This was not possible with prior art signs. 
   Although the present invention has been described with reference to a settop terminal, it should be understood by those of skill in the art that the invention is adaptable to other applications within the CATV environment, or even other communication applications which do not pertain to CATV systems. 
   For example, as shown in  FIG. 7 , a prior art headend  700  generally includes two pieces of equipment; a baseband section  702  and an IF section  704 . These two sections  702 ,  704  are typically designed to operate as “stand alone” units. Together, the two sections  702 ,  704  output a single RF channel. The baseband section  702  generally comprises a video section  706  and an audio section  708 . These sections  706 ,  708  receive audio and video baseband inputs and combine these inputs to an intermediate frequency for output to the IF section  704 . In the IF section  704 , the intermediate frequency is up-converted to the desired RF output channel. Since both sections  702 ,  704  comprise units of equipment which are designed to work independently, this requires the duplication of many components between units  702 ,  704 . 
   Referring to  FIG. 8 , a headend  800  made in accordance with the present invention is shown. The headend  800  includes an audio pre-processing section  802 , a video pre-processing section  804 , the universal modulator  808  of the present invention (which is coupled to two filters  810 ,  812 ), a transmitter  814  (if desired), and a microprocessor  806 , which controls all of the components of the headend  800 . As was previously described hereinbefore, since the universal modulator  808  can convert a baseband input signal to any desired RF output signal while avoiding ODBFs, the universal modulator  808  may be utilized to replace most of the components in the baseband section  702  and the IF section  704 . This significantly reduces the number of components required for a headend  800 . Accordingly, the cost and complexity are also thereby reduced. 
   It should be understood by those of skill in the art, with reference to  FIG. 8 , that the universal modulator  808  of the present invention may also be used to accept a baseband digital VSB signal and remodulate the signal to a desired RF output signal for use with broadcast HDTV television receivers. The universal modulator  808  could also be used to transmit RF signals to devices which require high frequency RF signals, including wireless appliances such as a cordless telephone or a wireless LAN receiver. In such an application, the second mixer up-converts the HI-IF signal to a higher frequency RF signal, instead of down-converting the HI-IF as previously described. The desired RF output signal would be:
 
RF output ═(HI-IF)+L03  (Equation 10)
 
   The RF output signal may then be transmitted directly to the wireless appliance.