Abstract:
An IF signal processing arrangement for processing both analog and digital signals is disclosed in the present application. The signal processing arrangement includes a signal source for providing one of digital and analog IF signals, a first SAW filter having an output for filtering the IF signal, digital signal processing circuitry coupled to the output for processing a filtered digital IF signal, and analog signal processing circuitry coupled to the output for processing which includes filtering a filtered analog signal.

Description:
This application claims the benefit of U.S. provisional application Ser. No. 60/130,167 filed Apr. 20, 1999, which is hereby incorporated herein by reference, and which claims the benefit under 35 U.S.C. § 365 of International Application PCT/US00/10724, filed Apr. 20, 2000, which was published in accordance with PCT Article 21(2) on Oct. 26, 2000 in English. 

   FIELD OF THE INVENTION 
   This application relates to intermediate frequency (IF) signal processing. 
   BACKGROUND INFORMATION 
     FIG. 1  is exemplary of a known tuner and IF signal processing apparatus indicated generally by the reference numeral  10 . This tuner and IF signal processing apparatus  10  is for use with analog signals of a given broadcast standard, such as NTSC, PAL, and SECAM. This application describes NTSC application of the invention as an exemplary embodiment. 
   The Tuner/IF system  10  comprises a tuner  12  (with RF input  14  and IF output  16 ), IF filter(s)  18 , and IF signal processor  20 . The tuner&#39;s IF output  16  is at the standard frequency (e.g., desired channel spectrum centered at 44 MHz, picture carrier at 45.75 MHz, and sound carrier at 41.25 MHz). The IF filter(s)  18 , which usually consist of one SAW (surface acoustic wave) filter for inter-carrier IF systems or two SAWF for parallel picture and sound IF systems, pass the desired channel and reject all others. In the parallel system one SAW filter passes the desired sound signal only, and the other passes the desired picture signal only. In either case, the filter characteristics include a “Nyquist Slope” through the double-side-band region of the picture IF spectrum. The filtered signal is applied to a conventional IF processing circuit  20  which performs such general functions as demodulation, AGC generation and the like and provides a processed baseband video output signal to a video processing circuit. The video processing circuit performs conventional functions such as color demodulation and other functions such as brightness, hue and tint control and the like. 
   With the advent of digital television (DTV), and specifically digital terrestrial television such as HDTV (high definition television), television receivers and their corresponding tuner/IF systems which provide proper tuning and filtering required for processing both NTSC and DTV signals are needed. 
     FIG. 2  illustrates a modification of the NTSC only system of  FIG. 1  to provide a tuner/IF system  22  able to be used for both NTSC and DTV reception. In  FIG. 2 , a tuner  24  is modified to provide reception of both NTSC and DTV signals. Conversion signals are appropriately selected such that both types of signals produce a common IF signal frequency (e.g., about 44 MHz). Two SAW filters  26  and  28  are coupled in parallel with the output of the NTSC/DTV tuner. SAW BPF # 1   26  has specific requirements for DTV signal reception and processing while SAW BPF # 2   28  has specific requirements for NTSC signal reception and processing. For example, both SAW BPF # 1   26  and SAW BPF # 2   28  have a center frequency of about 44 MHz. However, SAW BPF # 1  has a flat passband response, while SAW BPF # 2  has the characteristics described above for the IF Filter(s)  18  in  FIG. 1 . The DTV filtered signal is applied to Digital IF processing circuitry  30 . The digital IF processing circuitry  30  provides the filtered and processed DTV signal to Digital Link (i.e., decoder) circuitry (not shown). The NTSC filtered signal is applied to NTSC IF processing circuitry  32 . The NTSC IF processing circuitry  32  provides the filtered and processed NTSC signal to video processing circuitry (not shown). 
   Due to passband flatness requirements for DTV signals, the DTV/NTSC tuner  24  in  FIG. 2  has a wider bandwidth than the NTSC tuner  12  in  FIG. 1 . Consequently, the NTSC adjacent channel rejection of system  22  in  FIG. 2  is not as good as system  10  in  FIG. 1 . 
   SUMMARY 
   An If signal processing arrangement for processing both analog and digital signals is disclosed in the present application. The signal processing arrangement includes a signal source for providing one of digital and analog IF signals, a first SAW filter having an output for filtering the IF signal, digital signal processing circuitry coupled to the output for processing a filtered digital IF signal, and analog signal processing circuitry coupled to the output for processing which includes filtering a filtered analog signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated in the accompanying drawings, wherein: 
       FIG. 1  (Prior Art) is a block diagram of a conventional NTSC Tuner/IF configuration for processing NTSC television signals; 
       FIG. 2  is a block diagram of a parallel DTV/NTSC Tuner/IF configuration for tuning both DTV and NTSC signals; 
       FIG. 3  is a simplified block diagram of a dual DTV/NTSC Tuner/IF apparatus embodying the features of the present invention; 
       FIG. 4  is a detailed block diagram illustrating a practical implementation and additional features of the embodiment of  FIG. 3 ; 
       FIG. 5  is a tuner frequency response diagram comparing the response of a conventional NTSC only tuner with that of the embodiments of  FIGS. 3 and 4 ; 
       FIG. 6  is a tuner frequency response diagram comparing the selectivity of a conventional NTSC only receiver to that of the embodiments of  FIGS. 3 and 4  of the present invention; and 
       FIG. 7  is a tuner frequency response diagram illustrating certain aspects of sound intermediate frequency (SIF) processing for the embodiment of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention recognizes that passing the NTSC signal through SAW BPF # 1  (i.e., connecting the input of SAW BPF # 2  to the output of SAW BPF # 1  instead of the tuner output) provides dramatically better adjacent channel and spurious signal rejection than system  10  in  FIG. 1 . This change is illustrated in  FIG. 3 . These benefits are significant as NTSC adjacent channel rejection (1st adjacent, 2nd adjacent, etc.) is a parameter that becomes significantly more important during the HDTV transition period in which VHF and UHF “taboos” (i.e., restrictions) on adjacent channel frequency allocations may be discarded or significantly reduced to make more spectral room available for the new HDTV terrestrial transmission channels while still accommodating current NTSC terrestrial channel allocations. 
   A simplified diagram of the NTSC/DTV tuner/IF signal processor of the present invention is illustrated in  FIG. 3  and indicated generally by the reference numeral  40 . A more detailed diagram illustrating further features of the tuner/IF signal processor  40  of the present invention is shown in  FIG. 4 .  FIGS. 3 and 4  show that the input to the NTSC SAW filter is taken from the output of a DTV SAW filter. Thus, the received NTSC signal is “double filtered”. 
   Advantageously, the configurations of  FIGS. 3 and 4  provide significant improvement in NTSC adjacent channel rejection and rejection of other out-of-band undesired signals as compared with the examples of  FIGS. 1 and 2 . 
   In  FIG. 3 , the NTSC/DTV tuner/IF signal processor  40  is provided with conversion frequencies which result in a common IF output signal frequency (e.g., 44 MHz) for both NTSC and DTV reception modes. The NTSC/DTV tuner/IF signal processor  40  includes a single conversion tuner  42  able to receive both NTSC and DTV signals. The tuner  42  provides received NTSC and DTV signals to a first SAW filter  44 . The output of the first SAW filter  44  is provided to both a second SAW filter  46  and digital IF processing circuitry  48 . The digital IF processing circuitry  48  receives the filtered signal from the first SAW filter  44  and provides a near baseband output signal to digital “link” or decoder circuitry (see  FIG. 4 ). 
   When an NTSC signal is received, the received signal is provided to the first SAW filter  44 . The first SAW filter  44  filters the received NTSC signal and provides the filtered signal to the second SAW filter  46 . The IF signal thus passes through both filters for application to the NTSC IF processing circuitry  50  thereby reducing the undesirable effects of adjacent channel interference discussed above. 
     FIG. 4  is a practical implementation of  FIG. 3 , and includes more details. With the current state-of-the-art technology, the filter characteristics required for the DTV IF signal cannot be achieved with a single SAW filter as depicted by SAW BPF # 1   44  in  FIG. 3 . That is, the selectivity requirements cannot be met without excessive insertion loss. The excessive insertion loss would ultimately result in inferior system noise figure or severe linearity requirement for a preamplifier. In practice, the required filter and performance characteristics are achieved by cascading two identical SAW filters, Digital SAW # 1  and Digital SAW # 2 , with inter-stage amplifiers Post-amp  54  and Pre-amp  56  as shown in  FIG. 4 . Thus, when a DTV signal is selected by the NTSC/DTV tuner  42 , it is processed through the cascaded circuits comprised of Digital SAW # 1 , Post-amp  54 , Pre-amp  56 , and Digital SAW # 2   58 , to input of a 2 nd  converter circuit  60 . 
   The Post-amp  54  provides an optimum load impedance for Digital SAW # 1   52  and compensates for its loss. Similarly, the Pre-amp  56  provides an optimum source impedance for Digital SAW # 2   58  and compensates for its loss. The output of the 2nd converter circuit  60  is converted from an analog signal to a digital signal by the A/D Converter  76 , the digital processor  78  demodulates the digital signal and separates the picture and sound signals, and the signals are converted back to analog format by a pair of digital-to-analog converters  80 . 
   When an NTSC signal is selected by the NTSC/DTV tuner  42 , there are separate paths for the picture and sound signals. The picture signal is processed through Digital SAW # 1   52  and Post-amp  54  before being processed through the Pre-amp  64  and conventional NTSC Picture SAW # 3   66  to the NTSC processor  68 . Since the sound carrier frequency is at the band-edge of Digital SAW # 1   52  and the sloped frequency response through the sound channel will have undesirable effects (see  FIG. 7 ), the sound signal does not pass through Digital SAW # 1  and Post-amp  54 . Instead the sound signal is processed through Pre-amp  70  and NTSC Sound SAW # 4   72  to the NTSC IF processor  68 . The Pre-amp  64  provides an optimum source impedance for NTSC Picture SAW # 3   66  and compensates for its loss. 
   Similarly, the Pre-amp  70  provides the optimum source impedance for NTSC Sound SAW # 4  and compensates for its loss. Processing the picture signal through Digital SAW #  1   52  and Post-amp  54  provides the advantage of better selectivity (e.g., adjacent channel rejection and spurious signal immunity). The NTSC IF processor demodulates the picture and sound signals and provides composite video and audio baseband outputs. 
   Both the Digital IF 2 nd  Converter  60  and NTSC IF Processor generate RF AGC control signals that are applied to a RF AGC Switch  74 . The output of the RF AGC Switch  74  controls the gain of the NTSC/DTV tuner  42 . Similarly, the NTSC IF processor  68  and D/A converter  80  picture and sound signals are applied to an Audio-Video Selection and Display Processor  82  whose output drives an internal or external display unit  84 . When the system is installed in a new location, an automatic setup procedure determines which type of signal (NTSC or DTV) is present on each channel and stores the results in memory (not shown). Then whenever a new channel is selected, the system microprocessor (not shown) uses the data stored in memory to properly set the RF AGC switch  74  and the Audio-Video Selection Switch  82 . 
     FIG. 5  compares the frequency response of the NTSC only tuner  12  ( FIG. 1 ) and the NTSC/DTV tuner  42  ( FIG. 4 ) of the present invention. The wider bandwidth (i.e., poorer selectivity) of the NTSC/DTV tuner is a negative consequence of being able to maintain the passband flatness requirements for DTV signals. 
     FIG. 6  compares the frequency response of the NTSC only tuner  12  ( FIG. 1 ) to the frequency response through the NTSC/DTV tuner  42 , Digital SAW # 1   52 , and Post-amp  54  ( FIG. 4 ). Since the frequency response of the subsequent circuits is the sate for both systems, this illustrates the huge advantage in passing the NTSC picture IF signal through Digital SAW # 1   44  and Post-amp  54  instead of directly to the input of the Pre-amp  64  in the NTSC IF  50 . Doing so more than compensates for the relatively poor selectivity of the NTSC/DTV tuner  42 . The selectivity of the present invention, as seen in  FIG. 6 , is indicative of excellent adjacent channel rejection and spurious signal immunity. 
   It will be noted that the embodiment of  FIG. 4  has two SAW filters in the DTV signal path, and the NTSC picture signal is only passed through the first of these two SAW filters. This is the preferred configuration because it achieves the selectivity requirements with negligible degradation to system noise figure. That is passing the NTSC picture signal through both SAW filters in the DTV signal path is not necessary from a selectivity standpoint, and the additional degradation in system noise figure may be significant. 
     FIG. 7  is a graph illustrating the effects of passing the NTSC sound IF signal through the Digital SAW # 1   52  as opposed to bypassing the Digital SAW # 1   52 . The results obtained by passing the NTSC sound signal through the Digital SAW # 1   52  differ drastically from the results obtained by bypassing the Digital SAW # 1   52 . As can be seen from this figure, when the sound IF signal is provided from the single conversion tuner  42  directly to the NTSC sound SAW filter  72 , the frequency response through the sound channel is constant. Such is not the case for passing the NTSC sound IF signal through the Digital SAW # 1   52 . It is thus beneficial for the NTSC sound IF signal to bypass the Digital SAW # 1   52 . 
   Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of examples and that changes in details of arrangement may be made without departing from the spirit the invention. For example, the analog television signals may include PAL and SECAM television signals, and the digital television (DTV) signals may include QAM and digital VSB television signals. In addition, the DTV signal processing after Digital SAW # 2  might utilize another approach (e.g., A/D conversion and digital demodulation immediately after Digital SAW # 2 ).