Abstract:
The present application generally relates to apparatuses such as television signal processing apparatus that process radio frequency signals. More specifically, the present application is particularly useful in integrated circuits that must combine circuitry operating in a synchronous-sampling mode that must be adapted for use with a fixed rate sampling mode application. According to an exemplary embodiment, the television signal processing apparatus comprises a source of a fixed rate digital signal, signal processing circuitry operating in a synchronous-sampling mode wherein the signal processing circuitry comprises a signal representing a symbol rate, and an interpolator for processing the fixed rate digital signal to yield samples at the symbol rate.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/440,734, filed Jan. 17, 2003, and entitled “METHOD FOR USING A SYNCHRONOUS SAMPLING DESIGN IN A FIXED-RATE SAMPLING MODE”, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present application generally relates to apparatuses such as television signal processing apparatus, that process radio frequency signals. More specifically, the present application is particularly useful in integrated circuits that combine circuitry operating in a synchronous-sampling mode that must be adapted for use with a fixed rate sampling mode application.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present application generally relates to apparatuses such as television signal processing apparatus, that process radio frequency signals. More specifically, the present application is particularly useful in integrated circuits that must combine circuitry operating in a synchronous-sampling mode that must be adapted for use with a fixed rate sampling mode application.  
         [0004]     Modern signal processing apparatus typically include signal processing circuitry for processing a multitude of signal formats, such as NTSC, ATSC, QAM, or satellite signals. Such a signal processing apparatus typically includes various components such as a tuner for selecting a particular signal or channel from a plurality of signals or channels received by the apparatus. To process digital signals, such as ATSC or satellite signals, the signal processing circuitry, and in particular the tuner, must perform these functions with high-speed digital circuitry. Some digital signal processing apparatus operate in a synchronous-sampling mode, where the A/D converter takes samples coincident with the digital symbol locations. The digital symbols, and subsequently the sampling frequency are calculated by the demodulator and a rate control signal is output from the demodulator to control the sampling rate of the A/D. It is also possible to take samples using an A/D converter at a fixed time intervals.  
         [0005]     It is often a major design change in terms of time and expense to convert a design originally intended to operate in synchronous-sampling mode to operate in a fixed-rate sampling mode. This is primarily due to the requirement for an enable signal to be provided to all of the memory elements in the design. An enable signal is required throughout the design to identify when processing is to proceed since the demodulator is running at a high rate and not every clock signal is accompanied by a digital symbol. A thorough knowledge of the original design is usually required to effectuate the design change and re-verification is required to be carried out. In situations of design reuse, it would be advantageous to introduce a preprocessing block that can convert the fixed rate samples to synchronous samples with requiring the necessity of an enable line.  
         [0006]     Furthermore, in digital signal processing applications, there are typically many different clocks used to drive the processing circuitry. These clocks are typically derived from a phase-locked loop (PLL). When the data is gathered through an A/D converter, using the PLL output to clock the A/D converter can degrade its performance as high speed A/D converters are sensitive to clock jitter. When an external clock is used to drive the A/D converter, a synchronization problem arises because of the unknown phase between the A/D clock and the PLL output clock. Previously, designers have used clock resynchronizers or back to back flip flops on the reference clock and PLL clock lines. This solution is based on the assumption that a “bad phase” occurs only some of the time. However, if the system starts up in the “bad phase” it will continue to operate constantly at the bad phase. This results in the data latching and putting the system into an unstable state. Therefore the robustness of the back-to-back flip flop approach is questionable. It is desirable to have an AND clock to be used by the digital signal processing circuitry that is synchronized to the PLL output clock to facilitate latching the AND output and preventing problems associated with clock jitter.  
       SUMMARY OF THE INVENTION  
       [0007]     In accordance with an aspect of the present invention, a signal processing apparatus signal processing apparatus comprising a source of a fixed rate digital signal, a signal processor operating in a synchronous-sampling mode for producing a control signal representing a symbol rate, and an interpolator responsive to the control signal for processing the fixed rate digital signal to yield samples at the symbol rate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a block diagram of a television signal processing apparatus according to an exemplary embodiment of the present invention;  
         [0010]      FIG. 2 a  block diagram of an exemplary embodiment of digital signal processing circuitry utilizing an A/D converter operating according to a fixed rate sampling mode concurrently with subsequent signal processing circuitry operating according to a synchronous sampling mode;  
         [0011]      FIG. 3 a  block diagram of clock generator circuitry according to an exemplary embodiment of the present invention;  
         [0012]      FIG. 4  is a diagram of a clock divider circuitry of a clock generator according to an exemplary embodiment of the present invention; and  
         [0013]      FIG. 5  is a timing diagram of the clock divider circuitry according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     The exemplifications set out herein illustrate preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
         [0015]     Referring to  FIG. 1 , a block diagram of an exemplary embodiment of television signal processing apparatus  100  of  FIG. 1  is shown. In  FIG. 1 , television signal processing apparatus  100  comprises signal receiving means such as signal receiving element  110 , tuning means such as tuner  130 , demodulation means such as demodulator  140 , decoding means such as decoder  170 , processing means and memory means such as processor and memory  180 , audio amplification means such as audio amplifier  190 , audio output means such as speaker  135 , video processing means such as video processor  145 , and visual output means such as display  155 , a power supply  125  and a switch  115  responsive to the processor and memory  180 . Some of the foregoing elements may for example be embodied using integrated circuits (ICs). For clarity of description, certain conventional elements of television signal processing apparatus  100  including control signals may not be shown in  FIG. 1 . According to an exemplary embodiment, television signal processing apparatus  100  may receive and process signals in analog and/or digital formats.  
         [0016]     Signal receiving element  110  is operative to receive signals including audio, video and/or auxiliary data from signal sources, such as radio frequency broadcast signal transmission sources, or cable television transmission. Signal receiving element  110  may be embodied as any signal receiving element such as an antenna, input terminal or other element.  
         [0017]     Tuner  130  is operative to tune signals including audio, video and/or auxiliary data signals. Accordingly, tuner  130  may tune signals for the main picture of television signal processing apparatus  100 . According to an exemplary embodiment, television signal processing apparatus  100  may further include a picture-in-picture (PIP) function wherein the first channel includes audio and/or video signals for a main picture, and a second channel (not shown) includes audio and/or video signals for the PIP function. Demodulator  140  is operative to demodulate signals provided from tuner  130 , and may demodulate signals in analog and/or digital transmission formats.  
         [0018]     Decoder  170  is operative to decode signals including audio, video and/or auxiliary data signals provided from the demodulator  140 . According to an exemplary embodiment, decoder  170  decodes digital data that represents program guide data or emergency alert signals indicating an emergency event. Decoder  170  may also perform other decoding functions, such as decoding data which represents auxiliary data signals included in the vertical blanking interval (VBI) of an analog television signal.  
         [0019]     Processor and memory  180  are operative to perform various processing, control, and data storage functions of television signal processing apparatus  100 . According to an exemplary embodiment, processor  180  is operative to process the audio and video signals provided from decoder  170 , and may for example perform analog processing, such as National Television Standards Committee (NTSC) signal processing and/or digital processing, such as Motion Picture Expert Group (MPEG) processing.  
         [0020]     The processor and memory  180  is also operative to receive the auxiliary data signals from decoder  170  and determine what actions are required based on the auxiliary data received. For example, if EPG data is received, the processor  180  may decide to sort the EPG data and store the data in the processor&#39;s associated memory  180 . If the processor  180  receives auxiliary data associated with the emergency alert function of television signal processing apparatus  100 , it may compare data in the emergency alert signals to user setup data stored in memory  180  to determine whether the emergency alert function is activated to activate emergency alert signals.  
         [0021]     Audio amplifier  190  is operative to amplify the audio signals provided from processor  180 . Speaker  135  is operative to aurally output the amplified audio signals provided from audio amplifier  190 .  
         [0022]     Video processor  145  is operative to process the video signals provided from processor  180 . According to an exemplary embodiment, such video signals may include information based on the data contained in the received auxiliary data signals such as EPG information or emergency alert information. Video processor  145  may include closed caption circuitry that enables closed caption displays. Display  155  is operative to provide visual displays corresponding to processed signals provided from video processor  145 .  
         [0023]     Referring to  FIG. 2 , a block diagram of an exemplary embodiment of digital signal processing circuitry  200  comprising A/D converter  220  operating according to a fixed rate sampling mode concurrently with subsequent signal processing circuitry, such as a demodulator  240 , operating according to a synchronous sampling mode is shown. The digital signal processing circuitry further comprises a tuner  210 , interpolator  230 , clock generator  260 , and phase locked loop (PLL)  250  as well as a fixed rate clock  270 .  
         [0024]     In this exemplary embodiment shown in  FIG. 2 , the tuner  210  outputs an intermediate frequency (IF) analog signal. The A/D converter  220  takes samples of the IF analog signal at a fixed sampling rate. These fixed rate samples are taken at times corresponding to the digital clock signal input into the AND converter  220  by the fixed rate clock  270 . The fixed rate samples are read at the fixed rate by the interpolator  230  and a discrete number of samples are stored by the interpolator  230 , the number of samples depending on the interpolation method used by the interpolator  230 .  
         [0025]     The interpolated samples are then interpolated to yield samples at the symbol rate or some integer multiple thereof in the Interpolator  230  based on the rate control signal from the demodulator  240 . In a synchronous sampling mode of operation, the rate control signal might have originally been used to control the frequency of a voltage controlled oscillator (VCXO). As such, the interpolator  230  is designed such that its rate control input has the effect of mimicking the effect of this rate control signal going to a VCXO on the data samples delivered to demodulator  240 . The interpolator  230  operates using the fixed rate clock  270 , whereas the demodulator  240  runs on a burst clock generated by the clock generator  260 . The burst clock is enabled by the interpolator  230  when there are samples in the interpolator  230  ready to be processed by the demodulator  240 . There may be more than one clock frequency generated by the clock generator  260  going to the demodulator  240  and subsequent synchronous sampling mode circuitry. All of these clocks are allowed to run for 1 symbol of time for every symbol extracted from the interpolator  230 . For example, a clock running at 8 times the symbol rate would be allowed to run for 8 periods for every symbol taken from the interpolator  230 .  
         [0026]     Referring to  FIG. 3 , a block diagram of clock generator circuitry  300  according to an exemplary embodiment of the present invention is shown. In  FIG. 3 , the clock generator circuitry  300  comprises an AND converter  310 , a PLL  350 , a clock divider  360 , and a demodulator  340 . The clock divider  360  is used to synchronize the clock generated by the PLL and the reference clock, which is further explained in the discussion of  FIG. 4 , as well as creating multiple integers of the synchronized clock signal to be used by subsequent signal processing circuitry.  
         [0027]     Referring to  FIG. 4 , a diagram of a clock divider circuitry  400  of a clock generator according to an exemplary embodiment of the present invention is shown. In  FIG. 4 , the clock divider circuitry  400  comprises a plurality of D flip-flops  405 ,  410 ,  415 ,  420 ,  425 ,  460 ,  465 ,  470 , a plurality of AND gates  430 ,  435 ,  440 ,  445 , a plurality of OR fates  250 ,  255 . In the exemplary embodiment of the present invention shown in  FIG. 4 , five D flip-flops  405 ,  410 ,  415 ,  420 , and  425  are used to create a delay line for the reference clock. The PLL clock is used to advance the state of the delay line. The group of logic elements comprising the AND gates  430 ,  435 ,  440 ,  445  and the OR gates  450 ,  455  are used as a means for comparing the various output stages of the delay line  405 ,  410 ,  415 ,  420 ,  425 . For example, to generate the 1× clock, the state of the outputs of the first D flip-flop  405 , the second D flip-flop  410 , the fourth D filp-flop  420 , and the fifth D flip-flop are compared using the group of logic elements  430 ,  435 ,  440 ,  445 ,  450 ,  455 . The 1× clock is then passed through a final D flip-flop  460  to complete the synchronization of the reference clock with the PLL clock.  
         [0028]     Referring to  FIG. 5 , a timing diagram of the clock divider circuitry according to an exemplary embodiment of the present invention is shown. The timing diagram shown represents the signal state at indicated points on the clock divider circuitry of  FIG. 4   400 .  
         [0029]     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.