Abstract:
A method and apparatus for isolating a noise intolerant device, e.g., a phase-lock loop of a tuner within a television receiver, from source of noise. In one embodiment, the apparatus isolates a phase-lock loop integrated circuit (IC) from the bus, by providing an isolation buffer that allows the receiver to only pass data to the tuner&#39;s phase-lock loop IC when a tune command is issued by a processor. When not being tuned, the IIC lines to the tuner are held HIGH by a buffer until needed again to perform the tuning function. This allows the demodulation circuitry to use a setting for a carrier tracking-loop that optimizes bit error rate performance.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to a television receiver. More particularly, the invention relates to a method of reducing phase noise interference in a phase lock loop circuit of a television receiver.  
         [0003]     2. Description of the Background Art  
         [0004]     A typical high definition television (HDTV) system employs a front end comprising a tuner, a digital IF circuit and a digital demodulation integrated circuit (IC). The system is controlled from the digital decoder board using an inter-integrated circuit bus (I2C). The terminology IIC bus, I2C bus or I 2 C bus are equivalent, as used herein.  
         [0005]     The IIC bus is a two wire, bi-directional bus that permits only two integrated circuits (IC&#39;s) to communicate on a bus path at a time. An IC serving in a “master” mode of operation, initiates a data transfer on the bus and generates clock signals that permit the data transfer. An IC serving in a “slave” mode of operation is the IC being operated on or communicated to by the master IC, whereby the slave IC is instructed to either send or receive data. Each IC has its own unique seven bit address, wherein the master IC initiates the communications, and also terminates the communications.  
         [0006]     A serial clock line (SCL) propagates clock signals on the IIC bus from a master IC to a slave IC. Each master IC generates its own clock signals when transferring data on the bus. The second bi-directional wire of the IIC bus is a serial data line (SDA) that transfers data using eight bit serial transactions. Typically, a ninth bit is utilized as an acknowledgment bit. When both clock and data lines are held “HIGH”, no data can be transferred between two IC&#39;s. A HIGH to LOW transition on the SDA line, while the SCL line is HIGH, indicates a start condition for the exchange of data bits. Conversely, a LOW to HIGH transition on the SDA line, while the SCL line is HIGH, defines a stop condition. The master IC generates one clock pulse for each data bit transferred on the SDA line, and the HIGH or LOW state of the data line can only change when the clock signal on the SCL line is in a LOW state.  
         [0007]     Multiple IC&#39;s share the IIC bus. For example, a microprocessor, in a controller of a television receiver, communicates with numerous IC&#39;s within the television receiver via an IIC bus. A problem has been uncovered when down-converting a television signal to a specific intermediate frequency (IF) signal. Coincidental bus traffic by the microprocessor, which functions as a master IC, has been found to cause phase noise interference in a tuner of the receiver. Specifically, a phase-lock loop (PLL) integrated circuit is serially coupled on the IIC bus in the tuner of the television receiver, and acts as a frequency variable tone generator. The microprocessor controls the oscillator frequency of the PLL via the IIC bus. The phase-lock loop is susceptible to the bus traffic when the microprocessor sends commands to other IC&#39;s on the bus, so that instead of producing a tone locked at a specific frequency, a range of other frequencies around the desired tone frequency are produced. For example, in a PLL having a 4 Mhz oscillator, any incidental noise signals generated by the microprocessor may be received by other pins of the PLL integrated circuit connected to the IIC bus. This noise will be added to the resultant signal frequency. In an instance where a user selects a channel at 701 Mhz and the receiver system requires a down-converted IF signal at 44 Mhz, then the PLL must generate a tone locked at a frequency of 745 Mhz. Normally, the 701 Mhz television signal and the 745 Mhz tone signal are mixed to produce an IF signal locked at 44 Mhz. However, additional noise will generate other harmonic frequencies around the tone frequency, causing the IF frequency fluctuate in a range around 44 Mhz instead.  
         [0008]     Thus, the bus chatter is added to the incoming digital video/audio signal and causes a degradation in bit error rate (BER) performance of the television receiver. Ultimately, the bit errors manifest themselves as additional or missing luminance and chrominance pixel components in the video the user is viewing, as well as “clicks and pops” in the audio output. Similarly, when processing an analog television signal, the IIC bus chatter will manifest itself as a distorted picture and/or undesirable WOW and/or flutter in the audio output.  
         [0009]     Phase noise interference, caused by the IIC bus traffic, may be compensated somewhat by widening the bandwidth of the demodulation IC&#39;s carrier tracking-loop, to allow it to “track out” the corruption. However, such a method allows additional low frequency noise to combine with the video/audio signal, thereby degrading the bit error rate of the television receiver.  
         [0010]     Thus, there is a need to reduce the IIC bus chatter created by the IC traffic on the IIC bus. Furthermore, there is a need to reduce the IIC bus chatter before it influences the phase-lock loop circuitry of the tuner.  
       SUMMARY OF INVENTION  
       [0011]     The disadvantages heretofore associated with the prior art, are overcome by the present invention of a method and apparatus for isolating a noise intolerant device, e.g., a phase-lock loop of a tuner within a television receiver, from source of noise. In one embodiment, the apparatus isolates a phase-lock loop integrated circuit (IC) from the bus, by providing an isolation buffer that allows the receiver to only pass data to the tuner&#39;s phase-lock loop IC when a tune command is issued by a processor.  
         [0012]     When not being tuned, the IIC lines to the tuner are held HIGH by a buffer until needed again to perform the tuning function. This allows the demodulation circuitry to use a setting for a carrier tracking-loop that optimizes bit error rate performance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0014]      FIGS. 1A and 1B  depict a block diagram of a portion of a television receiver comprising a buffer for an IIC bus; and  
         [0015]      FIG. 2  depicts a flow diagram of a method for isolating the tuner from the controller assembly in accordance with the present invention.  
     
    
       [0016]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The invention will be primarily described within the context of a television receiver that receives audio and video television signals. However, it will be appreciated by those skilled in the art that the invention is well suited to any system in which digital signals are communicated on an I2C bus. Other signals and systems may illustratively include, but are not limited to, isochronous information transmitted to a television receiver, or digitized data that is transmitted between computers through cable modems on a cable system.  FIGS. 1A and 1B  together depict a block diagram of a portion of a television receiver (referred to hereinafter as receiver  100 ) comprising a buffer  114  for an IIC bus  118  in accordance with the present invention.  
         [0018]     The receiver  100  comprises a controller assembly  102  ( FIG. 1A ), for selecting television signals  120  using an input device  104 . The controller assembly  102  comprises a processor  106 , a bus expander  108 , and a buffer  114 . A tuner  140  ( FIG. 1B ) comprises at least one phase-lock loop  142  and at least one down-converter  146 , for tuning and down-converting the television signal  120 . The tuner  140  is coupled from an output of the buffer  114  to an input of the phase-lock loop  142 . The buffer  114  selectively controls data transmission to the tuner  140 . The terminology processor and microprocessor are considered interchangeable, for purposes herein.  
         [0019]     In this manner, the traffic noise from the microprocessor  106 , which is generated when the microprocessor  106  is communicating with other receiver IC&#39;s  105  in the system, will be excluded from passing to the phase-lock loop  142 . The elimination of noise from the phase-lock loop  142  permits the phase-lock loop  142  to lock at a specific frequency, and then generate a clear tone for mixing with the frequency down-converter  144 . Thus, a substantially noise free IF television signal is produced.  
         [0020]     Specifically, the receiver  100  comprises a front-end assembly  130  ( FIG. 1B ) having at least one tuner  140 , at least one digital demodulator  131  and at least one analog demodulator  132 , a digital IF down-converter  133  and an analog IF down-converter  134  to selectively tune, demodulate or otherwise “receive” at least one of a plurality of television signals  120 , having audio and video information transmitted in either analog or digital formats. The analog formats include conventional analog broadcasting systems such as those conforming to the NTSC transmission standards. The digital formats include Digital Broadcast Satellite (DBS), Digital Video Broadcasting (DVB), as well as terrestrial broadcast such as high definition television (HDTV) and cable formats, each conforming, for example, to the ATSC and MPEG transmission standards.  
         [0021]     A controller assembly 102  ( FIG. 1A ) is coupled to the front-end assembly  130  ( FIG. 1B ) through an input clock line  145  and an input data line  146  of the phase-lock loop  142 . The controller assembly  102  allows a user to select and tune the front-end assembly  130  to any one of the television signals  120 . The user makes a channel selection through an input device  104 , such as a remote control, and the controller assembly  102  sends a channel selection signal through the IIC bus  118  to the front-end assembly  130 .  
         [0022]     The front-end assembly  130 , in response to the channel selection signal provided by the controller  102 , operates to tune a selected television signal  120  for reception. In the instance where the selected television signal is a digital television signal, such as a HDTV channel, the digital demodulator  131  of the front-end assembly  130  demodulates the bit stream. The demodulated digital IF signal is then sent for processing at the MPEG transport processor  126 . The MPEG transport processor  126  separates the video and audio signal information and sends such video and audio information to a video processor  122  (via signal path  123 ) and an audio processor  124  (via signal path  125 ), respectively.  
         [0023]     In the instance where an analog television signal is selected, an analog demodulator  132  of the front end assembly  130  demodulates the video and audio information within the selected television signal, and provides the video and audio information to the video processor  122  and the audio processor  124 , respectively. Thus, in an analog signal circumstance, it is not necessary to send the demodulated audio/video IF signal to the MPEG transport processor  126 . Finally, after either the digital or the analog baseband signal has been recovered, the video processor  122  and the audio processor  124  process the video and audio information, and then send the video and audio information to their output devices, such as a display panel and speaker.  
         [0024]     In particular, the controller assembly  102  comprises a microprocessor  106  coupled to a bus expander  108 , via the IIC bus  118  serial clock and data signal paths  110  and  112 , and a plurality of other receiver IC&#39;s  105  also coupled to the IIC bus for other data processing purposes. The microprocessor  106  functions as a master IC on the IIC bus, and controls the IIC bus  118  at all times. As such, all of the remaining IC&#39;s coupled to the IIC bus  118  operate in a slave mode of operation.  
         [0025]     The bus expander IC  106  is coupled to the IIC bus  118  to enable non-IIC compliant IC&#39;s to interface with the IIC compliant IC&#39;s on the IIC bus  118 . The bus expander IC  106  is further coupled to a buffer  114 . The buffer  114  comprises a pair of OR gates  116  and  117 . However, a person skilled in the art for which the invention pertains will recognize that other buffering devices, such as discrete transistor circuits, may also be utilized.  
         [0026]     An output control port  109  of the bus expander  108  is coupled to both the first and second OR gates  116  and  117 , through first input ports  116  and  117  on each OR gate  116  and  117 . Additionally, a serial clock line  110  is coupled to a second input port  116   2  on the first OR gate  116 . Similarly, a seral data line  112  is coupled to a second input port  117   2  on the second OR gate  117 . The pair of OR gates  116  and  117  are then coupled to a tuner  140  in the front-end receiver  130 , through their respective output ports  116   3  and  117   3 . The output port of the first OR gate  116   3  is coupled to the input clock line  145  of the phase-lock loop  142 . Furthermore, the output port of the second OR gate  117   3  is coupled to the input data line  146  of the phase-lock loop  142 .  
         [0027]     In operation, the microprocessor  106  sends a command signal to the bus expander  108  to set the bus expander&#39;s output control port  109  to a logically HIGH state. The bus expander&#39;s  108  internal circuitry (not shown) sets the control port  109  HIGH, thereby setting the first input ports  116   1  and  117   1  of the pair of OR gates  116  and  117  to a HIGH state. Therefore, the Boolean logic for an OR gate dictates that no matter what the input signal may be at the second input ports  116   2  and  117   2  of the first and second OR gates  116  and  117 , the respective outputs  116   3  and  117   3  of the OR gates  116  and  117  will always be at a logical HIGH state, i.e., bus traffic noise will be precluded for being coupled to the tuner  140 .  
         [0028]     The tuner  140  of the receiver  100  comprises a phase-lock loop IC (PLL)  142  having an oscillator (e.g., voltage controlled oscillator (VCO))  143  coupled to a down converter  144  through a signal path  147 . In an instance where a user selects an analog channel, the tuner  140  produces the video and audio IF signals sent to the video and audio processors  122  and  124  for processing of the video and audio information. In an instance where a user selects a digital channel, the tuner  140  produces a digital IF signal (e.g., 5.38 Mhz) that gets forwarded to the digital demodulator  132  to separate out the carrier component of the signal and provide the baseband signal. The baseband signal is then forwarded to the MPEG transport processor  126  where the video and audio substreams are separated for processing at the video and audio processors  122  and  124 .  
         [0029]     The output of the first OR gate  116   3  is coupled to the input clock line  145  of the phase-lock loop IC  142  in the tuner  140 . Furthermore, the output of the second OR gate  117   3  is coupled to the input data line  146  of the phase-lock loop IC  142 . Thus, the buffer  114  functions to selectively isolate both phase-lock loop inputs  145  and  146  of the phase-lock loop IC  142  from the IIC bus  118  and the microprocessor  106 .  
         [0030]     In operation, when the bus expander  108  is sent a command signal from the microprocessor  106  to hold the output control port  109  “HIGH”, each OR gate  116  and  117  will logically hold their respective outputs  116   3  and  117   3  HIGH. Thus, the input clock line  145  and the input data line  146  of the phase-lock loop IC  142  are logically in a HIGH state. In an instance where both the serial clock and serial data lines on an IIC bus are held in a HIGH state, no data may be transferred between the two devices. Therefore, in this instance, the traffic noise from the microprocessor  106  that exists when the microprocessor  106  is communicating with other receiver IC&#39;s  105  in the system, will be excluded from passing through the input clock and data lines  145  and  146  of the phase-lock loop IC  142 , and will not affect the generation of tones by the VCO  143 .  
         [0031]      FIG. 2  depicts a flow diagram of a method for isolating the tuner from the bus in accordance with the present invention. The method begins at step  200  and proceeds to step  202 , where a user selects a television signal from a control device. At step  204 , the television signal is coupled to a down-converter or mixer for further processing.  
         [0032]     In step  206 , the microprocessor (master) signals the bus expander, via a first IIC command, to set the bus expander output to a LOW state. Henceforth, the phase-lock loop may receive data transmission from the microprocessor, via the SCL and SDA lines. To start data transmission, the microprocessor sets the SDA line of the IIC bus from a steady state HIGH, to a LOW transition, while the SCL is held in a steady state HIGH. Such transition indicates a start condition for the exchange of data bits. The HIGH or LOW state of the data line (SDA) may only change when the clock signal on the SCL line is LOW. At the next clock pulse from the microprocessor, the serial clock line is set LOW, causing the output clock line of the buffer (i.e., first OR gate of FIG.I) coupled to the clock input of the phase-lock loop, to a LOW state.  
         [0033]     At each subsequent LOW clock pulse, data may be transmitted over the SDA line, thereby permitting the phase-lock loop IC to receive data transmissions over the IIC bus from the microprocessor. The data transmissions contain information necessary for the phase-lock loop to generate a tone, which facilitates demodulation of a television signal located at a particular channel. As long as the microprocessor sends a repeated START signal, prior to transmitting each byte, the IIC bus remains in a “busy” state. When the microprocessor has completed the data transmission, the microprocessor generates a LOW to HIGH transition on the SDA line, while the SCL is HIGH, to define a stop condition.  
         [0034]     In step  208 , once the phase-lock loop receives the information necessary to generate the tone, the phase-lock loop locks at a specific frequency designated by the microprocessor. The method proceeds to step  210 , where the microprocessor, via a second IIC command from the microprocessor, causes the output on the bus expander to go into a HIGH state. The HIGH state from the output of the bus expander, thereby causes the buffer to also go into a HIGH state, in a similar manner as described above. At this point, the phase-lock loop is appropriately locked at a specific frequency, and the phase-lock loop is isolated from any further communications (noise) from the microprocessor&#39;s activities with other IC&#39;s on the IIC bus.  
         [0035]     In step  212 , the television signal and the locked frequency tone are combined in the down-converter, and an IF signal is produced. Thus, when the tone is mixed with the television signal in the down-converter, a IF signal is produced without any fluctuations in frequency due to microprocessor generated bus noise. In step  214 , the IF signal is demodulated (then sent to an MPEG processor to produce video and audio substreams in the instance where the signal is a digital IF signal), and processed to produce the desired audio and video output. At step  216 , the method ends, until a user requests a different television channel whereupon the inventive method is repeated.  
         [0036]     It should be apparent to those skilled in the art that a novel method and apparatus for excluding noise from an input of a terrestrial television signal tuner has been provided. The buffer device inventively functions to isolate the phase-lock loop IC from noise occurring on the IIC bus. Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.