Abstract:
A digital video communication device is provided. The digital video communication device includes a transmitter providing a spread-spectrum video signal including a predetermined frequency spread value and a frequency ratio and a receiver receiving the spread-spectrum video signal. The receiver includes a frequency synthesizer, a free-running clock generator configured to generate a free-running clock signal, wherein the free-running clock signal is used as a reference clock signal input to the frequency synthesizer. The receiver further includes a digital control logic circuit configured to separate the frequency ratio from the spread-spectrum video signal, and a line buffer coupled to the digital control logic circuit and the frequency synthesizer, the line buffer adjusting the frequency ratio and sending the adjusted frequency ratio to the frequency synthesizer, wherein the frequency synthesizer combines the free-running clock signal and the adjusted frequency ratio, and outputs a de-spread clock signal.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to digital telecommunications; in particular, to reducing clock jitter in spread-spectrum digital video data processing. 
       DISCUSSION OF RELATED ART 
       [0002]    Spread-spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic, or telecommunication signal) generated in a given bandwidth is deliberately spread in the frequency domain. The resulting signal has a wider bandwidth but often suffers from a poorer signal to noise ratio. In telecommunications, a typical spread-spectrum technique employs direct sequence, frequency hopping, or a hybrid of these, for multiple access and/or multiple functions. After generating and transmitting the spread-spectrum signal, a receiver receives the spread-spectrum signal and correlates the signal to retrieve the original information signal in a process known as de-spreading. One of the reasons to use spread-spectrum in telecommunications is to decrease the potential interference to other receivers while increasing the amount of privacy available to the users whose signals are being transmitted. 
         [0003]    Spread spectrum is being adopted extensively in the video industry to reduce electromagnetic interference (EMI) and to reduce power requirements. Modern mobile devices with increasing integration of high-resolution liquid crystal display (LCD) displays, such as laptop computers, smart phones, handheld mobile devices, demand faster clock speeds and smaller devices. These devices are designed to be lightweight and inexpensive, thus, passive EMI reduction measures such as capacitors and metal shielding are not viable options. 
         [0004]    One technique to reduce the EMI of these modern mobile devices is to use spread-spectrum clocking. Although spread-spectrum clocking provides improved active EMI reduction, spread-spectrum clocking creates additional challenges for designers. For example, when using spread-spectrum clocking there is a risk that the clock and the data will become misaligned during de-spreading. Further, when using a link symbol clock in accordance with the DisplayPort standard with spread-spectrum clocking, the amount of time required to synchronize the link symbol clock sent from the transmitter to the receiver will increase, thus creating more jitter at the receiver side. 
         [0005]    Consequently, there is a need for a spread-spectrum clocking scheme having little to no clock jitter. 
         [0006]    Consistent with embodiments of the present disclosure, a digital video communication device is provided. The digital video communication device includes a transmitter providing a spread-spectrum video signal including a predetermined frequency spread value and a frequency ratio, a receiver receiving the spread-spectrum video signal, wherein the receiver comprises a frequency synthesizer, a free-running clock generator configured to generate a free-running clock signal, wherein the free-running clock signal is used as a reference clock signal input to the frequency synthesizer. The receiver further includes a digital control logic circuit configured to separate the frequency ratio from the spread-spectrum video signal, and a line buffer coupled to the digital control logic circuit and the frequency synthesizer, wherein the line buffer adjusts the frequency ratio and sends the adjusted frequency ratio to the frequency synthesizer, wherein the frequency synthesizer combines the free-running clock signal and the adjusted frequency ratio, and outputs a de-spread clock signal. 
         [0007]    Further consistent with embodiments of the present disclosure, a method for reducing jitter of a spread-spectrum clock signal received by a digital video receiver is provided. The method includes receiving, by the digital video receiver, the spread-spectrum clock signal, generating, by a free-running clock generator in the digital video receiver, a free-running clock signal, determining an initial frequency ratio at the spread-spectrum clock signal, adjusting, by a line buffer in the digital video receiver, the initial frequency value; receiving, by one or more frequency synthesizers, the free-running clock signal and the adjusted frequency value, and generating, by the one or more frequency synthesizers, an output clock signal having minimized jitter based on the free-running clock signal and the adjusted initial frequency ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram illustrating an audio/video interconnection transceiver including a transmitter and a receiver. 
           [0009]      FIG. 2  is a schematic diagram showing a low-jitter audio/video interconnection transmitter and receiver having an independent free-running clock generator. 
           [0010]      FIG. 3  is a schematic diagram of a frequency synthesizer using a free-running clock signal as its reference clock input, consistent with some embodiments. 
           [0011]      FIG. 4  is a flow chart describing a method for reducing jitter of a spread-spectrum clock signal received by a digital video receiver, consistent with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In the following description, specific details are set forth describing some embodiments consistent with the present disclosure. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other material that, although not specifically described here, is within the scope and the spirit of this disclosure. 
         [0013]      FIG. 1  shows a schematic diagram illustrating an audio/video interconnection transceiver  100 . Consistent with some embodiments, audio/video interconnection transceiver  100  may be used as a transceiver that complies with standards such as Digital Visual Interface (DVI), Video Graphics Array (VGA), High-Definition Multimedia Interface (HDMI), Unified Display Interface (UDI), or DisplayPort (DP). As shown in  FIG. 1 , transceiver  100  includes a transmitter  110 , a receiver  180 , and perhaps a number of additional repeaters (not shown) or other devices (not shown). Transmitter  110  receives isochronous streams of data such as a set of uncompressed digital video data  101  and digital audio data  102 . Transmitter  110  and Receiver  180  are interconnected by one or more uni-directional, high-speed, and low-latency main serial link channels  118  for data transport. 
         [0014]    In many stream clock recovery systems, the digital video data  101  and digital audio data  102  being carried across the main serial link channel  118  may not retain the original video pixel clock rate or audio sample clock rate. The main serial link channel  118  is driven by a high speed clock running at a different rate than the original video pixel clock rate or the audio sample clock rate. For example, in accordance with the DisplayPort standard, the main serial link channel clock rate is fixed at either 1.62 Gbps or 2.7 Gbps, irrespective of the input video or audio clock rates. In accordance with the HDMI standard, the main serial link channel clock runs at the TMDS (Transition Minimized Differential Signaling) clock rate, which corresponds to the video pixel rate, but is independent of the audio sample clock rate. In both HDMI and DisplayPort standards, a fractional relationship time-stamped clock regeneration model can be defined. 
         [0015]    In serial communication systems, digital video data is often sent in a stream format without an accompanying clock signal over the serial link. In a clock and data recovery (CDR) process, the digital receiver generates a clock from an approximate frequency reference, and then the data stream is phase-aligned to transitions in the data stream with a device such as a phase-locked loop (PLL). A link or link symbol rate is the number of symbols transferred across the channel per second. The pixel bit rate is the number of data bits passed across the channel. A link or link symbol clock is the clock extracted from the incoming bit stream data, and it is the common clock shared by the transmitter and receiver. In some embodiments a link symbol clock runs at a lower frequency. The link rate is decoupled from the pixel rate, and the pixel rate is regenerated from a link symbol clock using the time stamp values M and N. 
         [0016]    The following equations conceptually explain how the stream clock frequency, also referred to as the pixel clock frequency must be derived from the link symbol clock frequency using the Time Stamps, M and N: 
         [0000]    
       
         
           
             
               
                 
                   
                     Stream_clock 
                      
                     _frequency 
                   
                   = 
                   
                     
                       M 
                       / 
                       N 
                     
                     * 
                     link_symbol 
                      
                     _clock 
                      
                     _frequency 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   where 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     N 
                     = 
                     
                       Reference_pulse 
                        
                       
                         _period 
                         / 
                         link_symbol 
                       
                        
                       _clock 
                        
                       _period 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     M 
                     = 
                     
                       Feedback 
                        
                       
                           
                       
                        
                       pulse 
                        
                       
                           
                       
                        
                       
                         period 
                         / 
                         stream_clock 
                       
                        
                       _period 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       M 
                       N 
                     
                     = 
                     
                       
                         stream_clock 
                          
                         _frequency 
                       
                       
                         link_symbol 
                          
                         _clock 
                          
                         _frequency 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0017]    This equation only holds true as long as the Reference pulse period is adjusted constantly to be equal to the Feedback pulse period. 
         [0018]    M is a dynamic parameter that is counted based on the incoming data signals, and N is a static parameter that depends on the relevant communication standard of the system. The values M and N are integers representing frequencies in clock recovery. 
         [0019]    Returning to  FIG. 1 , transmitter  110  includes a cycle time counter  111  to count the frequency parameter M from input video clock  101  and a frequency divider  112  drives a “divide by N” operation. The stream data including M and N is carried by main serial link channel  118  to receiver  180 . 
         [0020]    In receiver  180 , a SerDes (Serialization and De-serialization) device  120  takes input data from main serial channel  118  and outputs a link symbol clock signal  122 , an M/N ratio  125 , and a pixel data signal  126 . Link symbol clock signal  122  is used by a frequency synthesizer  130  as its reference clock input. M/N value  125  is calculated and sent to frequency synthesizer  130  as an input signal. Consistent with some embodiments, frequency synthesizer  130  may be a phase-locked loop (PLL) or a delay-locked loop (DLL), or other circuitry for synthesizing frequencies. Frequency synthesizer  130 &#39;s output signal  131  has a frequency based on equation (1): 
         [0000]      frequency=( M/N )*link_symbol_clock_frequency  (5)
 
         [0000]    where link_symbol_clock_frequency could be either 270 MHz or 162 MHz, for high bit rate and reduced bit rate respectively according to DisplayPort standard 1.1a. Consistent with other embodiments, link_symbol_clock_frequency could be other frequencies appropriate to other standards, such as HDMI or DVI. Video recovery device  140  coverts frequency synthesizer output signal  131  and pixel data signal  126  into video data  141 . Receiver  180  processes (not shown) the audio data into audio out  142 . 
         [0021]    In the ideal case, where the received M and N values are equal to the original M and N values, fractional long-term accuracy of the recovered stream clock is the same as that of the original. However, when the M and N values at receiver  180  are not equal to the original M and N values at the transmitter  110 , which is typically caused by bit errors in main serial link channel  118  or imperfect measurement mechanisms at transmitter  110 , frequency deviation occurs between transmitter  110  and receiver  180 . Over time, the frequency deviation propagates leading to a significant phase offset between transmitter  110  and receiver  180 . 
         [0022]    Spread-spectrum clocking can be used to provide a more accurate clock rate by dividing the electromagnetic spectrum into narrow frequency bands, and thus providing an improved signal to noise ratio. For example, if the system under test were to radiate all of its energy at one frequency, it would register a large peak at the monitored frequency band. However, spread-spectrum clocking distributes the energy so that it falls into a large number of the frequency bands, without putting enough energy into any one band to exceed the system specified limits. The spread-spectrum clocking process usually down-spreads a small fraction of link symbol clock frequency. For instance, down-spreading of 0.3% or 0.5% of the link symbol clock is commonly used in transmitters that are operating according to the DisplayPort standard. 
         [0023]    Returning to  FIG. 1 , assuming transceiver  100  is operating using spread-spectrum clocking, de-spreading occurs at receiver  180 . For de-spreading to work correctly, the transmitting and receiving sequences must be synchronized. This requires the receiver  180  to synchronize its sequence with the transmitter&#39;s  110  sequence via a timing search process. In a typical spread spectrum transceiver, the digital video signal transported at the main serial link channel  118  is time stamped by the M and N values generated in transmitter  110 . 
         [0024]    Although methods exist for recovering pixel data for a whole display line and even an entire display frame, in a transceiver that uses spread-spectrum clocking, the spread nature of the pixel data is problematic for older display devices, particularly those traditional CRT (cathode ray tube) or scaling devices that only have line buffers to scale up or scale down source image to fit into the display device. These older display devices rely on accurate and predictable DE/HSYNC/VSYNC timing to produce a generator lock (“gen-lock”), or to output a scaled image. However, using a link symbol clock in a transceiver using spread-spectrum clocking will inevitably make the period time of DE/HSYNC/VSYNC jitter constantly. For example, a spread-spectrum clock having down-spreading of about 0.2% will cause one line period of 2080 clock cycles (1920 pixels in line data plus 160 clock cycles for blanking) to jitter up to 4 clock cycles (2080*0.2%), if this line period is measured with an accurate single frequency clock. Moreover, jitter in the original main serial link channel  118  will spread to the output pixel clock signal  141  and audio signal  142  as well. 
         [0025]      FIG. 2  shows a low-jitter audio/video interconnection receiver using a de-spreading technique to reduce spread data noise, consistent with some embodiments. Transceiver  200  includes a transmitter  210 , a receiver  280 , and may further include a number of additional transmitter and receivers as repeaters or other devices (not shown). Consistent with some embodiments, audio/video interconnection transceiver  200  may be used as a transceiver that complies with standards such as Digital Visual Interface (DVI), Video Graphics Array (VGA), High-Definition Multimedia Interface (HDMI), Unified Display Interface (UDI), or DisplayPort (DP). As shown in  FIG. 2 , transmitter  210  and receiver  280  are interconnected by one or more uni-directional, high-speed, and low-latency main serial link channels  218  designed to transport uncompressed spread digital video data  201  and digital audio data  202 . Transmitter  210  includes a cycle time counter  211  to count the frequency parameter M from input video clock  201 , and a frequency divider  212  to drive a “divide by N” operation, where M and N are integer numbers defined in Equations (2) and (3) above. The spread-spectrum video input data  221 , time stamped with M and N, is transmitted by the main serial link channel  218  receiver  280 . Secondary signals such as the audio signals will also be transported via the main serial link channel  218 . 
         [0026]    Receiver  280  receives spread-spectrum video data  221  from main serial link channel  218 , which is input into a SerDes (Serialization and De-serialization) device  220 . SerDes device  220  receives a spread spectrum video data input signal  221  from main link channel  218  and outputs spread-spectrum video signal  222  which is stored in a line buffer device  250 . Consistent with some embodiments, spread-spectrum video signal  222  includes pixel data, a link symbol clock signal, and an M/N ratio, all included in video data input signal  221  transmitted over main link channel  218 . According to some embodiments, the M/N ratio is related to a frequency of the link symbol clock signal and a frequency of the video data input signal  221  (i.e., the input video data streamed from transmitter  210 ). Line buffer device  250  accommodates any clock discrepancy by providing a buffer read clock  235  for pixel signal  256 . Line buffer device  250  first sends an M/N value  255  to a frequency synthesizer  230 , which adjusts the frequency and sends a read clock signal  235  back to the line buffer  250 . The read clock  235  counts and outputs the pixel video signals  256  to video recovery circuit  240 . The process of adjusting the frequency of read clock  235  is continuous. Without a real-time adjusted read clock  235  in line buffer  250 , there will be unavoidable underflow or overflow of the frequency-spread video signal in the line buffer  250 . Consistent with some embodiments, frequency synthesizer  230  may be a phase-locked-loop (PLL) or a delay-locked loop (DLL), or other circuitry that is capable of synthesizing frequencies. 
         [0027]    The frequency synthesizer  230  may include one or more frequency filters (not shown in  FIG. 2 ), according to some embodiments. Frequency synthesizer  230  may also include at least one digital-to-analog converter (not shown in  FIG. 2 ). 
         [0028]    A free running clock generator  260  is used to generate a free-running clock signal  231  as the reference clock input for frequency synthesizer  230 . Consistent with some embodiments, free-running clock generator may be a crystal clock. By not using the recovered clock from the spread spectrum as the video input data, jitter from the spread-spectrum clock associated with the main serial link channel  218  may be removed from the frequency spread associated with signals transmitted through main serial link channel  218 . The M/N value  255  is dynamically adjusted at line buffer  250  to compensate for any clock discrepancies from the de-spread data received by receiver  280  through main serial link channel  218  and the adjusted M/N value  255  is used as an input for frequency synthesizer  230 . The structure of line buffer  250  depends on the clock accuracy from M/N value  255 , feedback clock signal  235  from frequency synthesizer  230 , used as the read clock signal by the line buffer  250 , and the frequency synthesizer  230  response times. Depth of line buffer  250  also depends on the M/N ratio, frequency synthesizer  230  clock accuracy, and a frequency synthesizer response time to an M/N value change. The M/N value is usually stable with very little change over time. Therefore, frequency synthesizer  230  is isolated from the spread spectrum noise from the data transmitted over main serial link channel  218 . In addition, since the free-running clock signal  231  from the free running clock generator  260  has very little jitter, frequency synthesizer  230  output clock  232  will have very little jitter as well. The dynamically adjusted output clock  232  is also fed back to line buffer  250  as its read clock  235 . Output clock  232  from frequency synthesizer  230  is then combined with output pixel data  256  from line buffer  250  at video recovery device  240  to provide a low jitter video signal  241 . Although the process of recovering the audio signal from main serial link channel  218  is not shown in  FIG. 2 , audio data output  242  is recovered similarly to the video signal, and the description is not repeated herein. 
         [0029]      FIG. 3  is a schematic diagram of a frequency synthesizer using a free-running clock signal as its reference clock input, consistent with some embodiments. The frequency synthesizer  230  in  FIG. 2  includes a frequency synthesizer circuit  330 , a multiplier circuit  320 , a divide_by_N circuit  310  for a frequency conversion operation on free-running clock  231 , and a circuit  340  to receive the M/N value  255  from line buffer  250 . 
         [0030]    A typical frequency synthesizer receives a standard link clock signal as its input reference clock signal. However, the reference clock signal received by frequency synthesizer  230  is a free-running clock signal  231  generated by the free running clock generator  260  shown in  FIG. 2 . The free-sunning clock signal  231  is divided by N in circuit  310 . The M/N value  255  from line buffer  250  is received by circuit  340 . The multiplier  320  combines the divided free-running clock signal  305  with M/N value  345  from circuit  340  into a voltage or current signal, which is sent to a frequency synthesizer circuit  330 . One example of a frequency synthesizer circuit is a voltage controlled oscillator, which will generate a clock signal having a clock rate related to the resulting voltage of signal  321 . The output clock signal  232  of frequency synthesizer  330  is fed back to line buffer  250  as its read clock  235  in  FIG. 2 . 
         [0031]    Returning to  FIG. 2 , a self-adaptive de-spreading scheme is thus introduced to filter out spread-spectrum jitter in digital video data stream. Even if the signals transmitted from transmitter  210  have jitter from temperature change or other conditions, receiver  280  tracks the clock changes of transmitter  210 . As a result, the average of clock frequency is maintained, and the periodic clock modulations (spread-spectrum) are removed from the data stream. In operation, a de-spreading operation according to some embodiments involves first finding an optimal initial M/N value to match the frequency down spreading, and continuously adjusting the M/N value to maintain the frequency down spreading. 
         [0032]    Since most spreading devices use down-spreading, the unspread M/N value from the transmitter  210  cannot be directly input into frequency synthesizer  230 . Thus, a corresponding adjusted M/N value  255  must be input to frequency synthesizer  230  to match the video signal modulated in a spread spectrum. For example, a link symbol clock&#39;s peak frequency is typically down-spreaded about 0.5% and its average frequency is down-spreaded about 0.25%, thus the M/N value should be about 0.25% lower, or 99.75% of the original M/N value at the transmitter  210 . 
         [0033]    There are several ways to find this optimal initial M/N value. One exemplary technique starts from by estimating an M/N value, and repeatedly measuring the line buffer entry  222  from a number of different lines. The measured M/N&#39;s FIFO status (first-in-first-out status) is compared to decide if the current M/N value is smaller or larger than the estimated value. Consequently, the M/N ratio can be increased or decreased based on this information. The optimization process repeats from the same lines in each frame until line buffer  250  can sustain multiple lines without any data underflow or overflow. 
         [0034]    After determining the optimal initial M/N value, the M/N value must be continuously adjusted to produce adjusted M/N value  255 . There are a couple of ways to adjust the M/N values. For example, an M/N value for each individual line of may be adjusted carefully to avoid data lost due to line buffer overflow or underflow. Typically the adjusted amount should be small, for example, only a few cycles for a 2080 clock-cycle video sequence should be adjusted to match the free-running clock signal  231  frequency at the frequency synthesizer  230  reference clock input. Otherwise clock jitter will be created again by over changing M/N values. 
         [0035]    Another example of continuously adjusting the M/N value is a frame-based adjustment: an M/N value for each frame of spread-spectrum video signal  222  can be adjusted based on the M/N value in one particular line in one frame. The M/N value is checked a few times in the particular line for a few consecutive frames to decide if an accumulated clock jitter is significant to warrant fine tuning of the frame&#39;s M/N values. The adjustment should be very small to avoid line buffer overflow or underflow from sudden large changes in the M/N values. 
         [0036]      FIG. 4  is a flow chart describing a method for reducing jitter of a spread-spectrum clock signal received by a digital video receiver, consistent with some embodiments. For the purpose of explanation, the method described in  FIG. 4  will be discussed with reference to  FIGS. 2 and 3 . First, digital video receiver  280  receives a spread-spectrum clock signal having an initial frequency ratio (Step  410 ). Consistent with some embodiments, a spread-spectrum clock may be included as part of video data input signal  221 . A free-running clock signal  231  is generated by free-running clock generator  260  (Step  420 ). The initial frequency ratio is determined by line buffer  250  and constantly adjusted to produce an adjusted frequency ratio (Step  430 ). Consistent with some embodiments, the frequency ratio is the M/N ratio discussed previously. The adjusted frequency ratio  255  and the free-running clock signal  231  are input into frequency synthesizer  230  (Step  440 ). The frequency synthesizer  230  combines the adjusted frequency ratio  255  with the free-running clock  231  and generates an output clock signal  232  having minimized jitter (Step  450 ). 
         [0037]    Consistent with the disclosed embodiments, a de-spreading scheme is provided to filter out spread spectrum noise and/or jitter in a digital video data stream. The scheme is self-adaptive such that any jitter from the transmitter due to temperature change or other adverse conditions is tracked by the receiver. As a result, an average clock frequency is maintained, and the spread spectrum noise is removed from the data stream. 
         [0038]    The foregoing description is intended to illustrate but not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the disclosure.