Abstract:
A method and apparatus for providing adaptive horizontal synchronization (sync) signal detection to identify whether a high quality television signal is being received by a television set. The method determines whether no signal is applied, whether a weak signal is applied or a strong signal is applied to the television set by sampling and processing the horizontal synchronization signal. The signals are classified by comparing the sample horizontal synchronization signals to a predefined threshold. The threshold is established based upon the type of source that produced the television signal.

Description:
This application claims the benefit under 35 U.S.C. § 365 of International Application PCT/US00//22472, filed Aug. 17, 2000, which was published in accordance with PCT Article 21(2) on Feb. 22, 2001 in English; and which claims benefit of U.S. provisional application Ser. No. 60/149,297 filed Aug. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for processing television signals, and more particularly, to a method and apparatus for providing adaptive horizontal synchronization (horizontal sync) signal detection. 
     DESCRIPTION OF THE RELATED ART 
     Modern television sets generally attempt to demodulate and display detected video signals that are undegraded and of sufficient quality that can provide an acceptable picture. As such, a television set must detect either the lack of a television signal or a degraded television signal that would provide a distorted picture, and then hide the poor or nonexistent picture from the viewer. A television set should also notify the user when the television signal is not present, degraded or not connected to the television. 
     To determine the presence of a usable television signal, a television set generally samples the input signal to determine the presence of a signal, or signal component, that would exist only if a television signal was being received by the television. One such signal is the horizontal synchronization signal (or horizontal sync) that is generated for each line in a video field. If a television set detects a horizontal sync signal, the television set may assume that a television signal is present at the input of the television set. If the amplitude of the horizontal sync signal is above a predetermined threshold level the television set can assume that the received television signal is of sufficient quality to produce acceptable picture. 
     The circuitry for sampling the horizontal sync signal generally comprises an analog-to-digital (A/D) converter (a video signal digitizer) the output of which is periodically sampled, in an asynchronous manner, to determine the amplitude of the horizontal sync signal. The digitized horizontal sync signal is coupled to a microprocessor via an IIC bus to provide to the microprocessor a sample of the horizontal sync signal. Since the sampling is periodically accomplished without regard to the timing of the video fields and the picture, the detection status samples can be spurious in that the horizontal sync status may be read as being valid on one occasion and invalid on another occasion depending on the location of the sample in the video field. For example, if the status is read at the top of a video field, the horizontal sync signal may be read as being invalid. However, if the status is read again a few microseconds later at a lower point in the same video field, then the horizontal sync status may be read as being valid. In such an arrangement, the television set cannot distinguish an invalid horizontal sync signal from a valid horizontal sync signal based on a single sample. If a television set uses such erroneous data, the television set may erroneously determine that a picture will be of poor quality and blank the picture screen even though the video signal was of sufficient quality to produce an accurate picture. Furthermore, the behavior of the horizontal sync status varies depending on the signal source, i.e., whether the source is cable television, terrestrial antenna, or a VCR playback signal. 
     SUMMARY OF THE INVENTION 
     Therefore, a need exists in the art for a method and apparatus for providing adaptive horizontal sync detection to accurately determine whether a television signal of sufficient quality is being received by a television set. 
     The present invention provides a method and an apparatus for providing adaptive horizontal sync detection to identify whether a television signal of sufficient quality is being received by a television set. 
     The invention determines whether no signal is being applied, whether a weak signal is being applied or a strong signal is being applied to a television set. The invention recognizes and accommodates certain anomalous behaviors of television sets that interfere with the normal detection of the horizontal sync signal. For example, the invention delays the horizontal sync sampling process for a predefined period of time after a channel change event has occurred to ensure that the signal is sampled at an appropriate time. Generally, channel changes will interfere with the horizontal sync signal for a short time after the channel change has occurred. By delaying the sampling of the horizontal sync signal, the detection circuitry avoids erroneous signal processing. 
     In addition, after every invalid horizontal sync signal is detected, the circuitry waits for 10 milliseconds and performs a second signal status reading to determine whether the horizontal sync signal becomes valid. This delayed double reading is especially important when the television signal source is a video cassette recorder (VCR). The amount of error that is allowed before the system deems the television signal to be poor is associated with the playback source of television signal, i.e., a VCR or digital versatile disk (DVD) player, will have a higher error tolerance than an over the air television station. Once the sync signal is processed, the user is notified whether the television signal is poor or whether no signal at all is detected. Notification is generally accomplished by blanking the video screen and providing a text message on the screen notifying the user of the weak signal. 
     An apparatus for processing a television signal according to the present invention comprises: a tuner for receiving a television signal; a signal processor for extracting a horizontal synchronization signal from the television signal; and a horizontal synchronization signal detector for sampling the horizontal synchronization signal, characterized by a horizontal synchronization signal processor, coupled to the horizontal synchronization signal detector, for adaptively processing the horizontal synchronization signal in response to the signal source type to determine the quality of the television signal. 
     A method of processing a television signal according to the present invention comprises the steps of: receiving a television signal; sampling a horizontal synchronization signal at a first location in a video field of the television signal; and processing the sample to determine a quality measure of the television signal using a predefined threshold, characterized in that the threshold is established in response to a type of source of the television signal, if the quality measure is less than a predefined threshold, re-sampling the horizontal synchronization signal at a second location in the video field, and processing the sample to determine a second quality measure of the television signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in detail with reference to the accompanying drawings, wherein: 
         FIG. 1  depicts a block diagram of a television apparatus in accordance with the present invention; 
         FIG. 2  depicts a flow diagram of a method of processing system change events in accordance with the present invention; 
         FIGS. 3A ,  3 B and  3 C together depict a flow diagram of a method of performing status polling in accordance with the present invention. 
     
    
    
     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 
       FIG. 1  depicts a block diagram of television apparatus  100  for processing television signals in accordance with the present invention. Television apparatus  100  receives an input signal at terminal  102  and provides a video output for display at terminal  122 . Television apparatus  100  comprises tuner  104 , signal processor  108 , central processing unit  114 , support circuits  116  and memory  118 . 
     Tuner  104  selects a particular channel within a frequency band for processing by signal processor  108 . Tuner  104  operates in a conventional manner well known by those skilled in the art. The intermediate frequency signal produced by tuner  104  is coupled to signal processor  108  for further processing to extract the video signal from the received television signal. Signal processor  108  contains many well-known circuits for demodulating and decoding a video signal. Within signal processor  108  is horizontal sync circuit  110  that extracts the horizontal sync signal from the television signal. 
     Television apparatus  100  according to the present invention also includes horizontal sync signal detector  112  that is coupled to horizontal sync circuit  110 . Generally, horizontal sync detector  112  is an analog-to-digital converter that samples the horizontal sync signal. The sampling occurs in response to a triggering signal from CPU  114 . When triggered, horizontal sync detector  112  writes the sample value to buffer  124 . The signal that triggers horizontal sync detector  112  is coupled from CPU  114  to horizontal sync signal detector  112  through IIC bus  106 . The sampled amplitude of the horizontal sync signal is digitized by horizontal sync signal detector  112 , stored in a buffer that is coupled to IIC bus  106  and, ultimately, the sample is propagated to CPU  114 . 
     CPU  114  is a general purpose microprocessor that is supported by support circuits  116  and memory  118 . Support circuits  116  are well known circuits such as cache, clock circuits, input/output driver circuits, power supplies and the like. Memory  118  comprises read only memory and/or random access memory. Memory  118  stores horizontal sync detection software  120  that causes television apparatus  100  to operate in accordance with the present invention. When executing horizontal sync detection software  120 , CPU  114  operates as a horizontal sync signal processor. 
     Horizontal sync signal detection software  120  controls horizontal sync detector  112  to timely sample the horizontal sync signal and then process the digitized signal to determine if the television signal is of sufficient quality to produce an appropriate video picture. Generally, the software classifies the quality of the video signal as viewable; weak, but viewable; and faulty. For viewable and weak signals, the video is displayed. For faulty signals, the video is not displayed and an error message is displayed. Details of the operation of software  120  are discussed below with respect to  FIGS. 2 and 3 . 
       FIG. 2  depicts a block diagram of interrupt routine  200  that is executed by CPU  114  when a system change event has occurred, such as when television tuner  104  has selected a new channel, television apparatus  100  has just been activated, the source of the television signal has been changed, and the like. Interrupt routine  200  begins upon the detection of a system change event at step  202  and proceeds to step  204 . At step  204 , routine  200  determines whether the selected channel is a non-NTSC channel (i.e., a non-analog television channel). If the channel is not an NTSC channel, the routine proceeds to step  206  where horizontal sync signal detector  112  (the device) is placed in a standby mode and CPU  114  ceases polling horizontal sync signal detector  112 . The routine is exited at step  208 . 
     If the query at step  204  is negatively answered, routine  200  proceeds to step  210  where routine  200  queries whether the television signal source is an external device. If the signal source is not an external device, routine  200  proceeds to step  214  where the routine queries whether the television signal source is an NTSC channel. If the query at step  214  is negatively answered, routine  200  proceeds to exit step  216  and stops. If the query at step  214  is affirmatively answered, routine  200  proceeds to step  218  where horizontal sync detector  112  is configured for cable or antenna mode, i.e., for reception of an NTSC channel. In the cable or antenna mode, the error threshold is set to a threshold commensurate with the error received in an NTSC signal having an appropriate signal level for proper detection and display. 
     If at step  210 , an external input source such as a video playback device, e.g., digital video disk (DVD) or video cassette recorder (VCR), is the source of the television signal, routine  200  proceeds to step  212  where horizontal sync signal detector  112  is configured for “VCR” mode. In the VCR mode, the error threshold is set to a level that is commensurate with a VCR signal that is detected and displayed properly. Generally, the current threshold for the NTSC signal is less than the error threshold for the VCR signal. However, in some instances, for example, when the VCR tape is worn or the VCR is in a fast forward or rewind mode, the threshold may be set to a lower level because the horizontal sync signal is distorted, yet the viewer may wish to see the image. The threshold levels may be set based on any appropriate method for determining the desired threshold levels, including, but not limited to, empirical testing and adjusting based on component specifications. 
     As such, the television apparatus according to the present invention may utilize multiple threshold values to classify the video signals. From either steps  212  or  218 , routine  200  proceeds to step  220 . At step  220 , a plurality of variables are set, including a weak signal count is set to 0, signal present is set to false, startup delay count is set to 1 and the start 100 millisecond status polling is begun such that horizontal sync detector  112  will be polled for a horizontal sync amplitude every 100 milliseconds by CPU  114 . Routine  200  is then exited at step  222 . Routine  200  then waits until another system change event has occurred. Meanwhile, buffer  124  of horizontal sync signal detector  112  will be polled every 100 milliseconds until the next change event occurs. 
       FIGS. 3A ,  3 B and  3 C depict a flow diagram of the horizontal sync detector status polling routine  300 . Routine  300  is executed every 100 milliseconds. Status polling routine  300  begins at step  302  and proceeds to step  304 . At step  304 , routine  300  determines the type of video signal digitizer that is used in signal processor  108 . Specifically, the routine checks an EEPROM containing the model number of the television set. From the model number, the routine derives the type of video signal digitizer. The parameters such as delays and threshold values may vary depending upon the accuracy of the digitizer. Thus, to create a versatile routine, routine  300  only sets the variables after confirming the digitizer type. The values used below are typical values. 
     Routine  300  is designed to operate with buffer  124  for horizontal sync detector  112 . Buffer  124  (also referred to as a status register) stores the samples of the horizontal sync signal that is sampled once in each of the video fields. In step  308 , buffer  124  is read. At step  310 , the horizontal sync status is queried as to whether the status is “OK” or not, i.e., whether the amplitude of the horizontal sync signal sufficient to deem that a television signal is present and of sufficient quality for display. If the answer to query  310  is negative, routine  300  proceeds to “B” in  FIG. 3B . If the query at step  310  is affirmatively answered, routine  300  proceeds to step  312 . 
     At step  312 , routine  300  queries whether the startup delay count is equal to zero. If the startup delay count is not equal to zero, routine  300  proceeds to step  320 . If the query at step  312  is affirmatively answered, routine  300  proceeds to step  314 . At step  314 , routine  300  queries whether the startup delay count is greater than or equal to 3. The value “3” is equivalent to 300 milliseconds i.e., three horizontal sync samples must be measured after channel change or source change before the “signal-present” variable is set to true. The value is empirically selected to provide a user friendly response to channel changes, i.e., the wait duration is selected to allow system transients to settle. If the query at step  314  is negatively answered, routine  300  proceeds to step  316  where the startup delay is implemented by one and routine  300  proceeds to “A”. If the query at step  314  is affirmatively answered, routine  300  proceeds to step  318  wherein the startup delay count is set to zero. Thereafter, a plurality of variables are set at step  320 . Specifically, “signal — present” is set to true, “weak — signal” count is set to zero, “error — detector” is set to false, and “confirm — error” is set to false. Routine  300  then proceeds to “A” in  FIG. 3B . 
     At step  322 , the horizontal sync status is deemed erroneous due to an error in the sample of the horizontal sync status register. At step  324 , routine  300  queries if the error detected is true. If the query at step  324  is negatively answered, routine  300  proceeds to step  326  where the variable query detected is set to true and, at step  328 , the 10 millisecond timer is set to re-poll for error confirmation. As such, once the status polling has been completed and an error has been detected, the status registers will be re-polled after 10 milliseconds to ensure that the status error was not caused by an error in reading the horizontal sync signal. As such, if the error in the horizontal sync signal was caused by a line being sampled early in the video field, the ten millisecond timer will re-poll the register and measure the horizontal sync signal later in the video field. 
     If, at step  324 , the query was affirmatively answered, routine  300  proceeds to step  330 . At step  330 , the variable confirmed error is set to true and the error detected variable is set to false. At step  332 , the routine queries if confirm error is set to true. If “confirm — error” is not set to true, then the routine proceeds to step  340  in  FIG. 3C . If the query at step  332  is affirmatively answered, the routine proceeds to step  334  where the “weak — signal” count is incremented by one and the “confirm — error” variable is set to false. At step  336 , routine  300  queries whether the “weak — signal — count is greater than or equal to the error threshold. The error threshold was previously set depending upon the type of source of the television signal, e.g., over the air versus playback device. If the query at step  336  is negatively answered, routine  300  proceeds to  FIG. 3C . However, if the query at step  336  is affirmatively answered, the variable “signal — present” is set to false and “weak — signal — count” is set to zero. 
     At step  340 , the “signal — present” variable is reported to the system on whether a television signal is present or not. At step  342 , the “signal — present” query is initiated to determine whether the “signal — present” variable is set to true or not. If the query at step  342  is affirmatively answered, an appropriate and proper television signal is deemed present and, at step  346 , the video display is enabled. If the query at step  342  is negatively answered, routine  300  proceeds to step  344  where the video display is disabled because a television signal is not available for display. At step  348 , routine  300  queries whether the “signal — present variables” has been false for more than two second. If the query is negatively answered, routine  300  proceeds to step  352  and exits. If the query is affirmatively answered, routine  300  proceeds to step  350  where a weak signal message is displayed on the display to the user. As such, the combination of steps  348  and  350  cause a weak signal display only after a weak signal has occurred for more than two seconds such that false weak signal displays are avoided. Routine  300  ends on step  352 . 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, it is to be understood that those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.