Abstract:
A dual HDTV/NTSC receiver for recovering one of a HDTV signal and a NTSC signal coexisting in a desired channel includes a tuning unit for converting a desired one of HDTV signals and NTSC signals into an intermediate frequency (IF) signal, a sync detector, coupled to receive the IF signal for detecting a sync signal of the NTSC signal, a timing recovery unit, coupled to receive the IF signal for self-recovering symbol timing of the applied HDTV signal, and providing a symbol timing lock signal corresponding to the IF signal and an analog-to-digital converted HDTV signal, a controller for judging whether a currently received television signal is either a NTSC signal or a HDTV signal based on the sync detection result of the sync detector and the symbol timing lock signal from the timing recovery unit, and for outputting a control signal according to the determination result, and a tuning controller for initially controlling the tuning unit so that one desired HDTV signal is received, and for subsequently controlling the tuning unit on the basis of the signal selected by the control signal among respective HDTV and NTSC signals output, respectively, from the tuning unit and the timing recovery unit. The receiver automatically determines whether a received signal is the NTSC signal or the HDTV signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high definition television (HDTV) receiver and, more particularly, to a HDTV receiver that is also capable of receiving NTSC signals. 
     The instant application is based on Korean Patent Application No. 94-37542, which is incorporated herein by reference for all purposes. 
     2. Brief Discussion of Related Art 
     Recently, the United States has elected the 8-VSB (Vestigial Side Band) modulation method as the designated method for HDTV transmission. It will be noted that HDTV broadcasting is initially planned to be put into service together with a conventional NTSC broadcasting. Thus, the HDTV receiver will be required to have a function capable of receiving both HDTV signals and conventional NTSC signals. 
     A dual-mode receiver for receiving HDTV signals and NTSC signals has been developed which is capable of processing the received signals by a HDTV or NTSC method according to a user&#39;s selection. One such receiver, which is disclosed in U.S. Pat. No. 5,283,653 entitled “DUAL HDTV/NTSC RECEIVER USING SEQUENTIALLY SYNTHESIZED HDTV AND NTSC CO-CHANNEL CARRIER FREQUENCIES” to Citta (Feb. 1, 1994) includes a dual HDTV/NTSC television receiver able to automatically select an appropriate signal when tuned to a selected television channel. This receiver selectively receives HDTV and NTSC television signals in one channel, among a plurality of channels, in which HDTV signals are of an inherently lower power than the co-channel NTSC signals. A tuner in the receiver initially synthesizes a carrier frequency associated with the HDTV signal of the selected channel, and a narrow band sync detector detects the HDTV signal using the synthesized HDTV carrier frequency. A microprocessor enables further processing of either the HDTV signal or the NTSC according to the detection result of the HDTV signal. 
     The present invention was motivated by a desire to provide an improved HDTV/NTSC receiver while avoiding the problems inherent in the conventional receiver discussed above. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a novel HDTV/NTSC receiver using symbol timing recovery and a sync signal detection. 
     Another object of the present invention is to provide a method of receiving both HDTV signals and NTSC signals using symbol timing recovery and sync signal detection. 
     These and other objects, features and advantages according to the present invention are provided by a receiver for selectively receiving HDTV television signals and NTSC television signals in a plurality of television channels. Preferably, the receiver includes: 
     a tuning unit for converting a desired one of HDTV signals and NTSC signals into an intermediate frequency signal; 
     a sync detector, coupled to receive the intermediate frequency signal output from the tuning unit, for detecting a sync signal of the NTSC signal; 
     a timing recovery unit, coupled to receive the intermediate frequency signal output from the tuning unit, for self-recovering symbol timing of the applied HDTV signal, and outputting a symbol timing lock signal and an analog-to-digital converted HDTV signal; 
     a controller for judging whether the currently received television signal is either a NTSC signal or a HDTV signal based on the sync detection result of the sync detector and the symbol timing lock signal from the timing recovery unit, and for outputting a control signal according to the determination result; and 
     a tuning controller for initially controlling the tuning unit so that one desired HDTV signal is received, and for subsequently controlling the tuning unit on the basis of the signal selected by the control signal among respective signals output from the tuning unit and the timing recovery unit. 
     These and other objects, features and advantages according to the present invention are provided by a method of receiving a signal of a desired channel in HDTV/NTSC television receiver. Advantageously, the method includes steps for: 
     (a) initially tuning a receiving signal to accept an HDTV signal of a desired channel and converting the tuned signal into an intermediate frequency signal; 
     (b) performing a self-recovery operation of symbol timing using the intermediate frequency signal generated during step (a) and generating a signal indicating a respective self-recovery result; 
     (c) tuning a receiving signal to accept a NTSC signal of the desired channel and converting the tuned signal into an intermediate frequency signal, if self-recovery operation of symbol timing is not accomplished during step (b); 
     (d) carrying out detection of the horizontal/vertical sync signal of the NTSC signal using the intermediate frequency signal generated in step (c), and providing a signal indicative of a detection result; and 
     (e) continuously receiving the HDTV signal when the self-recovery result signal indicates that symbol timing is self-recovered, and continuously receiving the NTSC signal when the detection result signal indicates that the horizontal/vertical sync signal is detected. 
     These and other objects, features and advantages of the invention are disclosed in or will be apparent from the following description of preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments are described with reference to the drawings in which: 
     FIG. 1 is a high level block diagram showing a dual HDTV/NTSC receiver in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a high level block diagram illustrating an exemplary internal structure of a symbol timing recovery unit which is usable in the HDTV/NTSC receiver of FIG. 1; and 
     FIG. 3 is a flow chart for illustrating an operation the apparatus of FIG. 1, with particular emphasis on the operation of a mode controller depicted in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described below in more detail with reference to the accompanying drawings of FIGS. 1 to  3 . 
     It will be appreciated that HDTV signals are suppressed so that video and audio information can be transmitted in frequency bands of 6 MHz for every channel. The HDTV signals provided by the transmitter are randomized for signal power, so as to be uniformly distributed in the television channel broadcasting bands of 6 MHz. This randomization makes a receiver perform a channel equalization operation, the randomized signal advantageously allows the receiver to recover symbol timing using a self-timing recovery procedure. 
     In the case of the HDTV signal according to the U.S. standard, i.e., the 8-VSB modulation method, a separate segment sync signal for symbol timing recovery and a reference signal having a constant frequency for carrier recovery are transmitted together with data. Preferably, the segment sync signal of four (4) bits is inserted between each 828 data symbols. Thus, the HDTV receiver can be tuned to frequencies of the input HDTV signal using the segment sync signal and reference signal. However, since a HDTV signal is transmitted at a low power level when compared with the NTSC signal, at the initial operation of the system, detection of the segment sync signal and the reference signal may be difficult. Therefore, almost complete recovery for the carrier from the low power HDTV signal should be carried out first. The more complete the recovery of carrier is, the more accurate the symbol timing recovery can be. 
     It should be noted that, in order to shorten an initializing time of the HDTV receiver, it is advantageous to, in the first place, adopt self-timing recovery by a symbol timing recovery unit rather than a sync signal. It will be appreciated that the NTSC signal is formatted so that the HDTV receiver can not extract data of the transmission rate of the HDTV signal from the NTSC signal passed through the symbol timing recovery unit. Accordingly, it is impossible that symbol timing recovery unit can be stably operated. The present invention proposes a dual receiver capable for receiving HDTV and NTSC signals, which automatically detects the type of signal source and determines an optimal receiving mode based on the difference of HDTV signals and NTSC signals. 
     It should be noted that a preferred embodiment of the present invention employs symbol timing recovery unit having a self-timing recovery operation. Of course, it is possible to develop an alternative embodiment of the dual HDTV/NTSC receiver employing monitoring of the operation of symbol timing recovery unit using the segment sync signal. 
     Turning now to the figures, FIG. 1 shows the dual HDTV/NTSC receiver in accordance with a preferred embodiment of the present invention. A tuner  1  of FIG. 1 advantageously receives a broadcasting signal of a desired channel among UHF/VHF broadcasting signals and converts it to an intermediate frequency (IF) signal. A microprocessor  2  selects a channel that is to be output by tuner  1  while also controlling the ancillary operations of tuner  1 . The IF signal output by tuner  1  is applied to a surface acoustic wave (SAW) filter  3 , which exactly filters the IF signal to produce a signal having 6 MHz band width. An intermediate frequency (IF) amplifier  4  advantageously controls the gain of the filtered IF signal input from SAW filter  3  according to the output signal from a NTSC signal processing unit  6  or a HDTV signal processing unit  12  feedback via a second switch  18 , and outputs a signal amplified to an appropriate level. Output signals from IF amplifier  4  are supplied to a horizontal/vertical sync detector  5 , mixer  9  and NTSC frequency phase locked loop (FPLL) circuit  15 , respectively. It will be appreciated that the above described tuner  1 , SAW filter  3  and IF amplifier  4  can be used for processing both HDTV signals and NTSC signals. 
     Horizontal/vertical sync detector  5  detects horizontal and vertical sync signals from the output signal of IF amplifier  4 , and signals a mode controller  14  whether a sync signal is detected or not. In case that the received signal is a NTSC signal, horizontal/vertical sync detector  5  denotes the detection of the sync signal to mode controller  14 . NTSC signal processing unit  6  processes the output signal of horizontal/vertical sync detector  5  so that the output signal can be displayed on a monitor  7 , and outputs the processed signal via a first switch  17 . FPLL circuit  15  for recovering a carrier of the NTSC signal receives the output signal of IF amplifier  4 , judges whether a carrier signal of the desired channel is accurately recovered, and outputs a correcting value of frequency error as the judgement result to a third switch  19 . 
     Preferably, mixer  9  multiplies the output signal of IF amplifier  4  by an oscillation signal having a constant frequency from a local oscillator  8 . It should also be noted that mixer  9  can alter the output signal of intermediate frequency amplifier  4  to a signal about the base band through frequency band conversion, so that full digital signal processing for the HDTV signal can be performed. Local oscillator  8  has a fixed oscillation frequency. A low pass filer (LPF)  10  removes unnecessary frequency components from the output signal of mixer  9 . 
     An analog-to-digital converter  11  samples a HDTV signal of the base band output from LPF  10  under the control of symbol timing recovery unit  13 . Since the symbol transmission rate Fs of the HDTV signal is about 10.76 MHz, A/D converter  11  samples the input signal about the base band responsive to frequency NFs, i.e., integer N times of symbol transmission rate Fs. The output signal of A/D converter  11  advantageously is applied to symbol timing recovery unit  13 , HDTV signal processing unit  12  and HDTV FPLL circuit  16 , respectively. Symbol timing recovery unit  13  receives the sampled HDTV signal and designates a sampling point to be used by A/D converter  11  in order to sample the input signal in the same interval and phase as that occurring at the transmitter. Detailed construction and operation of symbol timing recovery unit  13  will now be described with particular reference to FIG.  2 . 
     Referring to symbol timing recovery unit  13  of FIG. 2, timing error information generator  20  calculates an error between an optimized sampling point of time and a sampling point of time of the sampled HDTV signal supplied from A/D converter  11 . The calculation result, i.e., error information Pn is advantageously individually applied to a loop filter  21  and a symbol timing lock detector  22 . Loop filter  21  is employed for filtering the input error information P n , and preferably includes a phase locked loop (PLL) circuit (not shown). Characteristics of loop filter  21  may be represented by the following equation:            τ   s          (     n   +   1     )       =     {               τ   s          (   n   )       -       μ   1          P   n               (1st)                   τ   s          (   n   )       -       μ   1          P   n       -       μ   2            ∑     k   =   1     n                     P   k                 (2nd)                                       
     where τ s (n+1) and τ s (n) are output signals of the loop filter at (n+1)-th point of time and n-th point in time, respectively, P n  and P k  are error information, and the first and second cases represent the operation of loop filter of the first PLL and second PLL in (n+1)-th point of time, respectively. The output signal τ n  of loop filter  21  is supplied to both symbol timing lock detector  22  and a voltage controlled oscillator (VCO)  23 , respectively. Symbol timing lock detector  22  monitors either the output signal P n  of timing error information generator  20  or the output signal τ n  of loop filter  21 , and judges whether the symbol timing recovery is successfully made according to the variation degree of each value. More specifically, if an average value of the output signal P n  of timing error information generator  20  is about “O” or the output signal τ n  of loop filter  21  is constant without variation, symbol timing lock detector  22  determines that sampling is made at the optimized point and outputs the symbol timing lock signal. VCO  23  outputs an analog sine wave signal having an oscillation frequency which is varied according to the output signal τ n  of loop filter  21 . A clock generator  24 , which is connected to an output terminal of VCO  23 , compares the sine wave signal with a reference signal and outputs a clock signal of frequency NFs, i.e., integer N times the symbol transmission rate Fs, to A/D converter  11 , as previously described. 
     Referring again to FIG. 1, HDTV signal processing unit  12  receives the HDTV signal sampled by A/D converter  11 , and processes it for display on monitor  7  by output of the processed signal via first switch  17 . HDTV FPLL circuit  16 , which is a circuit for recovering a carrier of the HDTV signal, receives the output signal of A/D converter  11 , judges whether a carrier signal of a desired channel is accurately recovered, and outputs a correcting value representing frequency error. 
     Mode controller  14 , which is preferably coupled to output terminals of horizontal/vertical sync detector  5  and symbol timing recovery unit  13 , determines whether the receiver is to be operated in either a HDTV mode or a NTSC mode. The determination depends on whether a sync signal and a symbol timing lock signal are input or not, and control signals based on such determination are supplied to first through third switches  17 - 19 . Of these first through third switches  17 - 19 , first switch  17  is coupled to selectively supply the output signal of either NTSC signal processing unit  6  or HDTV signal processing unit  12  to monitor  7 . Second switch  18  is coupled to selectively output the output signal of either NTSC signal processing unit  6  or HDTV signal processing unit  12  to intermediate frequency amplifier  4 . Third switch  19  is connected to selectively output the output signal of either NTSC FPLL circuit  15  or HDTV FPLL circuit  16  to microprocessor  2 , which controls the oscillation frequency of tuner  1 in response to the correcting value of a frequency error value supplied from NTSC FPLL circuit  15  or HDTV FPLL circuit  16 . 
     The operation of the FIG. 1 apparatus will now be described in more detail with reference to the flow chart of FIG. 3 showing the signal processing process of mode controller  14 . 
     When the FIG. 1 apparatus initially operates according to the supply of system power, mode controller  14  controls first to third switches  17 - 19  so that the HDTV mode is ‘on’ and the NTSC mode is ‘off’ (step  101 ). During step  101 , first and second switches  17  and  18  select the output of HDTV signal processing unit  12 , and third switch  19  chooses that of HDTV FPLL circuit  16 . At this time, microprocessor  2  controls tuner  1  so that it is tuned for the recovery of the HDTV signal from the selected channel. 
     Symbol timing recovery unit  13  performs the self-recovery operation of symbol timing. By this operation, when the data transmitted at transmission rate Fs has a randomized characteristic, symbol timing can be exactly extracted within hundreds and thousands of symbols. 
     The symbol timing lock signal representative of the result of self-recovery is applied to mode controller  14  (step  102 ). Mode controller  14  judges whether the HDTV signal is input, based on the input symbol timing lock signal, and thereby judges whether the symbol timing lock signal is input from symbol timing recovery unit  13 , after a predetermined time T 1  during step  103 , at step  104 . When it is judged that the symbol timing lock signal is present at step  104 , mode controller  14  judges that symbol timing recovery unit  13  has stabilized and controls switches  17 - 19  in order to receive the HDTV signal continuously (step  105 ). During step  104 , if the symbol timing lock signal is absent, mode controller  14  controls first to third switches  17 - 19  so that the NTSC mode is ‘on’ and the HDTV mode is ‘off’ (step  106 ). During step  106 , first and second switches  17  and  18  select the output of NTSC signal processing unit  6  and third switch  19  chooses that of NTSC FPLL circuit  15 . Mode controller  14  judges the presence or absence of the NTSC signal based on whether a horizontal or vertical sync signal from horizontal/vertical sync detector  5  is present or not. When a predetermined time T 2  expires during performance of step  107 , it is judged whether the horizontal or vertical sync signal is input following the preset time T 2  (step  108 ). At step  108 , if the horizontal or vertical sync signal is present, mode controller  14  controls switches  17 - 19  in order to receive the NTSC signal subsequently. If the horizontal or vertical sync signal is not detected at step  108 , a set of the above processes (steps  101 - 108 ) are iterated, after determining as the NTSC signal is not present. During step  110 , when the number of iteration times becomes ‘N’, mode controller  14  determines that no signal is received and takes a set of predetermined actions, such as displaying information indicating that no signal has been received on a screen (step  111 ). 
     As described above, the present invention automatically determines whether a receiving television signal is either a NTSC signal or a HDTV signal to thereby improving convenience to the user, based on symbol timing self-recovered from the received HDTV signal and a sync signal detected from the received NTSC signal. Particularly, as the receiving signal is automatically determined, receivable signals with respect to the whole channels can be stored in advance in memory when setting up the television receiver. The apparatus of the present invention provides an effect that no checking process as to the receiving signal is input with regard to each channel, is required, whenever the user turns on the system. 
     While only certain embodiments of the invention have been specifically described herein, it will apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention.