Abstract:
A television signal receiver comprises: a tuner, an optional IF conditioner, an IF distortion canceller, and an IF demodulator. The tuner selects one channel from a radio frequency television signal to generate an intermediate frequency signal. The IF conditioner outputs an IF conditioned signal. The IF distortion canceller cancels a signal distortion in the IF signal or the IF conditioned signal to generate an IF distortion-cancelled signal. The IF demodulator demodulates the IF distortion-cancelled signal to output a baseband signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a television signal receiver and in particular to a television signal receiver capable of cancelling linear and non-linear distortion. 
     2. Description of the Related Art 
     Group delay distortion commonly existing in television signal receiver decreases signal quality, and needs to be cancelled. Please refer to  FIG. 1 .  FIG. 1  shows a block diagram of a conventional television signal receiver  100 . The television signal receiver  100  includes a tuner  110 , an intermediate frequency (IF) filter  120 , an intermediate frequency (IF) demodulator  130 , a baseband distortion canceller  140 , and a controller  150 . The receiver  100  receives a radio frequency (RF) television signal S RF  from a transmitter (not shown). The tuner  110  selects one channel from the RF television signal S RF , and down-converts the selected channel from RF band to IF band. The IF filter  120  filters the selected channel to generate an IF filtered signal S IF . The IF demodulator  130  demodulates the IF filtered signal S IF  to generate a baseband signal S BB . The baseband distortion canceller  140  cancels a group delay distortion in the baseband signal S BB  to output a distortion-cancelled signal S canc . After cancelling the group delay distortion, the distortion-cancelled signal S canc  meets the requirement of a flat group delay response. Finally, the distortion-cancelled signal S canc  is sent to the television. Additionally, a controller  150  generates a control signal S ctrl  to program the baseband distortion canceller  140  to cancel the group delay distortion. A detailed description of cancelling the group delay distortion is provided in the following. 
     Please refer to  FIG. 2 .  FIG. 2  shows a plurality of curves of the group delay response. The baseband distortion canceller  140  provides programmable group delay (such as curves GD I  and GD II ) to cancel the group delay distortion of the receiver circuitry  100 . After cancellation, the group delay response of the receiver circuitry becomes flat (see the curve GD canc ). Ideally, the group delay response of the receiver circuitry  100  should be flat (the curve of which is similar to curve GD canc ) and hence the baseband distortion canceller  140  would be unnecessary. In practice, due to the various group delay precorrection transmitted from various transmitters and the deviation of IF filter group delay of various IF filters  120 , the group delay response of the receiver circuitry  100  is not flat. The above-mentioned group delay precorrection is performed at the transmitter. Various broadcasters may send the RF television signals with various group delay precorrections (e.g. half, full, or no precorrection). The above-mentioned IF filter group delay is a characteristic of the IF filter (filter  120 ) at the receiver. Various IF filters may have various IF filter group delays. An adequate IF filter has an almost flat IF filter group delay while an inadequate IF filter has an uneven IF filter group delay. Therefore, the baseband distortion canceller  140  must provide various programmable group delay (such as curves GD I  and GD II ) to cancel various group delay precorrections and IF filter group delays in different situations. 
     As one can see, the conventional baseband distortion canceller is capable of cancelling the group delay distortion in the baseband stage. As to distortion characteristic, the group delay distortion is a type of linear distortion, and the characteristic of which is substantially invariant in each stage (RF, IF, and baseband) of the receiver circuitry. The characteristic of non-linear distortion, however, changes in different stages. In other words, the non-linear distortion is hard to be processed and cancelled in the baseband stage since the characteristic is already changed in the demodulation process performed in the IF demodulator. Hence, the conventional baseband distortion canceller is hard to cancel the non-linear distortion. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     A television signal receiver is provided. The television signal receiver comprises: a tuner, an optional IF conditioner, an IF distortion canceller, and an IF demodulator. The tuner selects one channel from a radio frequency television signal to generate an intermediate frequency signal. The IF conditioner outputs an IF conditioned signal. The IF distortion canceller cancels a signal distortion in the IF signal or the IF conditioned signal depending on the presence of IF conditioner to generate an IF distortion-cancelled signal. The IF demodulator demodulates the IF distortion-cancelled signal to output a baseband signal. 
     A method of receiving an RF television signal is provided. The method comprises: selecting one channel from the RF television signal to generate an intermediate frequency signal; optionally conditioning the IF signal; canceling a signal distortion in the IF conditioned signal to generate an IF distortion-cancelled signal; demodulating the IF distortion-cancelled signal to output a baseband signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of a conventional television signal receiver; 
         FIG. 2  shows a plurality of curves of the group delay response; 
         FIG. 3  shows a block diagram of a television signal receiver according to an embodiment of the present invention; 
         FIG. 4  shows a block diagram of the IF distortion canceller of  FIG. 3 ; 
         FIG. 5A  shows various transfer curves in a receiver amplifier; 
         FIG. 5B  shows various mapping curves in a mapping-level device; 
         FIG. 5C  shows the final transfer curve in a receiver circuitry after cancelling distortion; 
         FIG. 6A  shows various group delay precorrection curves in the transmitter; 
         FIG. 6B  shows an exemplary curve of the aggregate group delay response in the IF tuner and the IF conditioner; 
         FIG. 6C  shows various curves of the group delay response in the IF distortion canceller; 
         FIG. 6D  shows the final group delay response in the receiver circuitry after cancelling distortion. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Please refer to  FIG. 3 .  FIG. 3  is a block diagram of an embodiment of a television signal receiver  300 . The television signal receiver  300  comprises a tuner  310 , an optional IF conditioner  320 , an IF distortion canceller  330 , a controller  340 , and an IF demodulator  350 . The major difference between the conventional television signal receiver shown in  FIG. 1  and this embodiment is that the operational signal of the distortion canceller changes from the baseband signal (generated after the demodulation process in the IF demodulator) to the IF conditioned signal (before the demodulation process in the IF demodulator). The IF distortion canceller  330  of the invention can process not only the linear distortion but also the non-linear distortion prior to the demodulation process. In the demodulation process, the non-linear distortion changes its characteristic and is hard to be cancelled after that. Additionally, the IF distortion canceller  330  is programmable and the controller  340  sends a control signal S ctrl  to program the IF distortion canceller  330 . A detailed description of programming the IF distortion canceller  330  will be provided later. The optional IF conditioner  320  provides basic IF signal processing like filtering, amplification, mixing, or quantization. Depending on the discretion of those implementing this invention, the IF conditioner may include as simple as an IF filter to further reject unwanted interference, may include an amplifier to adjust the IF signal power level, may contain a frequency shifter to move the signal spectrum over the IF band, may enclose a quantizer to digitize the signal, or may constitute an arrangement of some or all the above practices, which are all well know and commonly adopted. As the circuitry used in the IF conditioner are potential sources of linear and non-linear distortion in the IF conditioned signal, it is preferred that the IF distortion canceller be placed after the IF conditioner to cancel the distortion generated by the IF conditioner. As to other elements ( 310  and  350 ), because the operation and functionality are similar to conventional elements, further discussion is omitted for the sake of brevity. A detailed description of the IF distortion canceller  330  is provided below. 
     Please refer to  FIG. 4 .  FIG. 4  is a block diagram of an embodiment of the IF distortion canceller  330  in  FIG. 3 . The IF distortion canceller  330  comprises a level-mapping device  410  and a programmable filter  420 . The level-mapping device  410  cancels the aggregate transfer curve distortion (a type of non-linear distortion) of the tuner  310  and the IF conditioner  320 , while the programmable filter  420  cancels the group delay distortion (a type of linear distortion) before demodulation. Detailed description of canceling the transfer curve distortion and the group delay distortion will be provided later (respectively in  FIG. 5  and  FIG. 6 ). Please note that the invention does not limit the type of distortion to be cancelled. The above-mentioned distortions (transfer curve distortion and group delay distortion) only serve as examples. In some embodiments, the IF distortion canceller  330  further comprises an amplitude distortion canceller for cancelling the amplitude response distortion (a type of linear distortion). In some other embodiments, the programmable filter  420  is implemented as a digital filter with adaptable coefficients and hence capable of simultaneously cancelling two linear distortions: amplitude response distortion and group delay distortion. A detailed description of the level-mapping device  410  for cancelling the transfer curve distortion (non-linear distortion) is provided below. 
     Please refer to  FIGS. 5A˜5C .  FIG. 5A  shows various transfer curves in various tuners and IF conditioners while  FIG. 5B  shows various mapping curves in the mapping-level device  410 .  FIG. 5C  shows the final transfer curve in the receiver circuitry  300  after cancelling distortion. A transfer curve is an aggregate characteristic of circuitry such as amplifiers or mixers implemented in the tuner  310  and the IF conditioner  320 . The slope of the transfer curve is the voltage gain of the circuitry. In an ideal circuit, the transfer curve is a straight line and the slope (voltage gain) is always constant (see the ideal transfer curve TC ideal  in  FIG. 5A ). In practice, there is no ideal circuit. The transfer curve of an actual circuit is straight in the beginning and saturates gradually. This is referred to as the transfer curve distortion (a type of non-linear distortion). Various circuits may have various transfer curve distortion characteristics (see the exemplary transfer curves TC I  and TC II  in  FIG. 5A ). Please refer to  FIG. 5B . In order to cancel the transfer curve distortion, the level-mapping device  410  is configured to provide the transposed curve of the non-straight transfer curve of the circuit as its mapping curve (see exemplary mapping curves MAP I  and MAP II  in  FIG. 5B ). In this example, the mapping curve MAP I  corresponds to the transfer curve TC I  while the mapping curve MAP II  corresponds to the transfer curve TC II . 
     Examples of cancelling the transfer curve distortion are provided as follows. If, for example, the transfer curve of the tuner and the IF conditioner circuitry is TC I , the level-mapping device  410  is configured to provide the mapping curve MAP I  to cancel the transfer curve distortion. If, as another example, the transfer curve of the tuner and the IF conditioner circuitry is TC II , the level-mapping device  410  is configured to provide another mapping curve MAP II  to cancel the transfer curve distortion. Please refer to  FIG. 5C . After cancellation, the final transfer curve turns into an actual linear line (see the curve TC canc ) and the transfer curve distortion in the receiver circuitry is cancelled. Additionally, the configuration of a mapping curve in the level-mapping device  410  is controlled by control signal S ctrl . The control signal S ctrl  may send a mapping table, a mapping function, or an instruction to the level-mapping device  410  to setup and utilize a proper mapping curve. 
     A detailed description of the programmable filter  420  for cancelling another kind of distortion (group delay distortion) is provided below. Please refer to  FIGS. 6A˜6D .  FIG. 6A  shows various group delay precorrection curves in the transmitter (not shown).  FIG. 6B  shows exemplary curves of the aggregate group delay response in the tuner  310  and the IF conditioner  320  while  FIG. 6C  shows various curves of the group delay response in the IF distortion canceller  330 .  FIG. 6D  shows the final group delay response in the receiver circuitry  300  after cancelling distortion. The broadcaster may provide the RF signals with full group delay precorrection (curve PRE full  in  FIG. 6A ), half group delay precorrection (curve PRE half  in  FIG. 6A ), or no group delay precorrection (curve PRE none  in  FIG. 6A ) from the transmitter. Please refer to  FIG. 6B . The group delay response of the tuner  310  may be the curve GD tuner , while the group delay response of the IF conditioner may be the curve GD cond . The aggregate group delay response curve GD IF  is the superposition of GD tuner  and GD cond  due to the linear nature of the group delay distortion. Please note that a group delay distortion results from a non-flat group delay response, which is a characteristic of the circuitry used in the tuner and the IF conditioner. Therefore, the aggregate group delay response depends on the specific implementation of the tuner and the IF conditioner and its curve may not resemble the exemplary curve GD IF  given in  FIG. 6B . 
     Please refer to  FIG. 6C . In this embodiment, if the half group delay precorrection (curve PRE half  in  FIG. 6A ) is cancelled by the group delay of IF conditioner (GD cond  given in  FIG. 6B ), the programmable filter  420  can be configured to provide group delays (see curves GD I , GD II , GD III ) to cancel the superposition group delay distortion introduced from the various precorrection in the transmitter (PRE full , PRE half , and PRE none  in  FIG. 6A ) and the aggregate group delay distortion of the tuner and the IF conditioner (GD IF  in  FIG. 6B ) respectively. In other words, the distortion introduced by the full group delay precorrection (curve PRE full  in  FIG. 6A ) and the aggregate group delay distortion of the tuner and the IF conditioner (GD IF  in  FIG. 6B ) is cancelled or compensated by programmable filter  420  which is configured to provide group delay curve GD I . The distortion introduced by the half group delay precorrection (curve PRE half  in  FIG. 6A ) and the aggregate group delay distortion of the tuner and the IF conditioner (GD IF  in  FIG. 6B ) is cancelled or compensated by programmable filter  420  which is configured to provide group delay curve GD II . The distortion introduced by the no group delay precorrection (curve PRE none  in  FIG. 6A ) and the aggregate group delay distortion of the tuner and the IF conditioner (GD IF  in  FIG. 6B ) is cancelled or compensated by programmable filter  420  which is configured to provide group delay curve GD III . Therefore, the group delay response at the output of the IF distortion canceller  330  is flat. 
     Please note that the programmable filter  420  cancels the group delay distortion to generate the distortion-cancelled signal S canc  before the IF demodulator, not after the IF demodulator. In other words, the distortion-cancelled signal S canc  is still an IF signal requiring further demodulation process to become a baseband signal acceptable to a TV. (The conventional distortion-cancelled signal S canc  is already a baseband signal like CVBS). The distortion-cancelled signal S canc  is then sent to the IF demodulator  350  (see  FIG. 3 ) to generate the baseband signal S BB . Additionally, the group delay response curve in the programmable filter  420  is configured by the control signal S ctrl . The control signal S ctrl  may send a plurality of coefficients (e.g. finite impulse response coefficients in case of programmable digital filters) or an instruction to the programmable filter  420  to setup and utilize a proper curve of group delay response. Please refer to  FIG. 6D . After cancellation, the group delay response of the receiver circuitry turns into a generally flat curve (see the curve GD canc ). 
     Compared with the conventional distortion cancellers, the operational band of the inventive distortion canceller moves from the baseband to the IF band. At the IF band, the characteristic of non-linear distortion has not yet been destroyed and hence the IF distortion canceller of the invention can cancel the non-linear distortion. In other words, the IF distortion canceller of the invention can cancel both linear and non-linear distortion while the conventional baseband distortion canceller can only cancel the linear distortion. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.