Abstract:
In an analog terrestrial mode of a TV having analog and digital capability, the RF gain is controlled using the TV&#39;s demodulator output. In a digital cable mode the RF gain is controlled using the output of the TV&#39;s IF SAW filter, and in an analog cable mode the RF gain is controlled using the output of an analog demodulator.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 10/781,289, filed Feb. 17, 2004 now U.S. Pat. No. 7,053,961. 

   I. FIELD OF THE INVENTION 
   The present invention relates generally to television automatic gain control (AGC). 
   II. BACKGROUND OF THE INVENTION 
   Both the RF and IF gain of a television may be automatically controlled to improve picture quality. The RF gain may be controlled under high signal strength conditions and the IF gain under low signal strength conditions. 
   Both gains may be controlled using a signal from the TV&#39;s digital demodulator and/or the TV&#39;s analog demodulator, but as recognized herein this scheme can result in large distortions particularly when cable signals are received (i.e., in a cable mode of the TV). The distortion in turn causes picture deterioration in the analog channel, and result in pixellation errors or no picture at all for digital channels. Alternatively, the RF gain can be controlled using the tuner output (i.e., the IF signal produced by the tuner prior to demodulation), but this can introduce undesirable noise, again resulting in, e.g., pixellation errors or no picture at all for digital channels. 
   Having recognized these drawbacks, the solutions below are provided. 
   SUMMARY OF THE INVENTION 
   A system includes a television tuner having at least one radiofrequency (RF) gain adjust amplifier and generating an intermediate frequency (IF) output signal. A filter such as a SAW filter receives the IF output signal and outputs a filtered IF output signal. A demodulator demodulates the IF signal to provide a signal for display on a television display. In the present invention, the RF gain adjust amplifier receives, as input, the IF output signal when the system is in a terrestrial mode and at least one of: the filtered IF output signal, and a signal from the demodulator, when the system is in a cable mode. Specifically, in non-limiting embodiments the RF gain adjust amplifier receives, as input, the filtered IF output signal in a digital cable mode and a signal from the demodulator in an analog cable mode. 
   If desired, an IF gain adjust amplifier can be provided to receive the IF output signal or the filtered IF output signal. The IF gain adjust amplifier may receive, as input, an output signal from the demodulator. One or both of an analog and digital demodulator may be provided. 
   In some embodiments a first power detection circuit includes a first control voltage amplifier interposed between the tuner and the RF gain adjust amplifier. The first power detection circuit receives the IF output signal. A second power detection circuit can be provided which includes a second control voltage amplifier interposed between the filter and the RF gain adjust amplifier, with the second power detection circuit receiving the filtered IF output signal. A second switch can receive the outputs of at least one control voltage amplifier. If desired, a first power detection line can connect the IF output signal to a single second switch and a second power detection line can connect the filtered IF output signal to the single second switch. 
   In another aspect, a method for controlling a radiofrequency (RF) gain of a television system includes, when the television system is in a terrestrial mode, controlling the gain using an intermediate frequency (IF) signal output by a television tuner. The method also includes, when the television system is in a digital cable mode, controlling the gain using a filtered IF signal output by a filter, and when the television system is in an analog cable mode, controlling the gain using a signal output by a demodulator. 
   In still another aspect, a system for controlling the radiofrequency (RF) gain of a television includes means for controlling an RF gain of the television, and means for generating an intermediate frequency (IF) signal. Means are provided for filtering the IF signal to render a filtered IF signal. Switch means select which signal to send to the means for controlling, based on a mode of the television. 
   In another aspect a system includes a television tuner having at least one radiofrequency (RF) gain adjust amplifier and generating an intermediate frequency (IF) output signal. A filter such as a SAW filter receives the IF output signal and outputs a filtered IF output signal. A demodulator demodulates the IF signal to provide a signal for display on a television display. In this embodiment, the RF gain adjust amplifier receives, as input, a signal from the demodulator when the system is in an analog terrestrial mode. The filtered IF output signal, or a signal from the demodulator, is used to control gain when the system is in a cable mode. 
   In another aspect of the above embodiment, a method is disclosed for controlling a radiofrequency (RF) gain of a television system. When the television system is in an analog terrestrial mode, the gain is controlled using a demodulated signal output by a demodulator. When the television system is in a digital cable mode, the gain is controlled using a filtered IF signal output by a filter, and when the television system is in an analog cable mode, the gain is controlled using a signal output by a demodulator. 
   The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the present TV system; 
       FIG. 2  is a block diagram of a first non-limiting power detection circuitry; and 
       FIG. 3  is a block diagram of a second non-limiting power detection circuitry. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring initially to  FIG. 1 , a system is shown, generally designated  10 , in which a TV content provider  12  (only one provider shown for clarity) provides terrestrial and cable TV signals to a television system, generally designated  14 , the relevant portions of which are shown in  FIG. 1  and described herein. It is to be understood that the content provider  12  can be one or more providers or sources such as cable/terrestrial boxes, wires, antennae, satellites, etc. 
   The TV system  14  includes a tuner  16  that can in turn include, in series from input to output, an RF gain control amplifier  18  that receives the input radiofrequency (RF) signal, a filter  20 , a mixer  22  that receives a downconvert signal from an oscillator  24 , and an output amplifier  26 . The skilled artisan will recognize that the output of the amplifier  26  is an intermediate frequency (IF) signal. 
   As shown in the preferred embodiment of  FIG. 1 , the IF signal from the tuner  16  is sent to an analog channel and a digital channel. More particularly, the IF signal is sent to an analog amplifier  28  and then to a surface acoustic wave (SAW) filter  30 , prior to being input to a National Television System Committee (NTSC) demodulator  32 . Also, the IF signal is sent to an amplifier  34  and then to a surface acoustic wave (SAW) filter  36  that generates a filtered IF signal, prior to being input after gain control amplification to a digital demodulator  38  such as a quadrature amplitude modulation (QAM)/eight level vestigial sideband (8VSB) demodulator. More particularly, an IF gain control amplifier  40  is in electrical series with the SAW filter  36  and digital demodulator  38  and receives its control signal from the digital demodulator  38  as shown by the dashed line. 
   In accordance with the present invention, two automatic gain control (AGC) switches or equivalent structure (e.g., logic gates in a system implemented in software) are provided. Specifically, as shown in  FIG. 1  a first switch  42  selectively outputs a gain control signal to the RF gain control amplifier  18  using two inputs. One input to the first switch  42  is selectively from the analog demodulator  32  via a second switch  44 , and one input is from the IF output via a first power detection circuit  46 , the details of which will be shortly disclosed. The second switch  44  receives input from the analog demodulator  32  and the filtered IF signal from the output of the SAW filter  36  via a second power detection circuit  48 . Also, the second switch  44  receives an input from the digital demodulator  38 , indicating whether the TV is in a terrestrial mode (i.e., whether the current channel is a terrestrial channel) or a cable mode (i.e., indicating whether the current channel is a cable channel) and indicating a digital or analog cable signal in accordance with principles known in the art. Additional processing circuitry  50  can receive the demodulated signal(s) to drive a TV display  52  in accordance with principles known in the art. 
   Based on what the signal from the digital demodulator  38  indicates, the circuit of  FIG. 1  can assume a terrestrial signal mode, wherein the first switch  42  operates to establish electrical connectivity between the first power detection circuit  46  (and, hence, the IF output signal) and the RF gain control amplifier  18 , for adjusting the balance between NF and distortion for the terrestrial signal receiver using the IF output signal. On the other hand, in a digital cable mode, the switches  42 ,  44  cooperate such that the signal from the second power detection circuit  48  (and, hence, the output of the SAW filter  36 ) is provided to the RF gain control amplifier  18 , for adjusting the balance between NF and distortion for the cable receiver using the SAW filter output. The third possibility—analog cable—invokes an analog cable mode, in which the switches  42 ,  44  cooperate to connect the analog demodulator  32  to the RF gain control amplifier  18  to control the amplifier  18 . In yet a fourth possibility—analog terrestrial—a mode can be invoked in which the switches  42 ,  44  cooperate to connect a demodulator such as the analog demodulator  32  to the RF gain control amplifier  18  to control the amplifier  18 . 
   Having described the components and operation of the system  10 , attention is directed to  FIGS. 2 and 3 , which show non-limiting examples of respective implementations of the power detection circuits  46 ,  48  and the switches  42 ,  44  shown in  FIG. 1 .  FIG. 2  shows that each circuit  46 ,  48  can include various resistors (labelled “R”) and a respective. control signal amplifier  54 ,  56 . Each control signal amplifier receives its own reference voltage (labelled “V c ” in  FIG. 2 ) that has a pulse width which is established as appropriate for the particular TV mode associated with the respective amplifier  54 ,  56 , i.e., terrestrial or cable. 
   In contrast,  FIG. 3  shows that in an alternate embodiment, each power detection circuit  46 ,  48  can include a respective power detection line  60 ,  62  that can be selectively connected through the switch or switches  42 ,  44  to a single control signal amplifier  64  to develop the control signal. As indicated in  FIG. 3 , the reference to the control signal amplifier  64  is pulse width modulated (PWM), with the pulse width being established as appropriate for the particular TV mode. 
   While the particular SYSTEM AND METHOD FOR TV AUTOMATIC GAIN CONTROL (AGC) is herein shown and described in detail the scope of the present invention is to be limited by nothing other than the appended claims.