Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/126,946 entitled “Methods and Apparatus for Transmitting analog and Digital Information Signals” filed Jul. 31, 1998 now U.S. Pat. No. 6,377,314, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods and apparatus for transmitting analog and digital information signals within a particular frequency bandwidth. 
     2. Description of the Background 
     Television broadcasters transmit standard analog television signals over channels regulated by the Federal Communications Commission (FCC). These signals conform to the requirements of the National Television Standards Committee (NTSC), administered by the FCC, and the signals are thus referred to as NTSC analog television signals. The current NTSC standard requires transmission of 525 lines of resolution transmitted as 30 interlaced frames per second (60 half frames per second). The FCC permits transmission of NTSC analog television signals over channels having a 25 megahertz bandwidth, an industry standard. 
     Recently, broadcasters have had a need to transmit digital television signals with the advent of high definition television (HDTV) and standard definition television (SDTV). These digital television signals are known as HDTV signals and SDTV signals, both of which conform to known industry standards. Obtaining additional bandwidth to transmit digital television signals can be difficult. Expanding a standard 25 megahertz channel is not be possible, at least since another channel likely exists adjacent the channel, and expansion of one channel would cause interference with another. Also, due to a limited number of channels available in the radio frequency (RF) spectrum used for transmission of television signals, broadcasters may have difficulty obtaining additional channels. 
     Therefore, a technique has been developed to transmit both analog and digital television signals within a standard 25 megahertz channel. This technology involves digitizing an NTSC analog television signal, combining it with an HDTV signal, and transmitting both as one digital signal centered on a carrier signal. However, digitizing an NTSC analog television signal often adversely affects its picture quality, resulting in what are referred to as “artifacts” in the picture. In addition, digitizing NTSC analog television signals tends to adversely affect picture quality in panning, involving moving the camera at least horizontally to record an event such as often occurs in recording sporting events. 
     Furthermore NTSC analog television signals have precise timing requirements that can be difficult to maintain when digitizing the signals. The timing requirements avoid, for example, delays or overlap between program broadcasts, commercials, and a broadcaster&#39;s identifying logo, all of which may arrive from different sources and thus must be precisely timed to generate a continuous uninterrupted picture. As a result, the technology required to digitize NTSC analog signals and combine them with HDTV signals can require a significant amount of processing capability and expensive components. 
     Accordingly, a need exists for transmitting analog television or information signals with digital television or information signals in the same bandwidth without significantly affecting the picture quality of the analog television signals. 
     SUMMARY OF THE INVENTION 
     A first method consistent with the present invention transmits combined analog and digital information signals. The method includes receiving an analog information signal and a digital information signal. The analog and digital information signals are combined for transmission within a particular frequency bandwidth while maintaining the analog information signal in analog form. 
     A second method consistent with the present invention transmits combined analog and digital information signals. The method includes receiving an analog information signal and a digital information signal. The analog and digital information signals are combined for transmission within a particular frequency bandwidth using a first carrier signal for the analog information signal and using a second carrier signal for the digital information signal. 
     A third method consistent with the present invention receives combined analog and digital information signals. The method includes receiving a signal, transmitted within a particular frequency bandwidth, having a first portion including an analog information signal maintained in analog form and having a second portion including a digital information signal. The analog information signal is separated from the digital information signal. 
     A fourth method consistent with the present invention receives combined analog and digital information signals. The method includes receiving a signal, transmitted within a particular frequency bandwidth, having a first portion including an analog information signal transmitted using a first carrier signal and having a second portion including a digital information signal transmitted using a second carrier signal. The analog information signal is separated from the digital information signal. 
     A first apparatus consistent with the present invention transmits combined analog and digital information signals. A first terminal receives an analog information signal, and a second terminal receives a digital information signal. A transmitter, coupled to the first and second terminals combines the analog information signal and the digital information signal for transmission within a particular frequency bandwidth while maintaining the analog information signal in analog form. 
     A second apparatus consistent with the present invention receives a combined analog and digital information signal. A terminal receives a signal, transmitted within an particular frequency bandwidth, having a first portion including an analog information signal maintained in analog form and having a second portion including a digital information signal. A receiver, coupled to the terminal, separates the analog information signal from the digital information signal. 
     A third apparatus consistent with the present invention transmits combined analog and digital information signals. A first terminal receives an analog information signal, and a second terminal receives a digital information signal. A transmitter, coupled to the first and second terminals, combines the analog information signal and the digital information signal for transmission within a particular frequency bandwidth using a first carrier signal for the analog information signal and using a second carrier signal for the digital information signal. 
     A fourth apparatus consistent with the present invention receives combined analog and digital information signals. A terminal receives a signal, transmitted within a particular frequency bandwidth, having a first portion including an analog information signal transmitted using a first carrier signal and having a second portion including a digital information signal transmitted using a second carrier signal. A receiver, coupled to the terminal, separates the analog information signal from the digital information signal. 
     Another system for providing a common transport signal comprising an analog information signal and a digital information signal, the system also including a transmitter for the common transport signal. The transmitter includes an analog information signal modulator, placing the analog information signal onto a first intermediate frequency, a digital information signal modulator, placing a digital information signal onto a second intermediate frequency, a first upconverter and amplifier for upconverting the analog information signal, and a second upconverter for upconverting a frequency translated digital information signal. The system also includes a power combiner for adding the upconverted analog information and the upconverted digital information signal to form a common transport signal. 
     The system also provides a means for reception of the common transport signal. The receiver includes a signal receptor and conditioner, a signal splitter and dual signal recovery chains; each chain including at least a downconverter, a bandpass filter and intermediate frequency amplifier, and a signal demodulator. 
     Another method of transmitting combined analog and digital information signals, includes receiving an analog information signal receiving a digital information signal shifting the frequency spectrum of the analog information signal a first amount, shifting the frequency spectrum of the digital information signal a second amount, and combining the frequency shifted analog information signal and the frequency shifted digital information signal for transmission within a standard television channel bandwidth such that the analog information signal is maintained in analog form during the combining step. 
     Another method of receiving a combined analog and digital information signal, the method including receiving a combined analog and digital information signal, splitting the combined signal to produce two such combined signals, downconverting the analog information signal by a first amount, downconverting the digital information signal by a second amount, frequency translating the frequency shifted digital information signal, and demodulating the analog information signal and the digital information signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings, 
     FIG. 1 is a diagram of a transmitter for transmitting analog and digital information signals consistent with the present invention. 
     FIG. 2 is a frequency spectrum diagram of an exemplary analog information signal; 
     FIG. 3 is a frequency spectrum diagram of an exemplary digital information signal; 
     FIG. 4 is a frequency spectrum diagram of an exemplary common transport signal including analog and digital information signals; 
     FIG. 5 is a diagram of a receiver for receiving a common transport signal including analog and digital information signals; 
     FIG. 6 is an exemplary block diagram of the transmitter shown in FIG. 1; 
     FIG. 7 is an exemplary block diagram of an IF translator shown in FIG. 6; 
     FIG. 8 is a frequency diagram of signals within the IF translator shown in FIG. 7; 
     FIG. 9 is an exemplary block diagram of the receiver shown in FIG. 5; 
     FIG. 10 is a diagram of a frequency response of a bandpass filter for filtering an analog information signal in the receiver shown in FIG.  9 ′ 
     FIG. 11 is an exemplary block diagram of an IF translator shown in FIG. 9; and 
     FIG. 12 is a frequency diagram of signals within the IF translator shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION 
     Overview 
     FIG. 1 is a diagram of a transmitter  100  for transmitting analog and digital information signals consistent with the present invention. Transmitter  100  receives on terminal  101  one or more analog information signals, receives on terminal  102  one or more digital information signals, and converts them to a composite transport signal on terminal  103  for transmission, typically as an RF signal. Transmitter  100  provides an advantage of maintaining analog information signals in analog form, thus avoiding, for example, digitizing the signals in order to transmit them with the digital information signals. In addition, transmitter  100  typically uses two carrier signals, one for the analog information signal and another for the digital information signal. 
     Transmitter  100  transmits common transport signal on terminal  103  within a particular frequency bandwidth. For example, it may transmit the common transport signal within a standard channel bandwidth for transmission of television signals, currently 25 megahertz. Therefore, transmitter  100  permits broadcasters to transmit digital information signals, such as HDTV, SDTV, or data, in addition to analog television signals, meaning that the broadcaster may include additional signals without requiring additional channels or expansion of a current channel. 
     FIG. 2 is a frequency spectrum diagram of an exemplary analog information signal  200  received on terminal  101 . In the grid shown in FIG. 2, each box represents 2.5 megahertz along the x-axis and 10 dB of amplitude along the y-axis. In this example, signal  200  includes an NTSC analog television signal spanning a 25 megahertz bandwidth and shown in baseband. NTSC analog television signals refer to analog television signals formatted consistent with current requirements of the NTSC. The phrase “analog information signal” refers to an electromagnetic signal transmitting information in analog form. Examples of analog information signals include, but are not limited to, to the following: analog television signals, NTSC analog television signals, analog audio signals, analog video signals, and analog video plus audio signals. 
     FIG. 3 is a frequency spectrum diagram of an exemplary digital information signal. In the grid shown in FIG. 3, each box represents 2.5 megahertz along the x-axis and 10 dB of amplitude along the y-axis. In this example signal  300  includes an HDTV signal spanning a 25 megahertz frequency bandwidth. Signal  300  may be provided from an industry standard 20 megabits/second modem, compressed from a direct data stream output of an HDTV camera. HDTV signals refer to digital television signals formatted consistent with current requirements of the Advanced Television Standards Committee (ATSC). The phrase “digital information signal” refers to an electromagnetic signal transmitting information in digital form. Examples of digital information signals include, but are not limited to, the following: HDTV signals, SDTV signals, digital data signals, and signals transmitted on T1 lines. 
     FIG. 4 is a frequency spectrum diagram of an exemplary common transport signal  400  including analog and digital information signals transmitted on terminal  103 . In the grid shown in FIG. 4, each box represents 2.5 megahertz along the x-axis and 10 dB of amplitude along the y-axis. Signal  400  is shown spanning a 25 megahertz bandwidth, providing an advantage of using the same channel bandwidth as the analog and digital information signals; alternatively, it may span a different channel bandwidth. Signal  400  includes a first portion  401  corresponding to analog information signal  200  and a second portion  402  corresponding to digital information signal  300 . In order to fit both the analog and digital information signals in the same channel bandwidth in this example, analog information signal  200  is bandpass filtered in order to band limit portions of the ends of its frequency spectrum and produce signal  401 . An NTSC analog television signal typically does not occupy the entire 25 megahertz channel bandwidth and therefore portions of the ends of the signal may be limited without sacrificing picture quality, permitting the NTSC analog television signal and digital information signal to both fit with the same standard television signal bandwidth. Alternatively, different channel bandwidths and types of analog and digital information signals (potentially occupying different bandwidths) may be transmitted using transmitter  100 . Although transmitter  100  typically transmits common transport signal  400  as an RF signal, it may alternatively transmit it within a different portion of the frequency spectrum. 
     FIG. 5 is a diagram of a receiver  500  for receiving a common transport signal including analog and digital information signals. Receiver  500  receives a common transport signal on terminal  501  and separates the analog and digital information signals, outputting one or more analog information signals on terminal  502  and one or more digital information signals on terminal  503 . The common transport signal received on terminal  501  may correspond t, for example, signal  400  shown in FIG. 4, and the analog and digital information signals output on terminals  502  and  503  may correspond, respectively, to NTSC analog television signal  200  shown in FIG.  2  and to HDTV signal  300  shown in FIG.  3 . Receiver  500  may alternatively receive common transport signals having different types of analog and digital information signals and potentially occupying different bandwidths, and it may separate and output those signals. Also, although receiver  500  typically receives a common transport signal as an RF signal, it may alternatively receive the common transport from a different portion of the frequency spectrum. 
     Accordingly, transmitter  100  and receiver  500  permit broadcasters, for example, to transmit analog television signals with digital information signals in a standard channel. Broadcasters may use transmitter  100  in a studio-to-transmitter link (STL). STLs are used to transmit television or other information signals from a broadcast studio to a television transmit site, which in turn transmits the signals for reception by consumers&#39; televisions. The analog television signals may be generated in the studio or transmitted from a broadcast vehicle to the studio. 
     Broadcasters may use receiver  500  on the transmitter side of an STL located at or proximate the television transmit site. In that situation, receiver  500  receives a common transport signal, and it separates the analog and digital information signals for transmission to consumers&#39; televisions. The analog and digital information signals typically require different transmitters for transmission to consumers&#39; television, in which case separating of the signals is necessary. In addition receiver  500  may also transmit the separated analog and digital information signals back to the studio from which the corresponding common transport signal was received. That communication is referred to as a transmitter-to-studio link (TSL). Transmitting the signals in a TSL provides a broadcaster with the signals it transmitted to the antenna and thus permits a broadcaster to receive feedback concerning the transmitted signals. 
     Transmitter 
     FIG. 6 is an exemplary block diagram of transmitter  100 . The block diagram shown in FIG. 6 is only one example of components for implementing a transmitter consistent with the present invention, and other types of components and configurations are possible for implementations consistent with the present invention. Transmitter  100  receives analog and digital information signals and processes them for transmission within a particular bandwidth or channel. A backplane  601  in transmitter  100  includes terminals for receiving analog and digital information signals. A terminal  602  receives an optional ATSC signal, one type of digital information signal. Typically, that signal may include an HDTV signal having data plus a clock signal; alternatively, it may include an SDTV signal. A terminal  603  includes an optional T1 (1T1) connection and receives a data stream from the T1 connection. A T1 connection, known in the art, is a dedicated line used, for example, by private networks and for providing a high-speed link to and from an Internet service provider. Such a data stream may be used, for example, by a broadcast studio for particular management functions relating to the transmitter. A terminal  604  is a service channel and receives signals on a standard RS-232 communication link; RS-232 is a known standard for serial transmission of information between computers and peripheral devices. A terminal  605  is an alarm input that monitors conditions external to the transmitter and provides a binary signal indicating an alarm or no alarm. For example, the alarms may indicate particular environmental conditions such as a temperature too high for optimum operation of the transmitter, or the alarms may implement a burglar alarm, indicating a breach of security related to the transmitter. 
     A terminal  606  receives an analog information signal, typically an NTSC analog video signal. Terminals  607  receive an audio signal corresponding to the video signal received at terminal  606  and optionally receive other audio signals as well. The three audio signals at terminals  607  each include three terminals, a ground connection and a balanced input connection. An alarm status terminal  608  receives signals indicating alarms internal to the transmitter to implement, for example, what is known as a “hot standby” feature, involving use of redundant transmitters and receivers. In operation using hot standby, if a transmitter or receiver fails, as detected by an internal alarm, the radio automatically switches over to the standby transmitter or receiver to continue operating and avoid a loss of the corresponding signal. 
     The following components combine and process received digital information signals. A modem card  609  receives digital information signals from terminals  602 - 605 . It includes an interface  609   a  which conditions the digital information signal to generate a common output; for example, it extracts the data from the digital information signal and converts it to a transistor transistor logic (TTL) signal, and it inputs the clock signal to modem care  609  to synchronize frames of the digital information signal. Modem card  609  combines all signals from terminals  602 - 606  into one signal using a multiplexing function, and it modulates the combined signal to a 70 megahertz intermediate (IF) frequency, an industry standard for television signals. Therefore, the output of modem card  609  is one data stream centered at 70 megahertz, signal  623 . Modem card  609  uses quadrature amplitude modulation (QAM), a known technique, to generate the modulated signal. An IF translator  610  receives 70 megahertz signal  623  and shifts it to a corresponding signal  624  centered at 82.5 megahertz. The functions of IF translator  610  are further explained below. 
     An image reject mixer (IRM) up converter  611  receives the signal from IF translator  610  and converts it to an RF frequency. Up converter  611  is controlled by a microwave local oscillator  621 , which provides the carrier signal for modulation. Up converter  611  includes an “I” (IF) terminal for receiving the 82.5 megahertz IF signal from module  610 , an “L” terminal for receiving the local oscillator signal from microwave local oscillator  621 , and an “R” (RF) terminal at which it outputs a corresponding up converted RF signal. Microwave local oscillators are known in the art and examples include those devices manufactured by Microlambda, Inc. Microwave local oscillator  621  typically includes a variable local oscillator signal for varying the up conversion frequency. The RF signal output from up converter  611  is transmitted through a power amplifier (PA)  612 , which provides a particular amount of gain, 30 dB in this example. Power amplifiers are known in the art and examples include devices manufactured by Aydin and Fujitsu. 
     A power combiner  613  receives the modulated and processed digital information signal output from power amplifier  612  and combines it with a processed analog information signal. A power combiner is a resistive network used to add together signals, typically implemented by using a power splitter and reversing the connections for the input and output signals. Power splitters are known components for receiving one signal and dividing it into two signals having the same frequency response as the input signal but usually reduced in amplitude. Therefore, by reversing the connections the power splitter functions as a power combiner, receiving two signals and adding them together. 
     A waveguide  614  receives the output of power combiner  613  and transmit it to an antenna  615  for RF transmission as an electromagnetic signal. 
     The following components process and provide analog information signals to power combiner  613 . An FM modulator  616  receives an analog information signal from terminal  606 , receives three audio subcarriers from an audio modulator  620 , and modulates those four signals into one carrier centered at 70 megahertz. Audio modulator  620  receives the three audio signals in baseband from terminals  607  and modulates each audio input signal onto a separate subcarrier. In this example, audio modulator  620  modulates audio signals onto subcarriers centered at 6.2 megahertz, 6.8 megahertz, and 7.5 megahertz. Therefore, FM modulator  616  outputs a 70 megahertz IF signal for the received analog information signals. 
     An IF filter and limiter amp  617  includes a standard bandpass filter, which band limits the IF spectrum of the 70 megahertz signal to fit, in this example, within a standard RF transmission channel. Therefore, it outputs a signal centered at 70 megahertz and having a 15 megahertz bandwidth for this example. The amplification function of filter  617  attempts to provide 10 dB of gain in order to amplitude limit the signal and produce an FM output signal. Bandpass filters having varying transfer characteristics may be used to filter the signal if different channel bandwidths are used. 
     The output of filter  617  is transmitted to an up converter  618 , which typically functions in a similar manner as up converter  611 . Up converter  618  receives the 70 megahertz analog information signal from filter  617  and, using a carrier signal received from microwave local oscillator  621 , converts the signal to an RF frequency. Up converter  618  outputs the modulated signal to a power amplifier  619 , which typically functions in a similar manner as power amplifier  612 , providing a particular amount of gain, 30 dB in this example. Examples of components for implementing power amplifiers are provided above. Up converters  611  and  618  may be implemented with mixers, which are known in the art. 
     The output of power amplifier  619  provides the analog information portion for combination with the digital information portion by power combiner  613 . Accordingly, if the analog and digital information signals include, respectively, an NTSC analog television signal and an HDTV signal, the output of power combiner  613  would typically resemble the spectrum shown in FIG. 4, each signal having its own carrier signal, having been shifted by the mixers to an RF channel frequency, and having been added together by power combiner  613 . The position of the analog and digital information signals within the channel may be reversed such that the digital information signal is at the lower frequency portion of the spectrum. Although transmitter  100  is shown as first separately modulating the input analog and digital information signals to an RF frequency and then combining the modulated signals, it may alternatively first combine the signals and then up convert the combination to an RF or other transmission frequency. 
     An alarm display board  622  receives the local internal alarms and transmits them via a ribbon cable to an LCD display. The LCD display may be located on or proximate the transmitter for indicating and displaying the internal alarms. 
     FIG. 7 is a block diagram of IF translator  610  in transmitter  100 . IF translator  610  functions to convert signal  623  centered at 70 megahertz to signal  624  centered at 82.5 megahertz. Because these two signals are close in frequency, a simple conversion from 70 megahertz to 82.5 megahertz would result in interference from harmonics produced during the conversion. Therefore, IF translator  610  performs an up conversion in order to isolate the signal and subsequently performs a down conversion in order to shift the signal back to 82.5 megahertz. This process is further explained with respect to FIG. 8 illustrating frequency spectrum diagrams of signals within IF translator  610 . 
     IF translator  610  receives signal  623  at terminal  700 , and transmits it through a pad  701  to an up converter  702 . Pad  701 , as well as other pads  703 ,  705 , and  707  within IF translator  610 , are implemented with resistive elements to ensure matching of resistance between components to optimize performance of the circuit. An up converter  702  receives signal  623  and mixes it with a local oscillator signal received from a voltage controlled oscillator  710 . In this example, signal  623  is mixed with a  430  megahertz IF signal, the result of which is shown in graph  800  (FIG.  8 ). Mixing signal  623  with the local oscillator signal produces a carrier signal  802  centered at 430 megahertz and also produces the sum and difference of the signals. The difference of the signals is a side band produced at 360 megahertz, as shown by signal  803 . A bandpass filter  704  receives the 430 megahertz carrier signal along with the resulting side bands, and the filter is centered at 360 megahertz to extract signal  803  and reject the other signals. Bandpass filter  704  may be implemented, for example, with a filter manufactured by Toko. 
     A down converter  706  receives the output of band pass filter  704  and mixes it with a local oscillator in order to down convert the signal to an 82.5 megahertz IF frequency. A voltage controlled oscillator  712  provides the local oscillator signal for down converter  706 , in this example a 442.5 megahertz signal. This operation is shown in graph  801 . Down converter  706  mixes the second local oscillator signal  804  with the 360 megahertz signal  803 , producing the difference between the two, signal  624  centered at 82.5 megahertz. Up converter  702  and down converter  706  may be implemented with mixers, which are known in the art. 
     Signal  624 , output from down converter  706 , is transmitted through an amplifier  708  in order to compensate for loss through the circuitry and is output to terminal  709 . Amplifier  708  provides sufficient amplification such that, in this example, signal  623  at input terminal  700  has an amplitude of approximately −10 dBm and signal  624  at output terminal  709  has an amplitude of approximately −3 dBm, providing for 7 dB of gain through IF translator  610 . 
     A dual phase lock loop (PLL)  711  synchronizes and controls voltage controlled oscillators  710  and  712 . It samples the outputs of voltage controlled oscillators  710  and  712  (signals Fsample) and outputs corresponding control signals (signals Vtune). PLL  711  includes a switch control  711   a , such that a user&#39;s setting of the switches is converted into a signal on one line, controlling the output. Dual PLLs are known in the art and may be implemented, for example, using the National Semiconductor dual PLL part number LMX2335. IF translator  610  typically always up converts to 360 megahertz but may down convert to different frequencies, as specified by switch control  711   a,  in order to swap positions of the analog and digital information signals within the channel. If the signals are switched in position, microwave local oscillator  621  may be adjusted in order to ensure centering of a common transport signal within a particular channel bandwidth. 
     Receiver 
     FIG. 9 is a block diagram of receiver  500 . The block diagram shown in FIG. 9 is only one example of components for implementing a receiver consistent with the present invention, and other types of components and configurations are possible for implementations consistent with the present invention. Receiver  500  receives a common transport signal at antenna  900 . The common transport signal is an electromagnetic signal typically transmitted as RF signal, although it may alternatively be received from other portions of the frequency spectrum. The received signal includes a digital information signal portion and an analog information signal portion and may resemble, for example, the common transport signal shown in FIG. 4. A waveguide  901  transmits the received signal to a low noise amplifier (LNA)  902 , providing gain to compensate for loss in a splitter  903 ; in this example, it provides 10 dB of gain. Splitter  903  is a resistive network which separates one signal into two corresponding signals; therefore, splitter  903  outputs two signals each having the same frequency spectrum as the input signal of splitter  903  but reduced in amplitude. One of the signals from splitter  903  is transmitted to a low noise converter (LNC)  904 , which converts the RF signal to a signal  924  centered at 82.5 megahertz using a signal received from a microwave local oscillator  913 . 
     An IF translator  905  receives the 82.5 megahertz signal  924  and shifts it to a corresponding signal  925  centered at 70 megahertz. The functions of IF translator  905  are further explained below. 
     The 70 megahertz signal  925  is transmitted through a bandpass filter  906 , which may be implemented, for example, using a standard filter centered at 70 megahertz and having a 10 megahertz bandwidth to reject the analog component and isolate the digital IF signal. An IF amplifier  907  receives the isolated digital IF signal and provides an output having a constant amplitude using automatic gain control. In this example, IF amplifier  907  provides an output having a constant 5 dBm amplitude. Amplifiers having automatic gain control are known in the art. A modem card  908  receives the 70 megahertz output of amplifier  907  at a constant amplitude and demodulates the digital information signal using a 70 megahertz demodulation signal and transmits the demodulated signal to an interface  908   a,  which combines frames in the digital information signal with a clock signal in the corresponding signal. 
     Modem care  908  also separates other types of digital information signals using the 70 megahertz demodulation signal. Therefore, the demodulated signals are output to a backplane  916 , including an HDTV or other such signal transmitted to an ATSC terminal  917 , a data stream transmitted to a T1 connection  918 , an RS-232 signal transmitted to a service channel  919 , and alarm outputs transmitted to terminal  920  (including three contact closures for each alarm output). These signals correspond with the signals described with respect to backplane  601  in transmitter  100 . 
     The other output from splitter  903  is transmitted to another low noise converter  909 , which uses the same oscillation signal from microwave local oscillator  913  to demodulate the analog component of the signal to a 70 megahertz IF signal. LNCs  904  and  909  may be implemented with mixers in series with low noise amplifiers, all of which are known in the art as an LNC. A bandpass filter  910  receives the 70 megahertz analog signal and provides a particular type of band limiting to separate and isolate the analog IF signal from the digital information signal. In particular, it provides filtering so that the digital information signal, such as an HDTV signal, does not interfere with the analog signal. The transfer characteristic of bandpass filter  910  is shown in FIG.  10 . In this example, transfer characteristic  1000  is shown with a “notch”  1001  in order to provide particular and limiting of the analog information signal. The dashed line  1002  approximates how the filter would function as a standard band pass filter without the notch. Bandpass filter  910  band limits the 70 megahertz signal by rejecting information between notch  1001  and dashed line  1002 . Notch  1001  is shown on the side of the spectrum adjacent the digital information signal. If the digital and analog information signals were switched in position, the notch  1001  may be located on the other side of transfer characteristic  1000 . Given a particular transfer characteristic, it is known in the art how to generate the corresponding bandpass filter. If different types of digital information signals are received, this particular band limiting may not necessarily be required, in which case a standard bandpass filter may be used. Alternatively, other types of bandpass filters having particular band limiting may be used depending upon the frequency response of the received signals. 
     An IF amplifier  911  receives the output of band pass filter  910  and provides an output having a constant amplitude using automatic gain control. In this example, IF amplifier  911  provides an output having a constant 5 dBm amplitude. An analog demodulator  912  receives the 70 megahertz signal at a constant amplitude and separates the analog video signal from the audio subcarriers using a 70 megahertz demodulation signal. Audio demodulator  914  receives the audio subcarrier signals from audio demodulator  912  provides demodulation at the same frequencies as modulator  620  in order to demodulate the three audio signals. Accordingly, the analog video signal is output to terminal  921  in backplane  916 , and the demodulated audio signals are output to terminals  922 . 
     An alarm display board  915  functions in the same manner as alarm display board  622  and provides alarm status signals at alarm status terminal  923 , presented on an LCD display. 
     FIG. 11 is a block diagram of IF translator  905  in receiver  500 . IF translator  905  functions in a similar manner as IF translator  610 . It uses different local oscillator frequencies, however, to shift an 82.5 megahertz signal to a 70 megahertz signal using an up conversion and down conversion, as further illustrated by the signal frequency diagrams in FIG.  12 . At terminal  1101 , IF translator  905  receives signal  924  at 82.5 megahertz. An up converter  1103  receives the 82.5 megahertz signal from pad  1102  and mixes it with a 442.5 megahertz local oscillator frequency received from a voltage controlled oscillator  1109 . Pad  1102 , as well as other pads  1104 ,  1107 , and  1112  within IF translator  905 , are implemented with resistive elements to ensure matching of resistance between the components to optimize performance of the circuit. 
     Diagram  1201  in FIG. 12 illustrates the output of up converter  1103 . Signal  924  is mixed with signal  1203  providing the sum and the difference. In particular, a side band is located at 360 megahertz as illustrated by signal  1204 , representing the difference between signals  924  and  1203 . The output of up converter  1103  is transmitted through a bandpass filter  1105  centered at 360 megahertz in order to isolate signal  1204  and reject other signals. A down converter  1106  receives the 360 megahertz signal and mixes it with a 430 megahertz local oscillator signal  1205  received from a voltage controlled oscillator  1111 . This down conversion is further illustrated in diagram  1202 . When signal  1204  is mixed with a 430 megahertz local oscillator frequency  1205 , both the sum and the difference are produced, and the difference is signal  925  centered at 70 megahertz. Up converter  1103  and down converter  1106  may be implemented with mixers, which are known in the art. 
     The signal from down converter  1106  is transmitted through pad  1112  to an amplifier  1108  providing amplification to compensate for loss within IF translator  905  and is output as signal  925  to terminal  1112 . In particular, amplifier  1108  provides sufficient amplification such that, in this example, signal  924  at input terminal  1101  has an amplitude of approximately −10 dBm and signal  925  at output terminal  1112  has an amplitude of approximately −3 dBm, providing for 7 dB of gain through IF translator  905 . Voltage control oscillators  1109  and  1111  are controlled by a dual PLL  1110 , which operates in a similar manner, and may be implemented with the exemplary component, as described with respect to dual PLL  711 . 
     While the present invention has been described in connection with an exemplary embodiment, it will be understood that many modifications will be readily apparent to those skilled in the art, and this application is intended to cover any adaptations or variations thereof. For example, different types of components, different types of signals, varying amounts of modulation and amplification, and various hardware embodiments for the signal processing may be used without departing from the scope of the invention. This invention should be limited only by the claims and equivalents thereof.

Technology Category: 5