Low distortion passthrough circuit arrangement for cable television set top converter terminals

Abstract

In a cable television converter terminal, NICAM signal passthrough is achieved using an alignment-free passthrough circuit including a NICAM SAW filter. The alignment-free passthrough circuit directly taps the output of the converter terminal's tuner to obtain the NICAM signal. As a result, the NICAM signal is not distorted by the components involved in obtaining the other signal components of the cable television signal. Consequently, sound quality is improved and rendered less vulnerable to degradation due to component aging, temperature fluctuations, and human error. The passthrough circuit can be implemented as a removable circuit module.

1 - 2 . (cancelled)  
     
     
         3 . For use in a cable television converter terminal, a passthrough circuit for passing a tuned signal from a tuner to a radio frequency modulator for output to external equipment, the passthrough circuit arrangement comprising: 
 a first signal path, arranged to receive the tuned signal from the tuner and to provide a NICAM signal component of the tuned signal to the radio frequency modulator; and    a second signal path, arranged to receive the tuned signal from the tuner and to provide at least one other signal component of the tuned signal to the radio frequency modulator;    wherein the first signal path comprises a NICAM surface acoustic wave filter, coupled to receive the tuned signal from the tuner and configured and arranged to pass a NICAM signal component of the tuned signal and to substantially reject non-NICAM signal components of the tuned signal; and    wherein the NICAM surface acoustic wave filter outputs a signal to a mixer which is set at a selected frequency using a crystal oscillator.    
     
     
         4 . (cancelled)  
     
     
         5 . For use in a cable television converter terminal, a passthrough circuit for passing a tuned signal from a tuner to a radio frequency modulator for output to external equipment, the passthrough circuit arrangement comprising: 
 a first signal path, arranged to receive the tuned signal from the tuner and to provide a NICAM signal component of the tuned signal to the radio frequency modulator; and    a second signal path, arranged to receive the tuned signal from the tuner and to provide at least one other signal component of the tuned signal to the radio frequency modulator;    wherein the first signal path comprises a NICAM surface acoustic wave filter, coupled to receive the tuned signal from the tuner and configured and arranged to pass a NICAM signal component of the tuned signal and to substantially reject non-NICAM signal components of the tuned signal; and    wherein the first signal path further comprises a mixer, coupled to receive the NICAM signal component passed by the NICAM surface acoustic wave filter, and configured to downconvert the NICAM signal component to a baseband frequency.    
     
     
         6 . A passthrough circuit as claimed in  claim 5 , wherein the baseband frequency is one of 6.552 MHz and 5.85 MHz.  
     
     
         7 . A passthrough circuit as claimed in  claim 5 , wherein the first signal path further comprises a low pass filter, coupled to receive the downconverted NICAM signal component from the mixer and configured and arranged to attenuate mixer harmonics from the downconverted NICAM signal and to provide a NICAM output signal to the radio frequency modulator.  
     
     
         8 - 9 . (cancelled)  
     
     
         10 . For use in a cable television converter terminal, a passthrough circuit for passing a tuned signal from a tuner to a radio frequency modulator for output to external equipment, the passthrough circuit arrangement comprising: 
 a first signal path, arranged to receive the tuned signal from the tuner and to provide a NICAM signal component of the tuned signal to the radio frequency modulator; and    a second signal path, arranged to receive the tuned signal from the tuner and to provide at least one other signal component of the tuned signal to the radio frequency modulator;    wherein the first signal path is constructed as a unitary circuit module.    
     
     
         11 . For use in a cable television converter terminal, a passthrough circuit for passing a tuned signal from a tuner to a radio frequency modulator for output to external equipment, the passthrough circuit arrangement comprising: 
 a NICAM surface acoustic wave filter, coupled to receive the tuned signal from the tuner and configured and arranged to pass a NICAM signal component of the tuned signal and to substantially reject non-NICAM signal components of the tuned signal;    a mixer, coupled to receive the NICAM signal component passed by the NICAM surface acoustic wave filter, and configured to downconvert the NICAM signal component to a baseband NICAM IF frequency; and    a low pass filter, coupled to receive the downconverted NICAM signal component from the mixer and configured and arranged to attenuate mixer harmonics from the downconverted NICAM signal and to provide a NICAM output signal to the radio frequency modulator.    
     
     
         12 . A passthrough circuit as claimed in  claim 11 , wherein the mixer is set at a selected frequency using a crystal oscillator.  
     
     
         13 . A passthrough circuit as claimed in  claim 12 , wherein the selected frequency is one of 45.75 MHz and 38.9 MHz.  
     
     
         14 . A passthrough circuit as claimed in  claim 11 , wherein the baseband NICAM IF frequency is one of 6.552 MHz and 5.85 MHz.  
     
     
         15 . A passthrough circuit as claimed in  claim 11 , wherein the NICAM surface acoustic wave filter, mixer, and low pass filter are constructed as a unitary circuit module.  
     
     
         16 - 17 . (cancelled)  
     
     
         18 . A method of making a passthrough circuit for passing a tuned signal from a tuner to a radio frequency modulator for output to external equipment, the passthrough circuit arrangement comprising: 
 providing a first signal path, arranged to receive the tuned signal from the tuner and to provide a NICAM signal component of the tuned signal to the radio frequency modulator;    providing a second signal path, arranged to receive the tuned signal from the tuner and to provide at least one other signal component of the tuned signal to the radio frequency modulator; and    constructing the first signal path as a unitary circuit module.