Switched signal processing circuit

Abstract

An expander circuit for decoding audio signals which were encoded in accordance with the BTSC multichannel sound system standard including DBX commanding is presented. A wideband expander circuit is utilized and a DBX expander can be accommodated. The wideband expander circuit is provided with a signal path having an input LPF (low pass filter), a stereo difference signal (L−R) demodulator, a second LPF, and a voltage controlled amplifier (VCA), the gain of which is controlled by a control signal derived from the demodulated difference signal which has been operated on by a bandpass filter (BPF) and an integrating peak detector. The output of the VCA is provided to a deemphasis network before being fed to a decoder matrix for combining with the sum stereo signal (L+R) for reconstructing the original L and R signals. When a DBX expander is connected, the DBX expander is substituted in place of the deemphasis network, the VCA control signal, which provides the wideband expansion, is overridden by a predetermined voltage, the bandpass filtering characteristic of the BPF is disabled, and the input is switched to bypass the input LPF. According to aspects of the present invention, the disablement of the BPF bandpass filtering characteristic actuates the switching of the input to bypass the input LPF.

1. Switching apparatus comprising:
 filter means including a frequency sensitive network for spectrally filtering a  an encoded audio signal, said filter means having at its output an audio signal and at least one member of the frequency sensitive network having a DC voltage thereacross, wherein the filter means is a bandpass filter, and changing the DC voltage across the member of the frequency sensitive network disables the bandpass filtering characteristic of the filter means;  
 a comparator means having an output signal responsive to a comparison of the amplitude of the DC voltage and a threshold voltage;  
 switch means responsive to the output signal of the comparator means for changing the DC voltage across the member of the frequency sensitive network by shunting the frequency sensitive network, said switch means also switching between a first audio signal and a second audio signal, as the encoded audio signal, said switch shunting said frequency network when a DBX expander is coupled to the switching apparatus.  
 
     
     
       2. The apparatus of  claim 1  wherein the filter means is a bandpass filter. 
     
     
       3. The apparatus of  claim 1  wherein the filter means is a bandpass filter, and changing the DC voltage across the member of the frequency sensitive network disables the bandpass filtering characteristic of the filter means. 
     
     
       4. The apparatus of  claim 1 , wherein the switch means includes a switch that switches between first and second terminals to select the signal. 
     
     
       5. The apparatus of  claim 1 , wherein the switch means includes a switch. 
     
     
       6. The apparatus of  claim 5 , wherein a position of the switch is responsive to the comparator. 
     
     
       7. The apparatus of  claim 6 , wherein the switch is coupled to a demodulator. 
     
     
       8. The apparatus of  claim 7 , wherein the switch is further coupled to a filter. 
     
     
       9. The apparatus of  claim 8 , wherein the switch is further coupled to an input terminal. 
     
     
       10. The apparatus of  claim 8 , wherein the filter removes SAP signaling. 
     
     
       11. The apparatus of  claim 7 , wherein the demodulator is coupled to first and second filters. 
     
     
       12. The apparatus of  claim 11 , wherein the first and second filters are low pass filters. 
     
     
       13. The apparatus of  claim 1 , wherein the switch means comprises a switch being coupled to an input of the comparator. 
     
     
       14. The apparatus of  claim 13 , wherein the switch is further coupled to ground. 
     
     
       15. The apparatus of  claim 14 , wherein the switch is coupled in parallel with a capacitance. 
     
     
       16. The apparatus of  claim 15 , wherein the switch short- circuits the capacitance.