US RE40960 E1
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 aan 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.
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The present application concerns television receivers having signal processing circuitry for decoding stereo signals which were encoded in accordance with the DBX® format.
The present application relates to wide band expansion of stereophonic signals as discussed in U.S. Pat. No. 5,091,957 of Anderson et al. The teaching of the '957 patent is expressly incorporated by reference herein. The '957 patent disclosed a wideband expander, and recognized that for practical purposes, satisfactory restoration of an (L−R) audio difference signal companded according to the DBX format can be achieved without spectral expansion. A decoder according to '957 provides a modified fixed deemphasis and wideband expansion of the received compressed L−R audio signal, but does not provide spectral expansion. Additionally, it was further recognized in '957 that the relatively complex RMS detectors of the DBX system can be replaced with simpler, lower cost integrating peak detectors.
It is desirable to design and build television receivers wherein different decoders can be used depending upon model and options.
Briefly, the present invention concerns an expander circuit for decoding audio signals which were encoded in accordance with the BTSC multichannel sound system standard including DBX companding. 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 bandpass filtering characteristic of the BPF actuates the switching of the input to bypass the input LPF.
In the DBX system of
Discussion of the significant differences between the arrangements of
The expanders of
As shown in
The present LPF 48 is of a twin-T topology (not shown) with the notch tuned to a frequency higher than 5H. In such an arrangement, the SAP signal can be sufficiently attenuated via the filter skirt without requiring close tolerance components to precisely determine the notch frequency. Additionally, placing the notch above the frequency of interest permits smaller capacitance values which are more practical to integrate and assures that R/C value variations will not change the skirt attenuation if a high enough “Q” is used. The present LPF 48 uses a portion of the twin-T arrangement as part of the feedback loop for an operational amplifier for increasing the Q of the circuit.
The output signal from LPF 48 is fed to a switch 50 for switching the input terminal of the (L−R) demodulator 52 between the output of LPF 48 and input terminal 46. In the exemplary embodiment, LPF 48 is bypassed when a DBX expander is used. When a DBX expander is used, a third order SALLEN-KEY LPF (not shown) can be used for providing sharper skirts at the notch frequency. The operation of switch 50 will be discussed in greater detail below.
The signal from switch 50 is fed to LPF 53 for deriving the stereo sum signal (L+R) which is subsequently fed to matrix 38. The signal from switch 50 is also provided to demodulator 52 which is a standard demodulator according to the MTS television standard. The output signal from demodulator 52 is fed to the series coupling of LPF 10 and VCA 18, as well as to the series coupling of BPF 20 and integrating peak detector 26, all of which correspond to similarly designated members shown in FIG. 2. Integrating peak detector 26 is comprised of an detector 54 and a integrating capacitor 56 for providing a integrated peak detected control signal at line 58 to VCA 18. The expanded output signal from VCA 18 is fed to a fixed deemphasis network 24 (see
The members so far described in connection with
A DBX expander 60 would be wired in place of the emphasis network 24 and such substitution is shown by switch IS1. This substitution is made because the DBX expander 60 provides its own deemphasis and thus, emphasis network 24 is not needed.
In wideband expander 40, BPF 20 and integrating peak detector 26 provide a control voltage at line 58 to VCA 18 to provide amplitude expansion of the (L−R) signal. Since DBX expander 60 provides its own amplitude expansion of the signal, the amplitude expansion provided by the control voltage at line 58 is defeated by providing a constant predetermined voltage 62 through a resistor 64 to the output of the integrating peak detector 26 at line 58 by way of illustrative switch IS2. In this way, any changes in the control voltage at line 58 which would be caused by the (L−R) signal processing of BPF 20 and integrating peak detector 26, are “swamped” out by constant voltage 62. Thus, in this arrangement, VCA 18 provides a constant amplification with the amount of said amplification being predetermined by the value of the constant control voltage 62 applied at line 58.
As discussed above, switch 50 switches the input signal for demodulator 52 to bypass LPF 48 when a DBX expander 60 is used. In the exemplary embodiment, the switching of switch 50 is effectuated by the defeating of the bandpass operation of BPF 20. Such an arrangement will not adversely effect the operation of the system since any control signal generated by the operation of BPF 20 (and integrating detector 26) is rendered ineffectual by voltage source 62 and resistor 64. The frequency selective operation of BPF 20 is provided by capacitors 21a and 21b which provide a frequency sensitive network, and will be more fully discussed below. In the exemplary embodiment, capacitor 21a is coupled to a reference potential such as ground, and is provided with a DC voltage thereacross. This DC voltage is compared against a reference potential by comparator 66, which has an output 67 to which the switch position of switch 50 is responsive. The DC voltage across capacitor 21a, is of a value above a threshold reference voltage VREF coupled to the negative terminal of comparator 66. In such an arrangement, switch 50 is switched for receiving an input signal from LPF 48. If the DC voltage at line 68 is below VREF, the output of comparator 66 causes switch 50 to be switched for receiving the input signal from terminal 46, bypassing LPF 48. In the exemplary embodiment, the DC voltage across capacitor 21a is changed by short circuiting the capacitor 21a to ground by illustrative switch IS3. However, a resistor can be used having a low enough value sufficient to change the voltage at line 68 through VREF from above VREF to below VREF.
The details of BPF 20 are shown in FIG. 5. The (L−R) signal derived from the output of demodulator 52 is provided to unity amplifiers 70, 72 and in turn is fed to a respective pair of frequency sensitive low pass filters comprised of resistors 74, 76, and respective capacitors 21a, 21b to ground for providing low pass filtering. The output signals from the low pass filters are fed to respective unity gain amplifiers 82, 84 and subsequently to an operational amplifier 86 wherein one of the low pass filtered signals is provided to the non-inverting input terminal and the other low pass filtered signal is provided to the inverting input terminal. In such a case, when the two signals into amplifier 86 are equal, there is no output signal at terminal 88 of amplifier 86. If the signal fed to the inverting terminal has a rolloff at a lower frequency than the signal to the non-inverting terminal, the subtraction process is reduced past the rolloff frequency and the signal at the non-inverting terminal will be passed through to terminal 88 until the non-inverting input signal reaches its own rolloff frequency. In this way a bandpass filter is generated.
In the exemplary embodiment, the DC voltage across capacitor 21a at line 68 is provided from the output line 90 of amplifier 72 which is DC coupled and has a DC voltage thereat. Thus, when capacitor 21a is shunted or short circuited to ground through illustrative switch IS3, the operation of that signal path 98 is defeated, and consequently, the operation of the bandpass function of BPF 20 is defeated. The change of the DC voltage at line 68 causes switch 50 to switch between input signal terminals as discussed above.