Abstract:
An SC filter with intrinsic anti-alias function for adjustably decreasing or increasing the amplitude of audio signals in a predetermined frequency range. The SC filter includes a filter module having an RC network with at least one frequency-response-determining RC member whose resistor component R is realized in SC technology. The SC filter also includes a setting means connected to the filter module such that its setting determines the frequency response of the SC filter. The setting means also renders possible a neutral setting in which the effective audio signal path of the SC filter circumvents the filter module so that no decrease or increase of the amplitude of individual frequency portions takes place. An anti-alias low pass filter unit is connected into the audio signal path when the setting means is not set to the neutral setting and the anti-alias low pass filter unit is not located in the audio signal path when the setting means is set to the neutral setting. Furthermore, an audio singal processor may include such a filter.

Description:
TECHNICAL FIELD 
     The invention relates to a switched capacitor (SC) filter with intrinsic anti-aliasing function and to an audio signal processor provided with such a filter. 
     BACKGROUND OF THE INVENTION 
     The document GB 2 138 231 A describes a filter circuit comprising an SC filter that is adjustable with respect to its frequency response, in which in a neutral setting the effective audio signal path circumvents the SC filter, so that no decrease or increase of the amplitude of individual frequency ranges takes place. It is known furthermore from said GB 2 159 014 A to connect an anti-alias filter upstream of an SC filter. 
     With apparatus in the field of entertainment electronics, such as for example radio receivers, cassette recorders, CD players and the like, it is usual to provide in the audio signal path adjustable filter circuits through which the user can take influence on certain sound characteristics, such as for example treble boost and treble cut or bass emphasis and bass cut. The adjustment of suitable filter characteristics is made by the user by means of operating members provided therefor. The adjustable filter circuits, for purposes of influencing the audio signals in entertainment electronics equipment, are designed as a rule such that they permit in particular also a neutral setting in which they have as little influence as possible with respect to frequency and phase response. 
     In case the user desires no emphasis or de-emphasis at all of certain frequency ranges of the audio signal, he will select this neutral setting of the respective filter circuit. 
     With respect to numerous apparatus to be associated with the field of entertainment electronics, such as for example with compact radio receivers as used in particular for installation in motor vehicles, a trend can be observed for a long time to achieve a cost reduction in manufacture by putting together the entire electrical circuit of the receiver from as little as possible highly integrated electronic components. 
     FIG. 6 shows a block diagram of an exemplary audio receiver. The radio frequency signals delivered from a radio frequency antenna  10  are supplied to a radio frequency receiver, hereinafter referred to as “radio frequency processor” RFP. Radio frequency processor RFP comprises radio frequency pre-stages, tuners, intermediate frequency amplifiers and intermediate frequency filters as well as demodulators. A low frequency (LF) signal constituting the useful signal of the station received is fed to an audio signal processor ASP. This low frequency signal does not only include the demodulated audio signal, but also auxiliary signals for stereo reception, ARI identification, RDS and so on. 
     The audio signal processor ASP in particular converts the low frequency signal delivered by radio frequency processor RFP into audio signals which are fed to an audio power processor APP. The audio power processor APP comprises in particular a power amplifier  12  amplifying the output signals delivered from audio signal processor ASP, to such an extent that these can be reproduced through loudspeakers  13 . Furthermore, the audio power processor APP comprises a voltage regulating module  14  for power supply. 
     The audio signal processor ASP may be fed, via additional analog inputs, with other audio signals, for example from cassette recorders, CD players or the like. 
     The audio receiver described is controlled in its entirety by a microcontroller  15  which is provided with an operating unit  20  that is accessible to the user. The exchange of data between microcontroller  15  on the one hand and radio frequency processor RFP, audio signal processor ASP and audio power processor APP on the other hand takes place through a conventional I 2 C bus. 
     FIG. 7 shows a schematic block diagram of the integrated audio signal processor ASP of the audio receiver shown in FIG.  6 . The audio signal processor ASP has a number of analog audio signal inputs  50   a  to  50   j  that are connected to corresponding input lines of an analog multiplexer  55 . A stereo output signal of analog multiplexer  55  is fed via output lines  57   a  and  57   b  to a chain of series-connected blocks  60 ,  62 ,  64 ,  66 ,  68  acting as signal processors. 
     The audio signals first enter a muting filter (“mute”)  60 , and then in succession a volume control with loudness filter  62 , a second muting arrangement with soft muting property (“soft mute”)  64 , a bass control (“bass”)  66  as well as a treble control  68  (“treble”). The stereo audio signals then are fed to a number of audio driver amplifiers  70 , where they are conditioned to such an extent that they are suitable for controlling the power output stages in power amplifier  12  within audio power processor APP. 
     In this respect, each channel of the stereo signal is split to a first sub-channel for a first loudspeaker to be installed in the front of the passenger compartment and a second sub-channel for a second loudspeaker to be installed in the rear of the passenger compartment. 
     The audio signal processor comprises furthermore a number of usual functional blocks  72  to  82  which are necessary for processing the stereo differential signal, the ARI signal etc. Finally, there is provided a pause circuit  90  and a power supply means  95 . 
     It is apparent to the expert that the audio signal processor ASP represented in FIGS. 6 and 7 does not constitute the sole possible embodiment of such a processor. In particular, the audio signal processor ASP may also be implemented without the functional units for processing FM stereo signals. The type and the number of the LF stages for taking influence on the audio signal may also vary for each particular case. 
     The circuit described hereinbefore involves the disadvantage that there is no complete integration of the audio filters  62 ,  66 ,  68  since numerous RC members with high resistances are realized by means of external components because of a too large space requirement of the resistors R. 
     When employing the so-called “switched capacitor” filter technology (“SC technology”), high resistances, which in case of direct implementation would cause consumption of much chip area, are substituted by arrangements with switched capacitors. Filter circuits with switched capacitors as such are known for example from “Analog MOS Integrated Circuits for Sigal Processing” by Roubik Gregorian and Gabor C. Temes, John Wiley &amp; Sons. This circuit technology facilitates indeed the complete integration of filter circuits on a semiconductor chip, but entails also other consequential problems. 
     In case of the “switched capacitor” filter technology, the useful signals are sampled de facto with the frequency at which capacitors are switched that are used for re-storing charges. According to the basic sampling theorem of Shannon, an analog signal s(t) is described completely by equidistant sampling values in a time distance Δt only when the frequency spectrum of s(t) above an upper limit frequency f max  is identical to zero:                f     m                 a                 x       =     1     2      Δ                 t               (   1   )                                
     The upper useful limit frequency thus is half of the sampling frequency, and signal portions of higher frequencies above f max  result in the so-called “aliasing effect”, i.e., the energy content of these higher-frequency signal portions makes itself felt in the baseband in the form of interference signals. The principles of the aliasing effect are known, for example, from “Nachrichtentechnik”, Vol. II: “Nachrichtenubertragung”, by W. Rupprecht, Springer-Verlag, Berlin et al., 1982, pages 75 to 78. 
     A conventional countermeasure against this aliasing effect consists in providing in front of a filter chain realized in the form of “switched capacitor” technology, an input low pass whose characteristics are suitably selected such that signal portions with frequencies higher than f max  are sufficiently strongly attenuated. 
     However, this circuit technology involves the disadvantage that background noise and distortions of the input low pass filter are superimposed on the useful signal also when the filter or filter chain is in the neutral setting. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to make available an adjustable SC filter with anti-alias function as well as audio signal processor provided with such a filter, which do not involve the prior art disadvantages and which in particular can easily be integrated completely and feature low noise and distortion also in the neutral position. 
     This object is met according to the invention by an SC filter according to claim  1 . The subclaims indicate developments of the subject matters of the alternative independent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an adjustable audio SC filter with intrinsic anti-alias function according to the invention, which is designed in SC technology. 
     FIG. 2 a  is a schematic view of a resistor member  200  designed as switched capacitor. 
     FIG. 2 b  is an equivalent circuit diagram of resistor member  200 . 
     FIG. 2 c  shows an exemplary implementation of a switching means of FIG. 2 b  utilizing two MOS-transistors. 
     FIG. 2 d  shows clock signals which are free from overlapping. 
     FIG. 3 shows a circuit diagram of a first embodiment of the filter module with anti-alias low pass of FIG. 1, namely for a loudness filter. 
     FIG. 4 shows a second embodiment of the filter module with anti-alias low pass of FIG. 1, namely for a bass filter. 
     FIG. 5 shows a circuit diagram of a third embodiment of the filter module with anti-alias low pass of FIG. 1, namely for a treble filter. 
     FIG. 6 shows a schematic block diagram of a radio receiver according to the prior art. 
     FIG. 7 shows a schematic block diagram of the audio signal processor of the radio receiver of FIG. 6 with some peripheral circuitry. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a block diagram of an adjustable SC technology audio filter  100  with intrinsic anti-alias function according to the invention, which is suitable in particular for use in an audio signal processor ASP as shown for example in FIG.  7 . The circuit shown in FIG. 1 then is suitable in particular for implementation of the loudness filter  62 , the bass filter  66  and of the treble filter  68  of FIG.  7 . 
     The audio filter  100  comprises an input IN connected to the non-inverting input of an operational amplifier OP 1  as well as an output OUT connected to the output thereof. The inverting input of operational amplifier OP 1  is connected to a tap  105  of an ohmic voltage divider  110  connected between the output of operational amplifier OP 1  and the output of a filter module  115  and realized in SC technology. The input of filter module  115  is connected via an anti-alias low pass  120  to the output of operational amplifier OP 1 . 
     The setting of the tap  105  of ohmic voltage divider  110  determines the effectiveness of the filter with anti-aliasing connected in accordance with the invention, as shown in FIG.  1 . When the tap  105  of ohmic voltage divider  110  is set such that it is directly short-circuited with the output of operational amplifier OP 1 , the circuit shown in FIG. 1 behaves like a voltage follower with a unity amplification factor. The filter module  115  as well as anti-alias low pass  120  are not located in the signal path. In this case, the filter  100  is in its neutral setting, and neither the filter module  115  nor the anti-alias low pass  120  feed noise voltages into the audio output signal at output terminal OUT or cause signal distortions. 
     In the extent in which the tap  105  of the ohmic voltage divider  110  is changed in its setting such that a partial resistance between the output of operational amplifier OP 1  and tap  105  becomes effective, the filter module  115  and the anti-alias low pass  120  enter into the signal path and take influence on the frequency characteristics of the filter  100 . As such, the ohmic voltage divider acts as a filter control unit that determines whether the filter module  115  and the anti-alias low pass  120  will affect the input audio signal received by the audio filter  100 . 
     The output signal at the output terminal of operational amplifier OP 1  is subjected to a negative feedback, via anti-alias low pass  120 , filter module  115  as well as voltage divider  110 , to the inverting input of operational amplifier OP 1 . Anti-alias low pass  120  prevents that signal portions with a frequency higher than a maximum frequency f max , i.e., half of the switching frequency of switched capacitors within filter module  115 , from reaching the input of filter module  115 . 
     Anti-alias low pass  120  may be designed in particular as a conventional RC low pass. It is also possible to combine the functions of anti-alias low pass  120  and of filter module  115  in a complex filter circuit. 
     The ohmic voltage divider  110  may be designed as a conventional potentiometer. However, when used in an integrated circuit, it turns out to be advantageous to design ohmic voltage divider  110  for example as a resistor chain switched by MOS switches. When the filter circuit according to FIG. 1 is used in an audio signal processor (as shown in FIG. 7, for example), it is possible to adjust the ohmic voltage divider  110  by a digital setting means, e.g., via the I 2 C bus. 
     FIG. 2 a  shows a schematic representation of a resistor member  200  implemented with a switched capacitor, as employed in the embodiments shown in FIGS. 3 to  5 . 
     FIG. 2 b  shows an electrical equivalent circuit diagram of the resistor member  200  shown in FIG. 2 a . Input IN of the circuit may be connected via a first switch S 1  to a first terminal of a capacitor  210 , or may be connected to ground. Output terminal OUT may be connected via a second switch S 2  to a second terminal of capacitor  210 , or may be connected to ground. When the switches S 1 , S 2  are not addressed, they are in an electrically open state. 
     FIG. 2 c  shows an exemplary implementation of the switches S 1 , S 2  of FIG. 2 b  using two MOS transistors  220 ,  230 . A first MOS transistor  220  connects, via its source-drain path, a common terminal  260  to a first switch output terminal  240 , whereas a second MOS transistor  230  connects, via its source-drain path, the common terminal  260  to a second switch output terminal  250 . The gate of first MOS transistor  220  is connected to the clock signal Φ 1 , whereas the gate of the second MOS transistor  230  is connected to clock signal Φ 2 . FIG. 2 d  shows clock signals Φ 1 , Φ 2  for controlling the switches S 1 , S 2  in FIG. 2 b  and the MOS transistors  220 ,  230  in FIG. 2 c , respectively. As is usual in SC technology, the two clock signals Φ 1 , Φ 2  are designed such that their high-level (active) intervals do not overlap at any time. The effect achieved thereby is that in the circuit according to FIG. 2 c  there is always only a single one of the two MOS transistors  220 ,  230  conducting and the common terminal  260  thus is open or is connected either to first switch terminal  240  or to second switch terminal  250 , but never to both simultaneously. 
     FIG. 3 shows an electric circuit diagram of a first embodiment  300  of a filter module  115 , namely for a loudness filter with anti-alias property, combining therein the functional blocks of filter module  115  and of anti-alias low pass  120  of FIG.  1 . The circuit embodies a low pass of the second order and is designed in SC technology. 
     An input IN is connected to the inverting input of an operational amplifier OP 2  via a first low pass resistor  305  and a first resistor  310  designed in SC technology. The second terminal of first low pass resistor  305 , which is not directly connected to input terminal IN, is connected to ground via a first low pass capacitor  315 . The non-inverting input of operational amplifier OP 2  is connected to ground. 
     The output of operational amplifier OP 2  is connected via a capacitor  320  to the inverting input thereof The inverting input of a further operational amplifier OP 3  is connected via a second resistor  325  designed in SC technology to the output of operational amplifier OP 2 . The non-inverting input of operational amplifier OP 3  is also connected to ground. 
     The output of operational amplifier OP 3  is connected via a third capacitor  330  to the inverting input thereof Connected in parallel to third capacitor  330  is a third resistor  335  in SC technology. A fourth resistor  340  in SC technology connects the inverting input of operational amplifier OP 2  to the output of operational amplifier OP 3 . 
     The output of operational amplifier OP 3  is connected to an output OUT. 
     The resistor  305  as well as capacitor  315  constitute anti-alias low pass  120  in FIG.  1 . Operational amplifier OP 2  as well as operational amplifier OP 3  and the remaining components correspond to filter module  115  in FIG.  1 . 
     FIG. 4 shows an electric circuit diagram of a second embodiment  400  of a filter module  115  with anti-alias low pass  120 . The circuit shown in FIG. 4 is employed in a bass filter. 
     The inverting input of an operational amplifier OP 4  is connected to an input IN via a first resistor  405  in SC technology and a first low pass resistor  410 . The terminal of low pass resistor  410  that is not directly connected to input terminal IN, is connected to ground via a first low pass capacitor  415 . The non-inverting input of operational amplifier OP 4  is also connected to ground. The output of operational amplifier OP 4  is connected via a first filter capacitor  420  to its inverting input. The inverting input of an additional operational amplifier OP 5  is connected via a second resistor  425  in SC technology to the output of operational amplifier OP 4 . The inverting input of operational amplifier OP 5  is connected furthermore via a second filter capacitor  430  to input IN and via a third filter capacitor  435  to the output of operational amplifier OP 5 . Finally, inverting input of operational amplifier OPS is connected via a third resistor  440  in SC technology and a fourth resistor  445  in SC technology to the inverting input of operational amplifier OP 4 . 
     The terminal of the third resistor  440  in SC technology that is not directly connected to the inverting input of operational amplifier OP 5  as well as the terminal of the third filter capacitor  435  that is not directly connected to the inverting input of operational amplifier OP 5 , are connected to each other via a second low pass resistor  450 . The connecting node between the SC technology resistors  440  and  445  is connected to ground via a second low pass capacitor  455 . The output of operational amplifier OP 5  is connected to output terminal OUT. 
     The filter circuit  400  constitutes a modified biquad filter. RC members  410 ,  415  as well as  450 ,  455  correspond to anti-alias low pass  120  in FIG. 1, whereas all other components, inclusive of the two operational amplifiers OP 4  and OPS, correspond to filter module  115  in FIG.  1 . 
     The first low pass  410 ,  415  filters higher-frequency signal portions in front of first SC technology resistor  405 . 
     Higher-frequency signal portions may nevertheless reach the inverting input of operational amplifier OP 5  and the SC technology resistors  440 ,  445  via capacitor  430 . In order to suppress these higher frequency signal portions there as well, a second low pass  450 ,  455  is provided. 
     FIG. 5 shows an electric circuit diagram of a third embodiment  500  of a filter module  115 , namely for a treble filter. An input IN is connected via a first capacitor  505  to an output OUT. Moreover, input IN is connected via a first resistor  510  and a second capacitor  515  to ground. The output terminal OUT is also connected to ground via a second resistor  520  and a third capacitor  525 . The coupling points between resistor  510  and capacitor  515  as well as resistor  520  and capacitor  525  are connected to each other via a resistor  530  in SC technology. The SC resistor  530  includes a capacitor  540  connected in parallel with a first switch  545  and in series with a second switch  550 . 
     The circuit shown in FIG. 5 corresponds to the functional blocks of filter module  115  and of anti-alias low pass  120  in FIG.  1 . 
     The circuit shown in FIG. 5 allows that a frequency-dependent voltage drop of the third order can be achieved across SC resistor  530 , corresponding to approx. 60 decibel per decade. 
     It should be understood that even though numerous features and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only. Changes may be made in detail and yet remain within the broad principles of the present invention.