Switched capacitor filter with a neutral bypass setting

Details Switched capacitor filter with a neutral bypass setting Switched capacitor filter with a neutral bypass setting

1997-07-25

2002-06-25

Korzuch, William (Department: 2641)

Electrical audio signal processing systems and devices

Including frequency control

C381S100000

Reexamination Certificate

active

06411717

ABSTRACT:

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 & 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 &Dgr;t 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

Affiliated with

Also associated with

Rating

Switched capacitor filter with a neutral bypass setting does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Switched capacitor filter with a neutral bypass setting, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switched capacitor filter with a neutral bypass setting will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2981857