Electrical computers: arithmetic processing and calculating – Electrical digital calculating computer – Particular function performed
Reexamination Certificate
1999-01-20
2001-12-18
Malzahn, David H. (Department: 2121)
Electrical computers: arithmetic processing and calculating
Electrical digital calculating computer
Particular function performed
C708S312000
Reexamination Certificate
active
06332151
ABSTRACT:
The present invention relates to a time discrete filter comprising a cascade connection of a plurality of decimating filter sections. The invention also relates to a tone detector using a filter according to the invention.
A filter according to the preamble is known from the article “Design of Multirate Bandpass Digital Filters with Complex Coefficients” by M. Ikehara and S. Takahashi in Electronics and Communications in Japan, Part 1, Vol. 71, No. 1, 1988, pp. 21-29, which is translated from Denshi Tsushin Gakkai Robunshi, Vol. 69-A, No. Aug. 8, 1986, pp. 950-957
Such filters can be used for selecting signals having a relative small bandwidth from an input signal having a larger bandwidth. This is e.g. the case in tone detectors which are used for detecting the presence of one or more tones in an input signal. Tone detectors are used in telephone systems for detecting signaling tones such as CAS tones, DTMF tones and dial tones.
CAS tones are used in the ADSI (Advanced Digital Signaling Interface) standard for signaling to an ADSI (screen) telephone terminal that the exchange will transmit digital information to said digital telephone terminal.
DTMF tones are used for sending dial information from a telephone terminal to an exchange. They can also be used for transmitting information such as caller-ID to a telephone terminal.
All these applications have in common that a signal having a small bandwidth has to be selected from an input signal having a much larger bandwidth. The selection of such a small bandwidth signal can be realized by sampling the input signal with an adequate sampling frequency, and by filtering the sampled signal with a band pass filter with a small bandwidth centered around the nominal frequency of the signal to be selected.
If such a filter is designed for using the same sample frequency from the input to the output, such a filter requires a substantial amount of computational resources (chip-area, power consumption or processor cycles). By using a cascade connection of a plurality of decimating filter sections the required amount of computational resources can be reduced, because the first decimating filter section runs on the sample frequency of the input signal. The other filter sections run on sampling frequencies decreasing after every decimation step.
In the filter according to the above mentioned article complex coefficients are allowed in order to reduce the required amount of computational resources. Furthermore the known filter comprises transformation means for transforming a real input signal into a complex output signal which is represented by real and imaginary signal components.
The transformation means known from the above article derive their output signals from the input signal by means of a Hilbert transformer. Such a Hilbert transformer suppresses all negative frequency components, and passes all positive frequency components. A Hilbert transformer requires a substantial amount of computational resources, which results in an expensive implementation of the filter.
An object of the present invention is to provide a digital filter according to the preamble which requires less computational resources.
For achieving said object the digital filter is characterized in that the transformations means have a transfer function with a transition band between a frequency in the pass band and a frequency to which a pass-band signal is transferred at the output of said transform means.
The present invention is based on the recognition that it is not necessary to suppress all negative frequencies as in done by the Hilbert transformer according to the prior art, but that it is sufficient to suppress only the frequencies in the input signal of the transformation means to which the signals to be passed are transferred by the decimation means. The above recognition leads in general to transformation means having a larger transition band. This results in a substantial decrease of the required amount of computational resources.
It is observed that the filter according to the invention can be a digital filter, but it is also possible that the filter according to the invention is implemented as a switched capacitor or as a switched current filter.
An embodiment of the present invention is characterized in that said at least one filter section has a transfer function being obtained by a transformation of a transfer function symmetrical around zero.
By using a filter with a transfer function which is obtained by a transformation of a transfer function symmetrical around zero, this filter can be based on a prototype filter which can easily be designed by using standard design tools.
A further embodiment of the invention is characterized in that the transformation is defined by shifting the transfer function according to a frequency shift. A frequency shift is a very easy transformation which can be performed easily.
A further embodiment of the invention is characterized in that the frequency shift is a multiple of one eighth of the sample frequency of the input signal of the decimating filter section.
If the frequency shift is a multiple of one eighth of the sample frequency, the filter section can have the same coefficients as the prototype filter it is derived from. If also such prototype filter is used in other filter sections the coefficients need to be stored only once.
A still further embodiment of the invention is characterized in that the frequency shift is a multiple of one fourth of the sample frequency of the decimating filter section.
If the frequency shift is a multiple of one fourth of the sample frequency of the input signal of the filter section, no additional multipliers are required in the filter, because a multiplication with j or −j can be obtained by taking a signal from a suitable branch in the filter.
A further embodiment of the invention is characterized in that the filter sections are arranged for decimating the input signal with a factor n, and in that the filter elements have delay elements with a delay value equal to n times the sampling period of the signal at the output of the preceding stage.
If the filter sections comprise delay elements with a delay value equal to the decimation factor and the sample period of the input signal, the filter section can be simplified by placing the decimator, which is normally placed behind the filtering element in the filter section, before the filtering element. Consequently, the clock frequency of the filter element can be reduced by a factor N.
A further embodiment of the invention is characterized in that the filter elements are of the bireciprocal type.
A bireciprocal filter is a filter with a transfer function which can be written as:
F
(
z
)=
C·{G
1
(
z
2
)+
z
−1
·G
2
(
z
2
)} (1)
In (1) G
1
and G
2
are equal to:
G
1
(
z
2
)
=
∏
j
=
1
K
B
j
+
z
-
2
1
+
B
j
·
z
-
2
and
G
2
(
z
2
)
=
∏
j
=
1
L
C
j
+
z
-
2
1
+
C
j
·
z
-
2
(
2
)
In (2) K is equal to L or equal to L+1. It is observed that G
i
reduces to 1 if B
j
or C
j
is equal to 1.
Due to the term z
2
in G
i
(z
2
) it is possible to place a downsampling means for reducing the sample rate with a factor of 2 in the filter section directly before the filter elements which realize the transfer fimctions according to (2) It can further be seen that for a third order filter (K=1, L=0) only one coefficient B
1
has to be stored. For a fifth order filter section (K=1, L=1) only two coefficients B
1
and C
1
have to be stored.
REFERENCES:
patent: 4896320 (1990-01-01), Gockler
patent: 5432511 (1995-07-01), Sadjadian et al.
patent: 5689449 (1997-11-01), Saramaki et al.
patent: 5757867 (1998-05-01), Caulfield et al.
patent: 6173302 (2001-01-01), Piirainen
By M. Ikehara and S. Takahashi in Electronics and Communications in Japan, “Design of Multirate Bandpass Digital Filters with Complex Coefficients”, Part 1, vol. 71, No. 1, 1988, pp. 21-29, which is Tra
Suermondt Rutgerus E. E. F.
van den Enden Adrianus W. M.
Malzahn David H.
Slobod Jack D.
U.S. Philips Corporation
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