Telephonic communications – Centralized switching system – Switching controlled in response to called station...
Reexamination Certificate
2000-11-22
2003-12-23
Tieu, Benny (Department: 2642)
Telephonic communications
Centralized switching system
Switching controlled in response to called station...
C379S361000, C379S386000, C708S311000
Reexamination Certificate
active
06668057
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-333371, filed Nov. 24, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a tone signal receiving apparatus for detecting a tone signal, a tone signal transmitting apparatus for generating a tone signal, and a tone signal transmitting/receiving apparatus having a function of receiving a tone signal and transmitting a tone signal, which are used in the field of communications and, more particularly, to a tone signal receiving apparatus, tone signal transmitting apparatus, and tone signal transmitting/receiving apparatus, which implement the above tone signal receiving apparatus, tone signal transmitting apparatus, and tone signal transmitting/receiving apparatus by digital circuits, respectively.
Conventionally, in a key telephone system used in an office building or business office, a key telephone main apparatus (to be referred to as a main apparatus hereinafter) having a switching function has the arrangement shown in FIG.
1
. Reference numeral
1
A denotes a main apparatus.
Referring to
FIG. 1
, the main apparatus
1
A comprises a trunk unit
11
, line card
12
, time switch section (to be referred to as a TSW hereinafter)
13
, control section
14
, and DTMF (Dual Tone Multi Frequency) signal receiving section
15
. These components are connected to each other through a voice bus (to be referred to as a PCMHW hereinafter)
16
and control bus (to be referred to as a DHW hereinafter)
17
.
The trunk unit
11
is connected to an external communication network NW through a subscriber's line ISL and has an interface function to the external communication network NW. The line card
12
is connected to extension terminals T
1
to Tm through a plurality of extension lines EL
1
to ELm and has an interface function to these extension terminals T
1
to Tm. Examples of the extension terminals T
1
to Tm are standard telephone sets and key telephone sets.
The TSW
13
selectively connects the trunk unit
11
to the line card
12
in accordance with an instruction from the control section
14
. The TSW
13
also selectively connects one of the trunk unit
11
and line card
12
to the DTMF signal receiving section
15
.
The DTMF signal receiving section
15
has a codec
15
a
and PB receiver
15
b
. The codec
15
a
converts an input digital signal into an analog signal and outputs the analog signal to the PB receiver
15
b
. The PB receiver
15
b
detects and identifies a DTMF signal from the input analog signal.
The operation of detecting a DTMF signal in the main apparatus
1
A will be described below.
When the user presses a dial key on the extension terminal T
1
, a DTMF signal according to the dial key is generated from the extension terminal T
1
This DTMF signal contains high- and low-frequency orthogonal components, as shown in FIG.
2
. The DTMF signal is transferred to the TSW
13
through the line card
12
and PCMHW
16
and then transferred to the DTMF signal receiving section
15
through the TSW
13
and PCMHW
16
.
In the DTMF signal receiving section
15
, a number of codecs
15
a
must be prepared in units of channels because each codec
15
a
extracts a signal on a predetermined channel of a number of channels multiplexed on the PCMHW
16
. The necessary number of codecs
15
a
is eight for 100 accommodated lines. Hence, the DTMF signal receiving section
15
has a large circuit scale, and integration for cost reduction is difficult.
In recent years, processing by the DTMF signal receiving section
15
may be implemented using a DSP (Digital Signal Processor).
FIG. 3
shows the internal block of a DTMF signal receiving section using a DSP.
This DTMF signal receiving section comprises a control bus interface section (to be referred to as a DHW I/F hereinafter)
21
, CPU
22
, and DSP
23
. The DHW I/F
21
has an interface function to the DHW
17
. The CPU
22
controls processing of the DSP
23
on the basis of a control signal supplied from the DHW
17
through the DHW I/F
21
. The DSP
23
has a storage section
231
which stores a Goertzel algorithm to be described below. More specifically, the DSP
23
detects and identifies a DTMF signal from a PCM signal supplied from the PCMHW
16
in accordance with the program stored in the storage section
231
upon receiving an instruction from the CPU
22
.
The Goertzel algorithm will be described below.
This Goertzel algorithm is optimum to DTMF signal detection by the discrete Fourier transform, in which a signal on the time axis is converted into a signal on the frequency axis and output, as in the Fourier transform. Generally, in the Fourier transform, when input signals at N sampling points on the time axis are calculated, output signals at N points are obtained on the frequency axis. In the Fourier transform, however, when only eight frequencies suffice as output points for such DTMF signal detection, the arithmetic operation is wasteful.
To prevent this, in the Goertzel algorithm, the number of samples is selected such that a spectrum only at a specific frequency is output, and the square of an output value is calculated to eliminate the complex number generated by the arithmetic operation so that only a real number can be output to make the processing easy. Note that the DTMF signal is a mixed wave of one frequency selected from four low-frequency components and one frequency selected from four high-frequency components, as shown in
FIG. 2
, and is represented by a mixed wave in a total of 16 combinations. Hence, when the DTMF signal is Fourier-transformed into a signal on the frequency axis, each of the two, high- and low-frequency components contained in the DTMF signal is represented by a peak value at one point. The DSP
23
can recognize and detect the type of DTMF signal from the combination of two frequencies corresponding to the peaks.
However, for the above-described method using the DSP
23
, a program for executing the Goertzel algorithm must be created. In addition, causing the CPU
22
and DSP
23
to execute the Goertzel algorithm requires to prepare a number of ROMs or RAMs or a large-capacity memory in the DSP
23
and also requires to change the CPU
22
to a processor compatible to high-speed operation. These pose a serous problem in integration and cost reduction of the DTMF signal receiving section and also increase the power consumption.
FIG. 4
shows the arrangement of another conventional key telephone system. In this key telephone system, a main apparatus
1
B has a tone signal generator
18
. The same reference numerals as in
FIG. 1
denote the same parts in
FIG. 4
, and a detailed description thereof will be omitted.
The tone signal generator
18
is connected to a TSW
13
and control section
14
and time-divisionally generates a tone signal formed from a plurality of kinds of waveforms in accordance with an instruction from the control section
14
. The tone signal is selectively sent to extension terminals T
1
to Tm by the TSW
13
.
To time-divisionally generate a tone signal, the tone signal generator
18
stores data in a ROM
181
, as shown in
FIG. 5
, and extracts data therefrom.
The ROM
181
has continuous areas for storing data of waveforms #1 to #n−1. The areas have the same size of m bytes. Each area stores the amplitude value data of the waveform to be generated. One waveform is generated by reading the m-byte data in accordance with the order from #1 to #n−1.
The tone signal generator
18
also has a counter
182
for waveform number switching and a counter
183
for data number switching. More specifically, in the tone signal generator
18
, data represented by an address value obtained by adding, by an adder
184
, an upper bit output from the counter
182
and a lower bit output from the counter
183
is extracted from the ROM
181
and output.
The operation in the tone signal generator
18
Hase Yoshiko
Otsuka Eiji
Yamashita Takeshi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Tieu Benny
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