CN113556434B - Method for realizing DTMF code transmission for airborne telephone - Google Patents
Method for realizing DTMF code transmission for airborne telephone Download PDFInfo
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- CN113556434B CN113556434B CN202110757606.XA CN202110757606A CN113556434B CN 113556434 B CN113556434 B CN 113556434B CN 202110757606 A CN202110757606 A CN 202110757606A CN 113556434 B CN113556434 B CN 113556434B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M7/00—Arrangements for interconnection between switching centres
- H04M7/12—Arrangements for interconnection between switching centres for working between exchanges having different types of switching equipment, e.g. power-driven and step by step or decimal and non-decimal
- H04M7/1205—Arrangements for interconnection between switching centres for working between exchanges having different types of switching equipment, e.g. power-driven and step by step or decimal and non-decimal where the types of switching equipement comprises PSTN/ISDN equipment and switching equipment of networks other than PSTN/ISDN, e.g. Internet Protocol networks
- H04M7/1295—Details of dual tone multiple frequency signalling
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04H—BROADCAST COMMUNICATION
- H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
- H04H20/53—Arrangements specially adapted for specific applications, e.g. for traffic information or for mobile receivers
- H04H20/61—Arrangements specially adapted for specific applications, e.g. for traffic information or for mobile receivers for local area broadcast, e.g. instore broadcast
- H04H20/62—Arrangements specially adapted for specific applications, e.g. for traffic information or for mobile receivers for local area broadcast, e.g. instore broadcast for transportation systems, e.g. in vehicles
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
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Abstract
Loading data for generating sine waves into a direct memory access DMA (direct memory access) module of a singlechip module, adjusting the conversion rates of two channels of a digital-to-analog conversion DAC (digital-to-analog conversion) module by using a timer TIM (time value) module so as to generate eight sine waves with different frequencies, and outputting the sine waves from ports of the two digital-to-analog conversion DAC modules of the singlechip; the single-chip microcomputer is connected with the path selection module, the on-off of the path selection module is controlled by the single-chip microcomputer, the signal coupling module is connected with an external power supply line in a coupling mode after direct current filtering, impedance matching and capacitance matching, and the DTMF signal is sent to a broadcast internal call system. The invention solves the problem of dependence on DTMF coding chip of the existing telephone, reduces the power consumption of the system and saves the cost.
Description
Technical Field
The invention belongs to the technical field of airborne broadcast internal calls, particularly relates to a method for realizing DTMF code sending of an airborne telephone, and particularly relates to a DTMF dial-up code communication circuit design without a DTMF code chip.
Background
The airborne broadcast intercom system is a communication system used for drivers and crewmembers in civil airliners to call each other and broadcast in a cabin, and terminal devices of the broadcast intercom system are a handheld microphone of the crewmember in the cabin, a handheld microphone of a cockpit and a cantilever microphone respectively. The crew member completes the functions of broadcasting in the passenger cabin, talking in the airplane, emergency calling and the like through the keys of the airborne telephone.
The on-board phone button and function include:
1) the "PA ALL" bond: triggering a full-passenger-cabin broadcasting function;
2) the "PA FWD" bond: triggering a front passenger cabin broadcasting function;
3) the "PA AFT" bond: triggering a rear cabin broadcasting function;
4) the "EMER CALL" bond: triggering an emergency call cockpit function;
5) the "CAPT" bond: triggering a communication function with a cockpit;
6) the "FWD ATTND" bond: triggering a call with a front cabin attendant;
7) the "ALL ATTND" bond: triggering a call function with all the crew members;
8) the "AFT L ATTND" bond: triggering a call function with a left rear cabin attendant;
9) the "AFT R ATTND" bond: triggering a communication function with a right rear passenger cabin attendant;
10) the "RESET" bond: triggering the FAH phone to clear the current state;
11) the "CAPT PRIOR" bond: and triggering a conversation function preferred by the driver.
After the crew presses the key, the telephone set can generate corresponding DTMF signals to communicate with the broadcast internal telephone system, and the broadcast internal telephone system receives the DTMF signals and then judges the required functions through decoding to complete the required control.
The DTMF signal is used to position and distinguish the keys of the keyboard, and a coding system is formed by using specific 8 frequencies. The 8 frequencies are divided into two frequency groups, i.e. high and low frequency groups. Wherein the high-frequency group comprises four frequencies of 1029Hz, 1336Hz, 1477Hz and 1633 Hz; and the low frequency groups comprise four frequencies of 697Hz, 770Hz, 852Hz and 941 Hz. Each character signal is formed by the superposition of sinusoidal signals with two frequencies in rows and columns, and a fixed group frequency mode is adopted. The specific keyboard and high and low frequency group division can be seen from the distribution setting in fig. 2.
The basic sample sizes of the DTMF low and high frequency bins form a completed baseline, as required by the associated CCITT. Meanwhile, the frequency deviation of the DTMF signal is required to be less than 1.5%, the signal limit value of each array is required to be longer than the time length of 40ms, the deviation of the equipment is required to be less than 2% at the receiving end, and reliable receiving is kept. Thus, the terminal can process the signal with 30-40 ms.
In conventional designs, DTMF coding is designed using an integrated circuit scheme, for example, using an MT8880 chip from MITEL corporation. However, such designs are subject to restrictions from foreign chip suppliers, and the integrated circuits and their peripheral circuits increase the power consumption of the system.
Aiming at the problems, the invention provides a low-power-consumption system design avoiding using a DTMF coding chip, and aims to solve the requirements of dependence of an existing airborne telephone on an imported chip and reduction of power consumption.
Disclosure of Invention
The technical problems solved by the invention are as follows: the invention provides a method for realizing DTMF code sending for an airborne telephone, which solves the problem of dependence on an imported coding chip of the existing airborne telephone and the requirement of the system on reducing the power consumption of the telephone by the design of a single chip microcomputer module, an adder module, a channel selection module and a signal coupling module, is suitable for a handheld microphone terminal of an airborne broadcast telephone system of a civil aviation airliner, and has the advantages of low power consumption, simple realization, high precision, high flexibility and the like.
The technical scheme adopted by the invention is as follows: a method for realizing DTMF code sending for an airborne telephone comprises a singlechip module, an adder module, a path selection module and a signal coupling module, and the specific sending process is as follows:
(1) the single chip microcomputer module is internally provided with embedded software, and the embedded software is internally provided with a sine wave buffer DATA DATA module, a timer TIM module, a direct memory access DMA module and a digital-to-analog conversion DAC module; the embedded software calculates parameters of a sine wave buffer DATA DATA module and a timer TIM module required for generating a sine wave signal, and pre-configures parameters of a direct memory access DMA module and the timer TIM module;
after the digital-to-analog conversion DAC module and the timer TIM module are initialized, a sine wave buffer DATA DATA module generating sine wave signals is loaded into a direct memory reading DMA module, the direct memory reading DMA module is mounted on the digital-to-analog conversion DAC module, then corresponding timer TIM module parameters are called according to key values, the counter period of the timer TIM module is reconfigured, the timer TIM module controls the conversion rate of the digital-to-analog conversion DAC module, and two DAC channels in the digital-to-analog conversion DAC module are called through the direct memory reading DMA module and the timer TIM module: DAC _ CHANNEL _1 and DAC _ CHANNEL _2, so that two CHANNELs of CHANNEL1 and CHANNEL2 directly read data in a buffer area of the direct memory reading DMA module and convert the data into two required sine wave signals, namely High _ SinWave and Low _ SinWave, which are sent out and connected to the adder module;
(2) the adder module synthesizes the two received sine wave signals into a DTMF signal used for communication of the airborne telephone and then sends the DTMF signal into the path selection module;
(3) the CHANNEL selection module gates the audio signals and the DTMF signals, the embedded software controls the gating of the audio signals and the DTMF signals through three signal lines of Select _ EN, Select _ A0 and Select A1, and simultaneously the embedded software controls DAC _ CHANNEL _1 and DAC _ CHANNEL _2 to stop sending sine waves;
(4) the signal coupling module filters direct current components of DTMF signals output by the path selection module, power supply and voltage stabilization are carried out on a direct current power supply, and the two paths of signals are processed and then connected and coupled respectively, so that the DTMF signals can be loaded on a direct current power supply signal line, and communication and control can be carried out through a power line conveniently.
In a further limitation of the above technical solution, the sine wave frequency output by the High _ SinWave is controlled by parameters of a timer TIM module, including 1209Hz, 1336Hz, 1477Hz and 1633Hz 4; the sine wave frequency output by the Low _ SinWave is controlled by parameters of a timer TIM module, and the parameters comprise 4 types including 697Hz, 770Hz, 852Hz and 941 Hz.
The technical scheme is further limited, the adder module is composed of an operational amplifier, an isolation filter circuit and an amplification configuration circuit, two paths of sine wave signals received by the adder module are filtered by the isolation filter circuit to remove direct current components, are subjected to voltage division through a voltage division resistor and then enter a Vin-end of the operational amplifier, the amplification configuration circuit is used for configuring the voltage of Vin + and V + to control the DTMF output amplitude, and after passing through the operational amplifier, the two paths of sine wave signals are combined into a fixed DTMF signal.
In a further limited aspect of the foregoing technical solution, the DTMF signal output by the adder module includes: 16 types of "1", "2", "3", "4", "5", "6", "7", "8", "9", "0", "B", "A", "C", "E", "F", "D".
In a further limitation of the above technical solution, the path selection module includes control software of the analog switch by the single chip, an analog switch chip, and a peripheral circuit; the signal coupling module comprises a direct current voltage stabilizer, a filter circuit and a direct current isolation circuit.
Compared with the prior art, the invention has the advantages that:
1. the scheme solves the problem of dependence on an imported coding chip of the existing airborne telephone and the requirement of the system on reducing the power consumption of the telephone by the design of the singlechip module, the adder module, the path selection module and the signal coupling module, meets the requirement of aviation standards and aviation standards on the airborne broadcasting microphone on the civil airliner, and has the advantages of simple circuit structure, high circuit integration level, high reliability, stable circuit performance, low power consumption, flexible design and cost saving;
2. in the scheme, the power supply interface of the airborne phone is connected with the current-limiting resistor, the phone is extremely sensitive to the power consumption requirement, the power consumption needs to be greatly reduced, an encoding chip and a peripheral circuit thereof are omitted, and the power consumption is reduced to a certain extent;
3. the scheme solves the problem that imported chips are often influenced by sale prohibition and production halt in China, avoids dependence on the imported chips, improves the localization rate of products, and avoids the risk of sale prohibition and production halt of the chips;
4. in the scheme, DTMF communication between the airborne telephone and the broadcast internal telephone system needs to set sending time and signal amplitude according to a receiving end design scheme, and the method can flexibly adjust the sending time and the signal amplitude through software and has strong portability.
Drawings
FIG. 1 is a schematic diagram of the principles of the present invention;
FIG. 2 is a prior art layout of DTMF high and low frequency components on a keyboard;
FIG. 3 is a diagram of a DTMF module power supply and filter circuit of the present invention;
fig. 4 is a circuit diagram of DTMF signal coupling in the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
Referring to fig. 1, an embodiment of the present invention is described in detail.
A method for realizing DTMF code sending for an airborne telephone comprises a single chip microcomputer module 1, an adder module 2, a path selection module 3 and a signal coupling module 4.
The specific sending process is as follows:
(1) the single chip microcomputer module 1 is internally provided with embedded software 1-1, and the embedded software 1-1 is internally provided with a sine wave buffer DATA DATA module 1-2, a timer TIM module 1-3, a direct memory access DMA module 1-4 and a digital-to-analog conversion DAC module.
The sine wave buffer DATA DATA module 1-2 combines the DATA requirements of CHANNEL11-5 and CHANNEL21-6 of the single-chip digital-to-analog conversion DAC module by adopting an interpolation and normalization method to form an array of 128 bytes, and the array is stored in the memory of the single-chip. The TIM module 1-3 of the timer of the singlechip is designed by adopting two timer channels, comparing output modes, counting upwards, initializing in an automatic reloading mode, and designing counting period parameters of the TIM module 1-3 of the timer according to eight frequencies of 1029Hz, 1336Hz, 1477Hz, 1633Hz, 697Hz, 770Hz, 852Hz and 941Hz, so that the parameters of each counting period can correspond to the generation of one frequency. The direct memory reading DMA module 1-4 of the single chip microcomputer is designed to use a memory-to-port output mode, and two DMA CHANNELs are adopted to respectively mount two CHANNELs, namely CHANNEL11-5 and CHANNEL21-6 of the digital-to-analog conversion DAC module, so that the digital-to-analog conversion DAC module can directly obtain conversion data from the memory, the response time is greatly reduced, and the frequency precision is improved.
The specific working process is as follows: the embedded software 1-1 calculates parameters of a sine wave buffer DATA DATA module 1-2 and a timer TIM module 1-3 required for generating a sine wave signal, and configures parameters of a direct memory access DMA module 1-4 and the timer TIM module 1-3 in advance.
After the digital-to-analog conversion DAC module and the timer TIM module 1-3 are initialized, the sine wave buffer DATA DATA module 1-2 which generates sine wave signals is loaded into the direct memory reading DMA module 1-4, and the direct memory access DMA module 1-4 is mounted on the digital-to-analog conversion DAC module, when a key is pressed down, the software judges the key value and judges which two frequencies need to be generated according to the key value, then calling the corresponding timer TIM module 1-3 parameter, reconfiguring the counter period of the timer TIM module 1-3, controlling the conversion rate of the digital-to-analog conversion DAC module by the timer TIM module 1-3, then the software control path selection module 3 is switched to a DTMF sending channel, and two DAC channels in the digital-to-analog conversion DAC module are called through the direct memory reading DMA module 1-4 and the timer TIM module 1-3: DAC _ CHANNEL _11-5 and DAC _ CHANNEL _21-6, so that the two CHANNELs CHANNEL11-5 and CHANNEL21-6 directly read data in the buffer area of the direct memory reading DMA module 1-4 without a single chip microcomputer and convert the data into two required sine wave signals, namely High _ SinWave and Low _ SinWave, which are sent out and connected to the adder module 2 through a GPIO. The sine wave frequency output by the High _ SinWave is controlled by parameters of TIM modules 1-3 of a timer, wherein the parameters comprise 4 types of 1209Hz, 1336Hz, 1477Hz and 1633 Hz; the sine wave frequency output by the Low _ SinWave is controlled by parameters of a timer TIM module 1-3, including 4 types of 697Hz, 770Hz, 852Hz and 941 Hz.
(2) Two paths of sine wave signals are sent out from two CHANNELs CHANNEL11-5 and CHANNEL21-6 of the single chip microcomputer and then need to be synthesized by the adder module 2. The adder module 2 is composed of an operational amplifier, an isolation filter circuit, and an amplification configuration circuit, wherein as shown in fig. 3, two sinusoidal signals received by the adder module 2 are filtered by the isolation filter circuit to remove a direct current component, and then enter a Vin-end of the operational amplifier after being subjected to voltage division by a voltage division resistor, the amplification configuration circuit configures the voltages of Vin + and V + to control the output amplitude of DTMF, and after passing through the operational amplifier, the two sinusoidal signals are synthesized into a fixed DTMF signal. The adder module 2 synthesizes the two received sine wave signals into a DTMF signal used for the communication of the airborne telephone and then sends the DTMF signal into the path selection module 3. The DTMF signal output by the adder module 2 includes: 16 types of "1", "2", "3", "4", "5", "6", "7", "8", "9", "0", "B", "A", "C", "E", "F", "D".
(3) Since the DTMF signal and the voice signal of the handset are transmitted by using the same set of audio signal line, the path selection module 3 is required to switch to the DTMF signal path when transmitting the DTMF signal, so that the DTMF signal can be transmitted through the audio signal line. The design is realized by controlling the channel selection module 3 through the embedded software 1-1, and the embedded software 1-1 realizes the signal gating function by controlling three signal lines of the channel selection module 3.
Specifically, the path selection module 3 includes control software of the analog switch by the single chip microcomputer, an analog switch chip, and a peripheral circuit. The channel selection module 3 gates the voice signals and the DTMF signals, the embedded software 1-1 controls the gating of the audio signals and the DTMF signals through three signal lines of Select _ EN, Select _ A0 and Select tA1, when the DTMF signals need to be sent, the embedded software (1-1) controls the three signal lines to switch the DTMF signals to a channel of the signal coupling circuit (4), and after the DTMF signals are sent, the embedded software (1-1) controls the three signal lines to switch to an audio output channel; while the embedded software 1-1 controls DAC _ CHANNEL _11-5 and DAC _ CHANNEL _21-6 to stop sending sine waves.
(4) In the airborne telephone, both the DTMF signal and the telephone voice signal are coupled to the external power line, so in the case of DTMF communication, the signal coupling module 4 is required to couple the DTMF signal to the power line, and the signal coupling circuit is shown in fig. 4. The signal coupling module 4 comprises a direct current voltage stabilizer, a filter circuit and a direct current isolation circuit. The signal coupling module 4 isolates the direct current power supply on the power supply signal wire through a VCO direct current voltage stabilizing chip, meanwhile, the capacitor is used for filtering the direct current component of the DTMF signal output by the path selection module 3, the linear voltage stabilizer is used for supplying power and stabilizing voltage to the direct current power supply, then a resistor, an inductor and a capacitor are required to be configured, and two paths of signals are respectively processed and then connected and coupled, so that the DTMF signal and the voice signal can be completely loaded on the direct current power supply signal wire without distortion, and communication and control can be conveniently carried out through a power line.
The related software modules of the scheme are as follows:
(1) frequency configuration module
In a word, the method loads data for generating sine waves into a direct memory access DMA (direct memory access) module of a singlechip module, and uses a timer TIM module to adjust the conversion rates of two channels of a digital-to-analog conversion DAC (digital-to-analog conversion) module so as to generate eight sine waves with different frequencies, and the sine waves are output from ports of the two digital-to-analog conversion DAC modules of the singlechip; the single-chip microcomputer is connected with the path selection module, the on-off of the path selection module is controlled by the single-chip microcomputer, the signal coupling module is connected with the external power supply circuit, the signal coupling module is coupled and connected to an external power supply circuit after direct current filtering, impedance matching and capacitance matching, and the DTMF signal is sent to the broadcast internal phone system. The invention solves the problem of dependence on DTMF coding chip existing in the existing telephone, reduces the power consumption of the system, saves the cost, and has the advantages of high flexibility, high frequency precision, low cost and low power consumption.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.
Claims (5)
1. A method for implementing DTMF code transmission for an airborne handset, characterized in that: the system comprises a singlechip module (1), an adder module (2), a path selection module (3) and a signal coupling module (4), and the specific sending process is as follows:
(1) the single-chip microcomputer module (1) is internally provided with embedded software (1-1), and the embedded software (1-1) is internally provided with a sine wave buffer DATA DATA module (1-2), a timer TIM module (1-3), a direct memory reading DMA module (1-4) and a digital-to-analog conversion DAC module; the embedded software (1-1) calculates parameters of a sine wave buffer DATA DATA module (1-2) and a timer TIM module (1-3) required for generating a sine wave signal, and pre-configures parameters of a direct memory access DMA module (1-4) and the timer TIM module (1-3);
after the digital-to-analog conversion DAC module and the timer TIM module (1-3) are initialized, loading a sine wave buffer DATA DATA module (1-2) generating a sine wave signal into a direct memory reading DMA module (1-4), mounting the direct memory reading DMA module (1-4) on the digital-to-analog conversion DAC module, calling corresponding parameters of the timer TIM module (1-3) according to key values, reconfiguring a counter period of the timer TIM module (1-3), controlling the conversion rate of the digital-to-analog conversion DAC module by the timer TIM module (1-3), and calling two DAC channels in the digital-to-analog conversion DAC module through the direct memory reading DMA module (1-4) and the timer TIM module (1-3): DAC _ CHANNEL _1(1-5) and DAC _ CHANNEL _2(1-6), so that two CHANNELs DAC _ CHANNEL1(1-5) and DAC _ CHANNEL2(1-6) directly read data in a buffer area of a direct memory reading DMA module (1-4) and convert the data into required two sine wave signals, namely High _ SinWave and Low _ SinWave, and send the two sine wave signals out to be connected to an adder module (2);
(2) the adder module (2) synthesizes the two received sine wave signals into DTMF signals used for communication of the airborne telephone and then sends the DTMF signals into the path selection module (3);
(3) the CHANNEL selection module (3) gates the voice signals and the DTMF signals, the embedded software (1-1) controls the gating of the voice signals and the DTMF signals through three signal lines of Select _ EN, Select _ A0 and Select A1, and the embedded software (1-1) controls DAC _ CHANNEL _1(1-5) and DAC _ CHANNEL _2(1-6) to stop sending sine waves;
(4) the signal coupling module (4) filters the direct current component of the DTMF signal output by the path selection module (3), supplies power and stabilizes voltage for a direct current power supply, and the two paths of signals are respectively processed and then are connected and coupled, so that the DTMF signal can be loaded on a direct current power supply signal line, and communication and control can be conveniently carried out through a power line.
2. A method for implementing DTMF code transmission for a handset according to claim 1, wherein: the sine wave frequency output by the High _ SinWave is controlled by parameters of a timer TIM module (1-3), including 4 types of 1209Hz, 1336Hz, 1477Hz and 1633 Hz; the sine wave frequency output by the Low _ SinWave is controlled by parameters of a timer TIM module (1-3), and the parameters comprise 4 types of 697Hz, 770Hz, 852Hz and 941 Hz.
3. A method for implementing DTMF code transmission for a handset according to claim 1, wherein: the adder module (2) is composed of an operational amplifier, an isolation filter circuit and an amplification configuration circuit, two paths of sine wave signals received by the adder module (2) are filtered by the isolation filter circuit to remove direct current components, then are subjected to voltage division through a voltage division resistor and enter a Vin-end of the operational amplifier, the amplification configuration circuit is used for configuring the voltage of Vin + and V + to control the DTMF output amplitude, and after passing through the operational amplifier, the two paths of sine wave signals are combined into a fixed DTMF signal.
4. A method for implementing DTMF code transmission for a handset according to claim 3, wherein: the DTMF signal output by the adder module (2) comprises: "1", "2", "3", "4", "5", "6", "7", "8", "9", "0", "B", "A", "C", "E", "F", "D" 16.
5. A method for implementing DTMF code transmission for a handset according to claim 1, wherein: the path selection module (3) comprises control software of the single chip microcomputer for the analog switch, an analog switch chip and a peripheral circuit; the signal coupling module (4) comprises a direct current voltage stabilizer, a filter circuit and a direct current isolation circuit.
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| US20080192623A1 (en) * | 2007-02-09 | 2008-08-14 | Mediatek Inc. | Methods and devices for dual-tone multi-frequency (dtmf) signaling |
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