Dual-tone Multifrequency Research Articles

Efficient robust DTMF decoding using the subband NDFT

Digital dual-tone multi-frequency (DTMF) decoding is often performed by applying a frequency transformation to the received DTMF signal to measure the energies present at the DTMF frequencies. A DTMF decoding algorithm is developed using the concepts of non-uniform frequency sampling and subband decomposition. Since DTMF signals occupy the low-frequency band of the telephone bandwidth, an efficient computation is achieved by retaining only the higher subband in a two-band decomposition of the received signal. Simulations based on the AT&T TouchTone standard are presented to compare the performances of DTMF decoding algorithms based on the subband non-uniform discrete Fourier transform (SB-NDFT), NDFT, SB-DFT and DFT. Results show that the SB-NDFT-based algorithm is more robust to the variation of DTMF frequencies within the specified tolerance bands. Besides, it also requires a significantly lower computational effort to attain a specified level of performance.

Rapid DTMF signal classification via parallel cascadeidentification

The parallel cascade identification of nonlinear systems is used for the rapid recognition of dual tone multi-frequency (DTMF) signals, i.e. the ‘dual tones’ used to encode the digits on a telephone keypad. The authors demonstrate that parallel cascades can correctly identify DTMF signals with half or less than half of the input data required by current methods.

VERDI: an acoustically programmable and adjustable CMOS mixed-mode signal processor for hearing aid applications

A CMOS mixed-mode fully-differential signal processor (VERDI), that constitutes the core of a hearing aid system, is introduced. Its characteristics (frequency response, compression level, and output sound pressure level) are wireless controlled by means of dual tone multi-frequency (DTMF) encoded signals transmitted by an external unit. A system description is provided, and the implementation and experimental results of the key components (preamplifier, antialiasing filter, automatic gain control, filter section, output amplifier, and DTMF receiver/decoder) are covered in more detail. The circuit has been fabricated in a conventional 1.2 /spl mu/m CMOS n-well process and operates from a single 1.3 V battery, although a dc/dc converter is required. It consumes typically less than 1 mA and occupies 28 mm/sup 2/ of silicon. The dynamic range is larger than 66 dB for the maximum input signal (54 mV/sub rms/=108 dB SPL).

Vocal frequency suppression apparatus and method

A vocal frequency suppression apparatus for suppressing selected portions of a vocal signal to prevent inadvertent control of components controlled by dual tone multi-frequency ("DTMF") codes. The vocal frequency suppression apparatus includes a DTMF code generator having a plurality of user-selectable switches. The apparatus further produces a vocal signal which may contain vocal frequencies which correspond to at least one frequency of at least one of the DTMF codes. The apparatus includes a device for suppressing one or more of the vocal frequencies which correspond to frequencies found in the DTMF codes to prevent inadvertent control of any DTMF controlled components by the vocal signal. The suppression device is operably located between the producing device and the DTMF generator.

Dual tone detector operable in the presence of speech or background noise and method therfor

A dual tone detector (100) for a single dual tone, a dual tone multi-frequency (DTMF), or similar system processes an input signal through both bandpass (103) and band reject (104) tone detectors. If both the bandpass (103) and band reject (104) tone detectors detect a tone, then the dual tone detector (100) provides a tone detect output signal. If only the bandpass tone detector (103), which is susceptible to false tones, detects a tone, then a voice input signal is muted and the tone detector (100) activates the tone detect output signal only if both the bandpass (103) and band reject (104) tone detectors subsequently detect a tone. In one embodiment, a dual bandpass/band reject tone detector (120) processes the input signal through shared front-end band reject filters (121, 122), limiters (124), resonators (127, 128), and a processing section (130) in order to save circuit area. Limiter and peak detector functions are also implemented in shared circuitry to further reduce circuit area.

PC remote control of appliances by using telephone lines

The idea of home automation using telephone lines and a PC is shown. The system is based on the dual tone multifrequency (DTMF) signals that could be sent through a loop of wire to switch on/off various appliances via a personal computer (PC). The system hardware and software are designed based on the telephone standards. The hardware includes the DTMF receiver, on/off hook detector, ring detector and an input/output PC interfacing card. An example of controlling an appliance is provided. The software control programming is written in Turbo Basic and their flow charts are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A wireless volume control receiver for hearing aids

The design of a practical wireless volume-control receiver for hearing aids is proposed in this paper. A trade-off design approach is introduced to solve the problem of minimizing the size of the receiver, reduce its power consumption, and maximize its performance. The receiver decodes standard dual-tone multifrequency (DTMF) signals and it requires only one multiplexed switched capacitor bandpass filter. Another unique component is the OTA design in weak inversion with reduced power supply, high voltage gain, and micropower dissipation. Most of the signals are processed using digital logic which contributes to the simplicity and power efficiency of the design. The IC layout of the receiver measures 1918 /spl mu/m/spl times/1109 /spl mu/m and it was implemented in a 2 /spl mu/m CMOS technology with a 0.5 V threshold voltage. Experimental results using discrete (commercial) components show that the receiver can detect each valid frequency within 0.41% of the nominal value. A modified version of the receiver may be used as a DTMF decoder. This approach has also been used to design a wireless programmable interface for hearing aids.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The analog display services interface

The trial implementation of Bellcore's advanced call management services (ACMSs) is reviewed. The ACMS used an experimental screen-based telephone with a context-sensitive visual display and context-sensitive soft keys. ACMS was based on a Bellcore research prototype, the customer premises equipment (CPE) network integrator (CNI), which consists of inband signal (tone) detection circuitry, software to interpret the detected signals and drive the screen, pulse (switch-hook flash) and tone generation capability, and software to translate user key activations to sequences of pulses and dual tone multifrequency (DTMF) tones. The narrowband information networking vision, the required changes in current practice and technology, the revenue potentials for telephone companies, and the risks associated with getting new customer premises equipment (CPE) are discussed. The author concludes that context sensitivity for displays and soft keys is critical to a successful user interface for complex services.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Potential future applications of direct, automated service activation

There is budding interest among telecommunications network and service providers in allowing subscribers to activate their telecommunications services by directly interacting with a fully automated service activation process. This capability facilitates both service turn-up for new customers and service addition for existing customers. Direct, automated service activation (DASA) is enabled by remote management of service attributes, which is governed by input from a variety of possible customer access channels. Some of the most promising direct input channels, beyond DTMF (dual-tone multi-frequency) keypad signaling, include centrally located video kiosks, portable analog display sets, and in-home video monitors. Another significant enabler for direct, automated activation is re-engineering key aspects of the existing service activation architecture. Three possible functional architectures for a DASA application, each progressively more complex, are out-lined. For any of these architectures, re-engineering must focus on several important aspects, such as the flexible sequencing of processing steps and closer alignment between network and administrative data.

A single-chip controller for digital answering machines

A single-chip controller for digital answering machines is introduced. Only two other devices, an audio quality DRAM (ARAM) and a codec, are needed to implement all the digital parts of such an answering machine. The device performs voice compression and decompression algorithms, dual-tone multifrequency (DTMF) detection, voice synthesis, and control functions of digital answering machines. >

Speech recognition for the disabled

A simple and inexpensive system for telephone communication that includes a modular speech recognition device developed for use by a disabled person is presented. User considerations are reviewed, and hardware details are given. The complete automatic speech recognition (ASR) system consists of five main blocks: a TCM29C18 codec (analog interface for the digital signal processor), an SRS-1 speech recognizer, an LED display, a dual-tone multifrequency generator, and tone enable plus data input circuitry. Results of a semiqualitative assessment in terms of recognition success for the limited vocabulary of numbers and commands from the user are reported. The results are described in chronological order, beginning with testing of the SRS-1 and the codec working as a stand-alone recognizer system, and leading to the testing of the recognition system combined with the tone generator. Finally, results were obtained when the whole ASR system was interfaced to the telephone line. Some further work is suggested.

A software programmable CMOS telephone circuit

A mixed analog-digital IC combining the speech and signaling functions of an analog telephone is described. The realization in a standard 2- mu m CMOS process offers the possibility of integrating complex digital functions, such as dual-tone multifrequency (DTMF) generation, in combination with many digital programmable analog circuits . Novel features, such as digital programmable level control, DC characteristics, frequency response, input impedance, and sidetone path, are implemented. These attributes offer a flexible solution for the different international requirements, with only a few external components. Software control of analog and digital functions and the high degree of integration enable telephone manufacturers to produce a quality feature phone with a minimum of components, and therefore with high reliability and low cost. >

SIGFRED: a low-power DTMF and signaling frequency detector

A CMOS circuit is presented containing an analog switched-capacitor processor for detection of dual-tone multifrequency, (DTMF) and signaling tones on a telephone subscriber line and a digital detection algorithm for postprocessing. The major issues for the design were that it should be compatible with a CMOS fabrication process for microcontrollers to be included as an on-chip component and that its power consumption should be as low as possible. It has been designed to work with a current of less than 0.5 mA at low supply voltages (2.5-3 V). The maximum supply voltage is limited to 6 V by the process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A robust approach to tele-control using DTMF signaling (telephone call transferring equipment)

A novel method is proposed which is effective in preventing malfunction in the DTMF (dual-tone multifrequency) signal decoder caused by the holding melody. When the monitoring time is one second, the probability of a malfunction is at most 0.01. The proposed approach is a double DTMF signal monitoring method using the statistical characteristics of a DTMF signal decoder for a holding melody. >

An oversampling-based DTMF generator

A circuit is described which uses one oversampling frequency synthesizer in an interleaved mode to generate dual-tone multifrequency (DTMF) telephone dialing tones. The synthesizer is based on direct digital synthesis using phase accumulation. The digital-to-analog conversion is carried out using a first-order sigma-delta converter. This sigma-delta converter requires only a simple second-order RC filter, eliminating the need for a switched capacitor filter. Measurements on the circuit confirm that all specifications of the DTMF are fulfilled. >

Enhanced telephone set on the Inter-IC bus

A microcontroller-based enhanced telephone set designed and developed using the Inter-IC (I/sup 2/C) bus system is described. Besides functioning as a subscriber telephone, this feature phone incorporates a software-controlled ringer function, pulse and DTMF (dual-tone multifrequency) dialing, redial and extended redial features, direct dialing for emergencies, on-chip memory for 10 repertory dial numbers of 18 digits, direct access repertory dialing, and a liquid-crystal display.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ensuring predictor tracking in ADPCM speech coders under noisy transmission conditions

The problem of predictor mistracking for narrowband signals in backward adaptive ADPCM (adaptive digital pulse code modulation) speech coders is shown to arise as a result of feedback from the signal reconstruction filter to the predictor adaptation process. A class of residual-signal-driven lattice predictors (PR) is defined that guarantees tracking for all signals without regard to the order of prediction. The LR predictor enhances speech and DTMF (dual-tone multifrequency) signal transmission performance in the presence of transmission errors. Under error-free transmission conditions, a segmental SNR (signal-to-noise ratio) drop for speech of nearly 2 dB may be encountered for the LR predictor relative to the classical signal-drive lattice predictor. In most practical telecommunication applications, however, this degradation is outweighed by the improved robustness of the predictor. >

Telecommunications device for the hearing impaired

First Page

A Dual-Tone Multifrequency Receiver Using Synchronous Additions and Subtractions

This paper describes a new detection algorithm for a dualtone multifrequency (DTMF) signal. The new algorithm basically uses only additions and subtractions but no multiplications. A signaling tone can be detected by sampling an input signal at four times each frequency involved in a signaling tone and accumulating these sampled values synchronously by additions and subtractions. This algorithm is referred to as the synchronous additions and subtractions (SAS) method. Many conventional methods have used various types of filters to detect signaling tones. In the SAS method in which the design of filters is unnecessary, only the generation of sampling pulses is required, and main operations are additions and subtractions. These features are useful to implement flexible receivers and various types of receivers. The simplicity of the SAS method is demonstrated by the implementation of a DTMF receiver using a conventional 8-bit microcomputer without any special hardware. The minimum signal-to-noise ratio is 5.7 dB when the receiver operates for the signaling tones to be accepted. Four easily measurable parameters are used to distinguish signaling tones from noise inputs. Only two false detections occurred for severe noise inputs of thirty hours.

Dual tone and modem frequency generator with on-chip filters and voltage reference

A single-chip CMOS circuit is described that contains a dual-tone multifrequency and modem frequency generator. For optimum performance and economy, switched-capacitor techniques are used for the on-chip bandgap reference voltage, digital-to-analog converters, and filter. CEPT recommendations on output level stability and distortion are met without recourse to external filtering and without a stabilized supply or external reference voltage. A self-aligned contact CMOS process with 4-/spl mu/m design rules and with 500-/spl Aring/-thick gate oxide is used to manufacture the circuit.