Echo Control Research Articles

Stereo acoustic echo cancellation: A review

Adaptive echo cancelers have been used successfully for over a decade to combat echoes that arise on a telephone network due to mismatches in impedance at various points in the circuit. Echoes arise in teleconferencing too, due to coupling between the loudspeaker and microphone in each room. This problem is similar to the network echo problem, although the impulse responses are longer and can change more rapidly. When echoes arise in a stereophonic system, however, the problem changes character due to an inherent nonuniqueness in estimating the echo paths. In this talk we will review the current status of the problem of stereo echo cancellation. After a brief introduction to the general problem of echo control, the nonuniqueness mentioned above will be discussed and then several approaches will be described to get around this nonuniqueness. At present echo path responses can be matched with an error of less than −20 dB, without any loss of the stereophonic effect.

Audio Technology Used in AT&T's Terminal Equipment

This paper discusses the audio technology of AT&T's terminal equipment. It describes electroacoustic and digital signal processing (DSP) techniques that improve the perceived audio quality and ease of use of telephones and speakerphones. The electroacoustic techniques include directional microphones, close-talking microphones, and loudspeakers of varying designs. The DSP techniques include echo control, speech enhancement, and active noise cancellation. These techniques increase signal-to-noise ratios, expand transmitted bandwidth, and reduce the effects of room reverberation, echo, and other common audio problems.

165 ARTERIAL THROMBOLYSIS IN THE NEONATE BY RECOMBINANT TISSUE PLASMINOGEN ACTIVATOR (rTPA)

In two neonates, aortic thrombosis induced by arterial catheter was located in abdominal aorta extended to the right renal artery; one induced right leg cyanosis and the other was identified by echo control of aortic catheter. In the third neonate, left atrial thrombosis was induced by the tip of a wire, positioned in left atria during aortic valvuloplasty and showed by post catheterim 2D echo control. The fourth neonate showed asystole related to myocardial ischemia after flushing a catheter inadvertently positioned in ascending aorta. In this case rtPA was immediately infused into the catheter which was then withdrawn. The three other neonates received a bolus rtPA 0.1 mg/kg/10 min followed by 0.9 mg/kg/3 hours then heparin 100 Ul/kg/h.Aortic thrombosis were relieved and echo control showed total disappearance of aortic thrombus. Blood pressure and renal urography; control were normal two months latter. Lysis of left atrial thrombosis was successful after a second injection of rTPA after failure of the first dose. In the fourth case rtPA did not avoid infarct, cardiac hypokinetic state and heart failure. However three months later cardiac function and EKG were normal. Except in the third case where minor bleedings were present after the second dose, there was not adverse effect during rtPA therapy.This short series demonstrated a very safe and rapid efficient action of rtPA in the treatment of severe neonatal systemic thrombosis.

Perfect sequence excitation of the NLMS algorithm and its application to acoustic echo control

Adaptive echo cancellers are currently being studied for applications such as audio teleconference systems or hands-free telephone sets with high speech quality. The purpose of the echo control is to eliminate the acoustic feedback from the loudspeaker to the microphone. One problem of echo cancellers using e.g. the normalized least mean square algorithm(nlms) for the adaptation of the coefficients is that the convergence properties degrade with colored signal input such as speech signals [8, 9, 17]. One approach to accelerate the convergence speed is to introduce linear prediction filters in order to decorrelate the speech signal [1, 2, 12, 15, 20]. This paper presents a new approach, named the excited lms algorithm orelms algorithm, which prewhitens the input signal applying perfect sequences. Coincidently, the proposed algorithm can be interpreted as a combination of the conventionalnlms algorithm and a system identification approach using m-sequences or related sequences.

Enhancement of hands-free telecommunications

Hands-free operation is an important feature of many communication terminals such as telephones and conferencing systems. In general, the quality of speech from a hands-free terminal is degraded by ambient noise and by acoustic echo. This paper presents some of the activities in acoustic echo control of a cec Esprit consortium. A short description is given of a database collected specifically for testing acoustic echo cancellers and of a simple methodology for comparative evaluation of adaptive acoustic echo control methods. Eight methods have been investigated so far. Annotated bibliographies are given for each method and comparative performance results are presented.

Performance evaluation of acoustic echo control: required values and measurement procedures

We consider in this paper the general problem of performance evaluation of acoustic echo control, and (with less emphasis) of speech enhancement, as basic functions to be integrated in terminals like hands-free telephones, videophones, teleconference systems, etc, globally named hands-free audio terminals. Performance requirements are first reviewed under the light of desirable correspondence between objective characteristics and subjective quality. Some typical examples illustrate the discussion. Then the problem of performance verification is considered, with emphasis on test signals. Outlines for future directions of research on the topic of performance evaluation and verification are finally sketched.

Target surface texture discrimination by the echolocating bat, Eptesicus fuscus.

Echolocating bats perceive the details of their surroundings through an active bisonar system having an acuity which rivals vision in other mammals [AnimalSonar, ProcessesandPerformance, edited by P. F. Nachtigall and P. W. B. Moore (Plenum, NY, 1988)]. To explore one facet of their acoustic imaging, big brown bats (Eptesicusfuscus) were trained to discriminate between targets based upon their surface roughness. Pairs of cylinders (7.6 cm diam by 20 cm) were covered with different grades of gravel or sandpaper. Individual bats were able to discriminate targets having a difference in mean grain size of about 0.5 mm, in general agreement with earlier results from a different bat species (Megadermalyra). Big brown bats actively control the parameters of their short (1–5 ms) broadband (100–20 kHz) echolocation sounds to shape the acoustic information available to their bisonar systems. To monitor the bat’s vocal control of echoes, vocalizations were recorded and compared to the sounds used by the same animals in a simple detection task. Echoes from the targets using synthesized bat sounds were also recorded and analyzed to explore the acoustic cues which the bats may use to perform texture discrimination. [Work supported by the Whitehall Foundation and NSF.]

Performance analysis of two echo control designs in ATM networks

An ATM call interworking with the public switched telephone network may experience echo problems due to impedance mismatch. The authors propose two echo control designs to assure the quality of voice services by reducing round-trip echo delay. Performance of these two echo control designs is then analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Status of the standardization activities on acoustic echo control

The standardization work on acoustic echo control (and more generally on audio terminals) is essentially accomplished in CCITT and ETSI. In ETSI, the work on acoustic echo control is still in an early state; it is shared between Sub-Technical Committees TE4 and TM5 with narrow band hands-free telephony as a first objective. Both technical aspects and quality performance are considered. In CCITT, the work is shared essentially between Study Group XII and Study Group XV. The applications aimed at are hands-free telephony, videophony, teleconference, mobile radio hands-free telephones, hand-held portables. Study Group XV is elaborating a recommendation giving equipment performance specifications, without specifying the use of particular techniques for acoustic echo control. The contents of this document, intended to cover a wide range of applications, are presented in this paper.

Can the VLSI technology meet the needs of acoustic echo control system?

Since the introduction of hands-free telephone sets, acoustic echo cancellation has been a challenge for engineers. While in the past the problem was overridden using gain switching techniques, recently many authors have shown that the best solution for the echo compensation problem is represented by an adaptive filter which iteratively tries to identify the unknown impulse response of the system loudspeaker-room-microphone. Unfortunately the VLSI technology was not suitable to meet the requirements of this solution in a single chip acoustic echo canceller. The present paper, starting from a CMOS 1.2μm technology, will show some figures in terms of speed and silicon area taken for some basic blocks (such as adders, multipliers, memories) required by a canonical solution based on an adaptive FIR filter of 1000 taps with coefficients updated by the NLMS algorithm. Moreover, it will be shown a more efficient solution for the acoustic echo compensation based on a sub-bands approach. Simulated experiments with this architecture will be presented, compared with the ones of the canonical solution for what concerns size, number of operations required and performances.

Is there an indication for embryo reduction?

The selective reduction of embryos in multiple pregnancies poses numerous medical, technical, ethical and psycho-social problems. In a retrospective study, we analysed nine hundred and twenty-two pregnancies obtained using medically assisted procreation between May 1982 and May 1990. Among 922 successful pregnancies, 372 were singleton, 102 were twin and 13 were triplet. Data from this analysis and from a French multicentre study of 262 embryo reduction procedures demonstrated the value of an embryo quality score for minimizing the risk of multiple pregnancy and the existence of extremely infrequent, ethically acceptable indications for embryo reduction. These indications included ultrasound-proven malformations of one fetus, multiple pregnancies in patients with extensive uterine scarring, and multiple pregnancy despite the appropriate use of preventative measures which can be expected to make this technique unnecessary in the future.

Sonar gain control and echo detection thresholds in the echolocating bat, E p t e s i c u s f u s c u s

The echolocating bat, Eptesicus fuscus, detects sonar echoes with a sensitivity that changes according to the time elapsed between broadcasting of each sonar signal and reception of echoes. When tested in an electronic target simulator on a two-choice echo-detection task, the bat's threshold improved by 11.5 dB as echo delay changed from 2.3 to 4.6 ms (target ranges of 40 and 80 cm). Earlier experiments measured the change in detection threshold for delays from 1 to 6.4 ms (target ranges from about 17 to 110 cm) and obtained about 11 dB of improvement per doubling of delay. The new experiments used electronic delay lines to simulate echo delay, thus avoiding movement of loudspeakers to different distances and the possible creation of delay-dependent backward masking between stimulus echoes and cluttering echoes from the loudspeaker surfaces. The slope of the threshold shift defines an echo gain control that keeps echoes from point targets at a fixed sensation level--reducing sensitivity by 11 to 12 dB as echo amplitude increases by 12 dB per halving of range during the bat's approach to the target. A recent experiment using loudness discrimination of echoes at 70 to 80 dB SPL (roughly 50 dB above threshold) found a slope of about 6 dB per halving of range, so the gain-control effect may be level dependent. The observed effect is operationally equivalent to forward masking of echoes by the transmission, but any events correlated with vocalization which impair hearing sensitivity for a short interval following transmissions could cause a decline in sensitivity to echoes. Contractions of the bat's middle-ear muscles synchronized to transmissions may account for the observed threshold shift, at least for a span of echo delays associated with the most critical portion of the approach stage of pursuit. Forward masking by the sonar transmissions may contribute to the threshold shift, too, but middle-ear muscle contractions do occur and must be a significant part of the cause.

Report on an international workshop on acoustic echo control held in Berlin, Federal Republic of Germany on 14 and 15 September 1989

Report on an international workshop on acoustic echo control held in Berlin, Federal Republic of Germany on 14 and 15 September 1989

Echo control in telecommunications.

This paper reviews echo control techniques for telecommunications, emphasizing the principles and applications of both circuit and acoustic echo cancellers. First, echo generating mechanisms and echo problems are described for circuit and acoustic echoes. Circuit echo is caused by impedance mismatching in a hybrid coil. Acoustic echo is caused by acoustic coupling between loudspeakers and microphones in a room. The echo problem is severe when the round-trip propagation delay is long. In this case, the echo must be removed. Next, the basic principle of the echo canceller, adaptive filter structure and adaptive algorithm are discussed. Emphasis is focused on the constructionand operation of an adaptive transversal filter using the NLMS (Normalized Least Mean Square) algorithm, which is the most popular for the echo canceller. Then, applications of circuit and acoustic echo cancellers are described. Circuit echo cancellers have been well studied and implemented in LSIs for many applications. Although acoustic echo cancellers have been introduced into audio teleconference systems, they still have some problems which must be solved. Therefore, they are now being studied intensely. Finally, this paper mentions the problems of echo cancellers and the direc tionof future work on them. The main targets for acoustic echo cancellers are improvingthe convergence speed, reducing the amount of hardware and bettering the double-talk control technique.

小特集 最近の音響技術 ２ 最近の電話の音響技術 エコー制御技術

小特集 最近の音響技術 ２ 最近の電話の音響技術 エコー制御技術

Microphones and feedback suppression in teleconferencing

The effect of room acoustics on microphone placement and selection for teleconferencing systems and current approaches to electroacoustic feedback and echo control in teleconferencing systems will be discussed.

An acoustic echo canceler implemented with AT&amp;T DSP16A

Acoustic echo control in teleconferencing and hand-free phone systems is not trivial because of its special characteristics. Many acoustic cancellation algorithms have been published in recent years but they are either of theoretical consideration or are computer simulations with complicated computation burdens, which makes real-time implementation impossible. In this paper, a two-band scheme is proposed to realize an acoustic echo canceler with an AT&amp;T WE DSP16A microprocessor. The structure of the algorithm is optimized so it can give an echo return loss enhancement (ERLE) of more than 25 dB. A double talk detector is developed with the subband adaptive algorithm so that it will ensure full duplex communication. Performance of this canceler is evaluated with both computer simulation results and real-time test results.

Teleconferencing—The acoustical environment and directional transducers

The performance of teleconferencing systems are largely a function of the acoustical environment, the type, and the location of loudspeakers and microphones. Acoustical treatment in most conference rooms is limited to carpeted floors and an acoustical ceiling. For the most part, present microphones are designed to work best under close talking conditions and have omnidirectional, or first-order unidirectional characteristics. Two small experimental conference rooms have been constructed in which the effects of room parameters such as reverberation time, noise level, and discrete echo control can be independently studied. Broadband (8-kHz) directional microphones and steerable directional arrays, mostly of second order, are being investigated for a variety of acoustical environmental conditions. The easily adjustable conference rooms allow us to investigate these microphone systems with respect to their directional properties, placement, and steering algorithms. Loudspeaker directional characteristics, and advantages of stereo transmission are also under investigation. The experimental conference rooms and new directional microphones will be described and a discussion of trade-offs between major parameters for speech reception and transmission will be given.

All-digital network transmission planning and terminal requirements

The development of a proposed transmission plan for an all-digital network is described. The main objectives of the plan are to obtain optimum end-to-end loudness grade-of-service, bit transparency, and echo control on all-digital connections and to ensure adequate transmission performance on hybrid connections, i.e. mixed analog/digital. These objectives are met by migrating connection loss, traditionally applied in the network, to the analog side of the digital/analog interface in the digital terminal. Thus the network transmission plan and digital access terminal characteristics are strongly interrelated. The proposed plan provides additional advantages including elimination of switched losses in the network when digital terminals are used and easy interconnection between networks (public and private).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Echo control for the speech communication systems

Echo control for the speech communication systems