E1 / T1 and STM1 / OC3 Echo Canceller Systems
The Importance of Echo Cancellation
In the current deregulated market, voice quality is a key competitive differentiator. One of the primary challenges faced by service providers in delivering quality voice communication is the elimination of echo. Echo results from signal reflection at the "hybrid," commonly the point where the two wires of the local network meet the four wires of the long distance network. The hybrid is not completely efficient in carrying the electrical energy from the four-wire network to the two-wire network, and a certain amount of the electrical energy or voice signal is reflected back from the hybrid towards the speaker. Echo is present whenever the one-time delay of a rebounded voice signal exceeds 16 milliseconds. If the delay exceeds 32 milliseconds, the quality of the voice call begins to degrade and an echo is reflected back to the speaker.
Delays, either due to a long transmission path, as in a long distance telephone call, or due to the complex signal hand-off from one network to another, for instance wireless to wireline, exacerbate the effect of echo. Therefore, most long distance, digital wireless and satellite voice calls require echo cancellation.
Delays are also introduced as intelligent processing equipment is increasingly incorporated into networks. Digital processing of voice signals requires time to compress, decompress and route signals through networks. As there is a greater shift towards voice-over packet, which is a means of packaging voice signals into units of digital data for efficient transmission, these processing delays will continue to increase. Voice-over Internet, voice-over ATM and voice-over Frame Relay are examples of voice-over packet networks.
Service providers deploy echo cancellation technology to address these delays. Put simply, an echo canceller samples the voice signal, estimates the echo and subtracts most of it away, leaving the original speech. Typically, a linear digital signal processor (DSP) models the echo and performs the main echo cancellation task. A non-linear DSP processor is employed to subtract out any residual echo or acoustic noise.
VQoS - Multi-Dimensional Echo Cancellation
Wireless and voice-over-packet technologies introduce extended delay into the network. As deregulation accelerates the build-out of this new global telecommunications infrastructure, it becomes difficult to predict where and when voice quality problems will arise. Where the classic echo canceller solved only one key voice quality problem: hybrid echo, Voice Quality of Service (VQoS) addresses voice quality issues and transforms the one-dimensional echo canceller into a multi-faceted programmable, voice quality assurance device.
Ditech's Voice Quality of Service (VQoS)™ technology platform is a suite of software programs embedded in the firmware of Ditech's echo cancellers. VQoS redefines the classic, one-dimensional echo canceller product. Where the classic echo canceller solved only hybrid echo, VQoS is a suite of software programs that solves several key problems of voice quality, including acoustic echo control, background noise control, volume control, and voice quality monitoring.
Telephone service providers struggle to provide quality voice calls while trying to handle more complex call routing caused by worldwide deregulation, and incorporating new varieties of wireless and voice-over-packet communications. VQoS enables service providers to future proof their networks for voice quality, as the VQoS functionality can be activated as various voice quality issues arise.
Today's service providers must offer quality voice communication over a shifting matrix of wireline, wireless, satellite networks and an emerging voice-over-packet network. As wireless and voice-over-packet technologies introduce extended delay into the network, voice quality can quickly deteriorate. And as deregulation accelerates the build-out of this new global telecommunications infrastructure, it becomes more difficult to predict where and when a voice quality problem will arise. VQoS addresses this unknowable future by transforming the echo canceller into a programmable, voice quality assurance device. VQoS features are explained in the following sections.
Echo Control
Acoustic Echo Control (AEC) In digital wireless networks, an acoustical echo is generated in the handset of the mobile user such that the person speaking to the mobile user hears an echo of his or her own voice. When enabled, AEC detects acoustical echo and renders it inaudible.
Far End Echo Cancellation (FEEC) The long transmission distances that are involved in satellite communications introduce severe echo problems into voice calls. As a result, service providers spend substantial time and expense to deploy echo cancellers in land-based relay stations in the countries where the calls are initiated or received. When enabled, FEEC allows satellite and international service providers to deploy very powerful echo cancellation in their major relay stations located in the United States and Europe, eliminating the need to deploy echo cancellers in remote areas of the globe.
Noise Control
Advanced Noise Control (ANC) When making a call in a wireless network, the mobile user's handset microphone often picks up background noise that can severely degrade voice quality. When enabled, NR detects background noise and eliminates it.
Voice Level
Automatic Level Control (ALC) When voice calls are routed between the networks of different service providers, particularly on international calls, the voice of the speaker is often too high or too low for comfortable listening. When enabled, ALC detects high or low voice levels and then automatically adds or subtracts voice energy to return the voice call to an ideal range.
High Level Compensation (HLC) HLC performs very much like ALC does when ALC is lowering high voice levels. HLC is another technical means of reducing high voice levels to an ideal range.
Fixed Gain Fixed gain detects high or low voice levels and compensates to return the voice level to an ideal range. As the name suggests, "fixed gain" raises or lowers the voice level by a pre-defined amount. Fixed gain might be preferred over ALC in certain situations where the voice level problem is consistent and well-defined.
Monitoring
Reflectometry To ensure voice quality, service providers must estimate the strength of the echo signal and the amount of delay introduced into the voice call. Reflectometry eliminates the guesswork on this critical voice quality issue. Ditech's reflectometry feature enables service providers to conduct real-time monitoring of the echo signal and the amount of delay on the circuit.
Telephone service providers are struggling to provide quality voice calls while trying to handle more complex call routing caused by worldwide deregulation, and incorporating new varieties of wireless and voice-over-packet communications. VQoS enables service providers to future proof their networks for voice quality, as the VQoS functionality can be activated as various voice quality issues arise.
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