A fundamental feature of professional VTC systems is acoustic echo cancellation (AEC). Echo can be defined as the reflected source wave interference with new wave created by source. AEC is an algorithm which is able to detect when sounds or utterances reenter the audio input of the VTC codec, which came from the audio output of the same system, after some time delay. If unchecked, this can lead to several problems including 1) the remote party hearing their own voice coming back at them (usually significantly delayed) 2) strong reverberation, rendering the voice channel useless as it becomes hard to understand and 3) howling created by feedback. Echo cancellation is a processor-intensive task that usually works over a narrow range of sound delays.
The term echo cancellation is used in telephony to describe the process of removing echo from a voice communication in order to improve voice quality on a telephone call. In addition to improving subjective quality, this process increases the capacity achieved through silence suppression by preventing echo from traveling across a network.
Two sources of echo have primary relevance in telephony: acoustic echo and hybrid echo. Speech compression techniques and digital processing delay often make these echoes more severe in telephone networks.
Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted or received signal. Once the echo is recognized, it can be removed by ’subtracting’ it from the transmitted or received signal. This technique is generally implemented using a digital signal processor (DSP), but can also be implemented in software. Echo cancellation is done using either echo suppressors or echo cancellers, or in some cases both.
In mathematics, computing, linguistics, and related subjects, an algorithm is an effective method for solving a problem using a finite sequence of instructions. Algorithms are used for calculation, data processing, and many other fields.
Each algorithm is a list of well-defined instructions for completing a task. Starting from an initial state, the instructions describe a computation that proceeds through a well-defined series of successive states, eventually terminating in a final ending state. The transition from one state to the next is not necessarily deterministic; some algorithms, known as randomized algorithms, incorporate randomness.
A partial formalization of the concept began with attempts to solve the Entscheidungsproblem (the “decision problem”) posed by David Hilbert in 1928. Subsequent formalizations were framed as attempts to define “effective calculability”[1] or “effective method”[2]; those formalizations included the Gödel-Herbrand-Kleene recursive functions of 1930, 1934 and 1935, Alonzo Church’s lambda calculus of 1936, Emil Post’s “Formulation 1″ of 1936, and Alan Turing’s Turing machines of 1936–7 and 1939.
In radio and television, broadcast delay refers to the practice of intentionally delaying broadcast of live material. A short delay is often used to prevent profanity or other undesirable material from making it to air, including more mundane problems such as technical malfunctions or coughing. In this instance, it is often referred to as a seven-second delay or profanity delay.
Longer delays can also be introduced, often to allow a show to air at the same time for the local market as is sometimes done with nationally-broadcast programs in countries with multiple time zones. That can sometimes be simply achieved with a video tape recorder or similar technology. In the context of modern digital video recorders, this can now be considered a class of time shifting. In this instance, it is often called a tape delay or west-coast delay in the United States, so-called because special events (including award shows) broadcast live in the rest of the US are often tape-delayed on the west coast. This includes Southern California (with the exception of the Academy Awards, which airs live in Los Angeles and the rest of the west coast), despite the fact that Southern California is where many live televised events (American Idol, Dancing With The Stars), take place.
Tape delay also refers to the process of broadcasting an event at a later scheduled time. This is because either a scheduling conflict prevents a live telecast, or a broadcaster seeks to maximize ratings by airing an event in a certain timeslot. A prominent example of the latter is coverage of the Olympic Games, when a broadcaster televises a high-profile event to air in prime time, even though that event is being shown at the “wrong time” because it actually took place when fewer people are likely to be at the TV. Sporting events aired on tape delay are often edited down for time considerations, highlighting the most interesting portions of the event.

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