Echo suppression and cancellation

In telephony, echo is the reflected copy of one's voice heard some time later. If the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. If the delay is very small (tens of milliseconds or less), the phenomenon is called sidetone. If the delay is slightly longer, around 50 milliseconds, humans cannot hear the echo as a distinct sound, but instead hear a chorus effect. which then resulted in laboratory echo cancelers in the late 1960s and commercial echo cancelers in the 1980s. An echo canceller works by generating an estimate of the echo from the talker's signal, and subtracts that estimate from the return path. This technique requires an adaptive filter to generate a signal accurate enough to effectively cancel the echo, where the echo can differ from the original due to various kinds of degradation along the way. Since invention at AT&T Bell Labs echo cancellation algorithms have been improved and honed. Like all echo cancelling processes, these first algorithms were designed to anticipate the signal which would inevitably re-enter the transmission path, and cancel it out. Rapid advances in digital signal processing allowed echo cancellers to be made smaller and more cost-effective. In the 1990s, echo cancellers were implemented within voice switches for the first time (in the Northern Telecom DMS-250) rather than as standalone devices. The integration of echo cancellation directly into the switch meant that echo cancellers could be reliably turned on or off on a call-by-call basis, removing the need for separate trunk groups for voice and data calls. Today's telephony technology often employs echo cancellers in small or handheld communications devices via a software voice engine, which provides cancellation of either acoustic echo or the residual echo introduced by a far-end PSTN gateway system; such systems typically cancel echo reflections with up to 64 milliseconds delay. ==Operation==

, is to route incoming speech from the far end xk to the local telephone and route speech from the telephone to the far end. However, the hybrid is never perfect, so its output dk contains both the desired speech from the local telephone plus filtered speech from the far end. The echo canceller is the adaptive filter fk, which attempts to minimize the error signal εk by filtering the incoming far end speech into a replica yk of the far end speech that leaks through the hybrid. Once the adaption is complete, the error signal consists mostly of speech from the local telephone. The echo cancellation process works as follows: • A far-end signal is delivered to the system. • The far-end signal is reproduced. • The far-end signal is filtered and delayed to resemble the near-end signal. • The filtered far-end signal is subtracted from the near-end signal. • The resultant signal represents sounds present in the room excluding any direct or reverberated sound. The primary challenge for an echo canceller is determining the response characteristics of the filter to be applied to the far-end signal such that it resembles the resultant near-end echo. The filter is essentially a model of speaker, microphone and the room's acoustical attributes. Echo cancellers must be adaptive because the characteristics of the near-end's speaker and microphone are generally not known in advance. The acoustical attributes of the near-end's room are also not generally known in advance, and may change (e.g., if the microphone is moved relative to the speaker, or if individuals walk around the room causing changes in the acoustic reflections). By using the far-end signal as the stimulus, modern systems use an adaptive filter and can converge from providing no cancellation to 55 dB of cancellation in around 200 ms. Echo cancellation alone may be insufficient in many applications. Echo cancellation and suppression can work in conjunction to achieve acceptable performance. Quantifying echo Echo is measured as '''''' (ERL). This is the ratio, expressed in decibels, of the original signal and its echo. High values mean the echo is very weak, while low values mean the echo is very strong. Negative indicate the echo is stronger than the original signal, which if left unchecked would cause audio feedback. The performance of an echo canceller is measured in echo return loss enhancement (ERLE), which is the amount of additional signal loss applied by the echo canceller. Most echo cancellers are able to apply 18 to 35 dB ERLE. The total signal loss of the echo (ACOM) is the sum of the ERL and ERLE. == Current uses ==

Sources of echo are found in everyday surroundings such as: • Hands-free car phone systems • A standard telephone or cellphone in speakerphone mode • Dedicated standalone speakerphones • Installed conference room systems which use ceiling speakers and microphones on the table • Physical coupling where vibrations of the loudspeaker transfer to the microphone via the handset casing In some of these cases, sound from the loudspeaker enters the microphone almost unaltered. The difficulties in canceling echo stem from the alteration of the original sound by the ambient space. These changes can include certain frequencies being absorbed by soft furnishings and reflection of different frequencies at varying strength. Implementing AEC requires engineering expertise and a fast processor, usually in the form of a digital signal processor (DSP), this cost in processing capability may come at a premium, however, many embedded systems do have a fully functional AEC. Smart speakers and interactive voice response systems that accept speech for input use AEC while speech prompts are played to prevent the system's own speech recognition from falsely recognizing the echoed prompts and other output. Modems Standard telephone lines use the same pair of wires to both send and receive audio, which results in a small amount of the outgoing signal being reflected back. This is useful for people talking on the phone, as it provides a signal to the speaker that their voice is making it through the system. However, this reflected signal causes problems for a modem, which is unable to distinguish between a signal from the remote modem and the echo of its own signal. For this reason, earlier dial-up modems split the signal frequencies, so that the devices on either end used different tones, allowing each one to ignore any signals in the frequency range it was using for transmission. However, this diminished the amount of bandwidth available to both sides. Echo cancellation mitigated this problem. During the call setup and negotiation period, both modems send a series of unique tones and then listen for them to return through the phone system. They measure the total delay time, then configure a delay line for that same period. Once the connection is completed, they send their signals into the phone lines as normal, but also into the delay line. When their signal is reflected back, it is mixed with the inverted signal from the delay line, which cancels out the echo. This allowed both modems to use the full spectrum available, doubling the possible speed. Echo cancellation is also applied by many telcos to the line itself and can cause data corruption rather than improving the signal. Some telephone switches or converters (such as analog terminal adapters) disable echo suppression or echo cancellation when they detect the 2100 or 2225 Hz answer tones associated with such calls, in accordance with ITU-T recommendation G.164 or G.165. ISDN and DSL modems operating at frequencies above the voice band over standard twisted-pair telephone wires also make use of automated echo cancellation to allow simultaneous bidirectional data communication. The computational complexity in implementing the adaptive filter is much reduced compared to voice echo cancelling because the transmit signal is a digital bit stream. Instead of a multiplication and an addition operation for every tap in the filter, only the addition is required. A RAM lookup table based echo cancelling scheme eliminates even the addition operation by simply addressing a memory with a truncated transmit bit stream to obtain the echo estimate. Echo cancellation is now commonly implemented with Digital Signal Processor (DSP) techniques. Some modems use separate incoming and outgoing frequencies or allocate separate time slots for transmitting and receiving to eliminate the need for echo cancellation. Higher frequencies beyond the original design limits of telephone cables suffer significant attenuation distortion due to bridge taps and incomplete impedance matching. Deep, narrow frequency gaps which cannot be remedied by echo cancellation often result. These are detected and mapped out during connection negotiation. ==See also==