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Old 07-02-2013, 01:39 PM
pdgavin pdgavin is offline
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3.3 Echo
There are only two physical sources of echo in telephony: electrical echo (or network echo), and
acoustic echo. Electrical echo is caused by a reflection of the speech signal at 2-to-4-wire hybrid
circuitry. This circuitry is present in analog trunk cards, and it also exists deep within the PSTN
(at customer premises, for example). Acoustic echo is caused by the physical coupling (air path,
appliance-body path) between a loudspeaker and a microphone, for example, in a
speakerphone, a handset and a headset. Whether or not a talker actually perceives electrical or
acoustic echo depends on the loudness of his/her reflected voice signal and the roundtrip delay
that that reflection suffers. The loudness of the reflection at the point of reflection depends upon
the electrical impedance mismatch, for electrical echoes, and the acoustic gain of the
loudspeaker-to-microphone path, for acoustic echoes. The roundtrip delay is a function of the
path the reflected signal traverses, which in turn is a function of the call topology.
3.3.1 Electrical echo, also called network echo: reflection of a talker's speech signal at a
point of 2-to-4-wire conversion caused by an impedance mismatch at the point of
analog-to-digital conversion.
3.3.2 Acoustic echo: reflection of a talker's speech signal at an acoustic endpoint caused by
the acoustic coupling between the loudspeaker and microphone.
3.3.3 Constant echo: when talking, the perception of echo with every utterance. Such cases
occur when there is a physical electrical or acoustic echo path but no echo controller in
the call topology to control echo. Additionally, constant echo may result even though an
echo controller is known to be in the call path; this indicates a complete failure of the
echo controller, usually because the capabilities of the echo controller are exceeded
(e.g., the echo tail length exceeds the specifications of the echo controller).
3.3.4 Intermittent echo: when talking, the occasional perception of echo. Intermittent echo
often caused by the intermittent failure of an echo controller in the call path. The echo
suppressor within the echo controller may fail to engage (to apply echo attenuation)
when necessary, with the result that short bursts of echo become audible. In acoustic
echo control applications (speakerphone) in which people or objects close to the
speakerphone are moving, the change to the physical echo path often results in audible
intermittent acoustic echo to listeners at the other end of the call.
3.3.5 Residual echo: when talking, the perception of very low-level (quiet) echo. The echo
could be either constant or intermittent. Residual echo can be caused by PSTN
electrical echo that is not entirely removed by the echo controller in the call path.
3.3.6 Distorted or buzz-like echo: when talking the perception of a distorted echo or buzzlike
sound. This can be caused by a non-linear echo source. An example of this is
saturation distortion at an analog trunk interface. In this case, signals low in amplitude
are reflected cleanly, but signals high in amplitude are returned with significant distortion
making it difficult for an echo canceler to control echo. Such distorted echo can be
perceived constantly or intermittently, depending on the degree of distortion and the
echo canceler(s) involved.
3.3.7 Slapback or kickback acoustic echo: this is strictly a phenomenon of acoustic echo.
With speakerphones, slapback or kickback echo is the intermittent echo perceived at the
ends of one's utterances. This can occur with both older-model half-duplex
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speakerphones and newer-model acoustic-echo canceling speakerphones. For
example, a talker speaking into a handset utters the phrase “Please send me the check”
and perceives echo primarily at the end of his/her sentence. This echo is described as
hearing just the sound “eck” or “k” of the word “check,” or as a slapping sound such as
that made by slapping one’s palm against a desktop. Commonly, slapback/kickback
echo is caused by acoustically reverberant rooms. Large offices and conference rooms
can have long reverberation times. In such rooms, the speakerphone senses at its
microphone a reverberated version of the word “check” (our prior example) several tens
or even hundreds of milliseconds after the far talker has finished saying the word
“check.” The speakerphone algorithm detects this reverberated speech at its
microphone, detects no speech at its receive-path driving the loudspeaker, and decides
to transition to transmit mode. The reverberated version of "check" is transmitted back to
the far talker, where it is perceived as echo.
3.3.8 Sidetone: in handsets and headsets, a portion of the microphone energy is fed back to
the earpiece so that the user of the handset/headset does has a psychoacoustic
experience that simulates the case in which the user's ear is not occluded by an object
(the handset earpiece). Without sidetone injection, the user experiences the
psychoacoustically bothersome condition that can be demonstrated to oneself by
pressing a finger into one ear while speaking. With one ear occluded, the sound of one’s
own voice is dominated by the path through the interior of the head (skull, etc.) instead
of around the head, an effect that most people find objectionable.
3.3.9 Hot sidetone: in a handset or headset, microphone-to-earpiece sidetone injection is not
normally noticed. Some digital phones, in particular, IP phones in which the internal
audio processing frame rate is 5 ms or greater, inject sidetone with an appreciable delay
(e.g., 5 ms) in the microphone-to-earpiece signal path. This delay causes the sidetone to
sound reverberant and/or louder than normal, or hot. Though hot sidetone is a type of
echo source – because some people may use the term “echo” to describe hot sidetone
– it is generated local to the telephone, not at some point within the telephone network.

Pete G
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