Avaya Audio Quality Terminology User's Manual
Contents
1. distinguish between hot sidetone see definition and short path echo 2005 Avaya Inc All Rights Reserved Page 62. AVAYA Audio Quality Terminology ABSTRACT The terms described herein relate to audio quality artifacts The intent of this document is to ensure Avaya customers business partners and services teams engage in effective communication involving audio quality related issues 2005 Avaya Inc All Rights Reserved 1 Introduction This document defines a variety of terms used to describe voice related artifacts experienced in telephony It is expected that this terminology will be used primarily by Avaya business partners and Avaya Global Services teams to facilitate the interpretation and understanding of voice related problems experienced in the field 2 Audio processing components and terminology In a typical telephony call speech from talker to listener often passes through the following processing components and in the order identified in Figure 1 speaker Echo controller Automatic Gain control Expander noise reduction Speech _ PEE TET EETA Speech _ Packet loss encode decode conealment Figure 1 Components of the end to end speech path The upper path is identical to the lower path but reversed in order The network could be TDM packet VoIP or a combination of the two The talker s voice enters at the microphone on the left side of Figure 1 then to the microphone expander voice coder network transport voice decoder packet loss concealment ech
3. ainst 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 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 Hot sidetone in a handset or headset microphone to earpiece sidetone injection is not normally noticed Some digital phones in p
4. articular 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 Short path acoustic echo short path electrical echo acoustic or electrical echo that occurs in a very short roundtrip call topology This type of echo is commonly described as a hollow sound or sound of speaking in a barrel see 2 1 3 In a digital to digital phone call think DCP to DCP station to station the roundtrip delay is usually very small less than 10 ms Some digital speakerphones produce significant acoustic echo which is not canceled suppressed or otherwise controlled in this simple call topology In these cases and depending on the volume setting of the far party s speakerphone and near party s listening handset the near party may perceive echo and refer to this as hot sidetone Again this is truly acoustic echo from the speakerphone but is returned to the talker with such a short roundtrip delay that it is perceived as hollowness or reverberance rather than as classic echo Because of the short roundtrip delay in this case it can be difficult to
5. ase when there is network echo e g in a teleconference without echo control 2005 Avaya Inc All Rights Reserved Page 3 3 1 9 3 2 3 2 4 Synthetic Mechanical or Robotic Voice this can be very subtle or very severe or very consistent or intermittent In the most severe case the pitch information has been lost making the speech sound monotonic and robotic Recognizing who is speaking is often difficult Amplitude clipping see definition for distortion above Clipping portions of the speech signal are not heard This can occur in packet switched networks when for example large numbers of successive speech packets are not received because of excessive network congestion Common in wireless phones where the RF signal strength fades as the user moves within the environment Clipping during double talk clipping as defined above but heard only when both parties of a telephone call talk at the same time When it occurs this effect is almost always caused by the excessive use of echo suppression see definition at some point within the network In this case clipping of speech utterances is not caused by lost speech packets or in the case of wireless phones RF fades though those artifacts may also be present in the same call Stutter this is often used to describe an effect caused by repetition of short bursts of noise or speech such as da da da da or fa fa fa fa Stutter distortion can occur in packet s
6. ause of network failures In such cases a concealment algorithm attempts to fill in missing speech samples Concealment can work well when the rate of lost speech is very low say less than 2 of transmissions Automatic gain control automatic gain control devices apply signal gain or loss automatically in an attempt to keep the speech sound level at the listener s ear relatively constant Therefore AGC boosts low level speech while reducing speech levels that are too loud Such devices have been used for decades in audio broadcasting and recording applications Terminology for voice related artifacts Speech distortions Distorted speech speech accompanied by an unnatural buzzing or raspy sound A classic example of distortion occurs in the case of a far party who is speaking too loudly or too close to the handset or headset microphone The far party s speech saturates either the mechanical or electrical capabilities of the handset causing overload distortion or amplitude clipping Muffled speech speech that has an unnatural loss of high frequency content Muffled speech may be caused by for example poorly designed microphone assemblies in handsets in particular wireless handsets and low bit rate speech coders Reverberant speech also hollowness or speaking in a tunnel effect sounds like the person speaking is in a barrel or large empty room This can be the case when the talker is using a speakerphone but it can also be the c
7. causes other audio artifacts including chopping or clipping of speech utterances and or pumping of the loudness level of a caller s speech Microphone expander and or noise reduction a microphone expander is a traditional and relatively simple method of improving the speech signal to background noise ratio emanating from the microphone path An expander attenuates weak room background noises while passing unaltered the relatively loud speech of the talker addressing the handset or headset or speakerphone Speech coder encoder and decoder the raw speech signal once digitized is often digitally encoded for transmission into the telephone network Encoding has one purpose namely to reduce the bits per second rate of transmission required to communicate voice from one end to the other Highly compressive codecs such as a G 729 codec reduce speech to a low transmission rate 8000 bits per second but sacrifice voice quality in doing so Higher voice quality is experienced in systems using the traditional G 711 codec mu law codec since G 711 s higher transmission rate of 64 000 bits per second better captures the nuances of speech Regardless of coding scheme at the receiving side the speech decoder reconstructs an approximation to the original speech for playback Packet loss concealment often combined with speech decoders When the network path includes packet speech transmission links like VoIP speech packets can be lost bec
8. o controller automatic gain control and finally the listener s ear 2 1 Audio Processing Components 2 1 1 Echo controller broad term meaning an echo canceler echo suppressor or a combination of the two Speakerphone algorithms are also included An echo controller prevents a talker from hearing distant reflections echoes of his her own voice reflections caused by acoustic or electrical reflection points within the telephone network and end user equipment Echo controllers are often only partially successful and this is why echo is sometimes heard even though the call path is known to include echo controllers Often people use the term echo canceler when in fact what is being referred to is an echo controller 2 1 2 Echo canceller a software or hardware implementation of a digital signal processing algorithm designed to model and subtract out or cancel the reflection or echo of a speech signal Strictly speaking an echo canceler does not introduce attenuation or suppression into the speech paths to reduce the loudness of echo The term canceler refers to an adaptive digital filter that models the physical echo path and subtracts that excited model from the return speech path 2005 Avaya Inc All Rights Reserved Page 2 3 1 3 1 1 Echo suppressor like echo canceler above except the echo level is reduced or eliminated by applying suppression or attenuation to the return speech channel The use of attenuation
9. o 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 perce
10. or Dialtone low bit rate codecs such as G 729 and G 723 were created to efficiently encode and transport speech but not music or other non speech signal such as tones Thus the usage of these and other codecs may distort 2005 Avaya Inc All Rights Reserved Page 4 and ruin the music signal or non speech signal This can be subtle or severe depending on the music source 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 ech
11. peech networks when one or more network elements e g router or switch become a bottleneck to the timely transmission of speech packets Speech level pumping pumping is often used to describe a varying speech loudness level that is were the speech gets louder softer then louder again etc over the course of a call often over a period of just several seconds Automatic gain control devices can cause audible and distracting pumping Noise and Other Phenomena Hiss or white noise relatively natural sounding noise containing energy at all frequencies Low level idle channel hiss noise can be perceived on nearly every telephone call when no person is speaking Static impulsive ticking noise similar to the sound of an AM radio when tuned to a very weak or nonexistent radio station In a packet speech network can be caused by lost speech packets and or bit errors May also be used to describe power line hum see definition below Motor boating repetitive noise that is separate and distinct from the talker s voice Motor boat noise differs from static in that it is repetitive or non random Hum sounds like humming as in Hmmmmm Hum noise often occurs when a source of 50 Hz or 60 Hz electrical power is located near a telephone The power source emits an RF radio frequency field that induces a hum like noise that is heard through the phone s handset headset earpiece or speakerphone loudspeaker Distorted Music on Hold
12. ption 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 buzz like 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 2005 Avaya Inc All Rights Reserved Page 5 3 3 9 3 3 10 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 ag
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