Voice Quality Troubleshooting Guide
Customer Premise Equipment (CPE) The telephone equipment on the customer site. On a Mitel customer
site, the MiVoice Business system is the CPE. Incoming calls that
arrive at the MiVoice Business system over the PSTN typically
originate at CPE equipment located on the other side of the PSTN.
Digital Service Processor (DSP) A specialized microprocessor that optimizes the processing of signals
such as audio or video.
Double Talk Occurs when more than one speaker is talking at the same time.
Echo (acoustic) When acoustic echo is present on a call, one or both parties hear what
they said tens or hundreds of milliseconds later. acoustic echo
originates at the phone.
Acoustic echo is caused by sound from the speaker being reflected
back into the handset microphone or by a user’s voice being reflected
off nearby surfaces and picked up by the microphone.
If the round-trip delay of the reflection is less than 50 ms, these
reflections result in sidetone (see “Sidetone” on page 7). However, if
the round-trip delay is greater than 50 ms, these reflections can result
in echo. Echo will be heard by the far-end user if the echo suppressor
or canceller in the phone fails to remove all acoustic or mechanical
reflection, or if the required echo suppressor or echo canceller is not
present.
Echo (electrical) When electrical echois present on acall, oneorboth parties hearwhat
they said a few tens or hundreds of milliseconds later. The reflections
that cause electrical echo originate at line hybrids. These reflections
cause echo if the round trip delay is long (typically greater than 50 ms)
and the echo is not removed by an echo canceller.
Echo (line) Line echo is another term for electrical echo.
Echo cancellers A device that removes echo from a signal by adaptively predicting the
echo signal and then subtracting the predicted echo from the signal
Since the mathematical model used to predict the amount of echo to
be removed is not an exact model and since there are always some
non-linearities in the echo path, not all echo will be removed. An echo
canceller normally includes an echo suppressor, also called a
non-linear processor (NLP), to remove this residual echo.
Echo suppressor A device, often called a non-linear processor, that detects the
presenceofecho andblocksor suppressesthesignalthatcontainsthe
echo.As aside effect,echo suppressioncan potentially blocka portion
of the valid signal. An echo suppressor is effective at removing low
levels of echo without a significant effect on audio quality.
Handset Audio Tap A device for recording the audio signal that is heard by a phone
listener. You can use the recording to check for voice quality issues,
such as residual echo in an echo canceller or acoustic echo from a
handset.
Hub A hardware device that connects multiple twisted pair or fiber optic
Ethernet devices together and makes them act as a single network
segment. Hubs work at the physical layer (layer 1) of the OSI model.
MiVoice IP Phones should not be connected to the network through
hubs.
Table 1: Voice Quality Terminology
TERM DEFINITION
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