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Echo Analysis for Voice over IP
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Echo Analysis for Voice over IP
Echo Analysis for Voice over IP
Version History
Echo Analysis for Voice over IP defines echo and describes where it occurs in a voice network. It
examines the basic aspects of echo analysis and describes the effects of various network elements on
echo. This document also explains how echo is measured and how echo cancelers work to estimate and
eliminate echo. It looks at customer and service provider expectations about echo, and explains how to
configure gateways to minimize echo. It points out that the normal characteristics of packet-based
networks might unmask preexisting problems in the time-division multiplexing (TDM)-based voice
infrastructure. Finally, it outlines a process for locating and eliminating loud echos and long echos, and
concludes with a real-life case study involving PBX-based echo in an international voice network.
This document contains the following sections: Echo Analysis Overview, page 1 Echo Analysis Basics, page 2 Effects of Network Elements on Echo, page 6 Echo Canceler, page 11 Customer Expectations About Echo, page 18 Service Provider Expectations About Echo, page 18 Configuring Gateways to Minimize Echo, page 18 Process for Locating and Eliminating Echos, page 20 Echo Analysis Case Study, page 22 Related Documents, page 27
Echo Analysis Overview
In a voice telephone call, an echo occurs when you hear your own voice repeated. An echo is the audible
leak-through of your own voice into your own receive (return) path.
Version Number
Date
Notes
1
June 22, 2001
This document was created.
2
November 30, 2002 Added information on loudness. Echo Analysis Basics
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Echo Analysis for Voice over IP
This document discusses basic concepts applicable to echo analysis, explains echo cancellation, and
provides a process for locating and eliminating echos.
Echo Analysis Basics
Every voice conversation has at least two participants. From the perspective of each participant, there
are two voice paths in every call: Transmit pathThe transmit path is also called the send or Tx path. In a conversation, the transmit
path is created when a person speaks. The sound is transmitted from the mouth of the speaker to the
ear of the listener. Receive pathThe receive path is also called the return or Rx path. In a conversation, the receive
path is created when a person hears the conversation. The sound is received by the ear of the listener
from the mouth of the speaker.
Figure 1
shows a diagram of a simple voice call between Bob and Alice. From Bob's perspective, the
transmit path carries his voice to Alice's ear, and the receive path carries Alice's voice to his ear.
Naturally, from Alice's side these paths have the opposite naming convention: the transmit path carries
her voice to Bob's ear, and the receive path carries Bob's voice to her ear.
Figure 1
Simple Telephone Call
As mentioned, an echo is the audible leak-through of your own voice into your own receive (return) path.
Figure 2
shows the same simple telephone call where Bob hears an echo.
Figure 2
Simple Telephone Call with an Echo
Bob hears a delayed and somewhat attenuated version of his own voice in the earpiece of his handset.
Initially, the source and mechanism of the leak are undefined.
One significant factor in echo analysis is the round-trip delay of the voice network. The round-trip delay
of the network is the length of time required for an utterance to go from Bob's mouth, across the network
on the transmit path to the source of the leak, and then back across the network again on the receive path
to Bob's ear.
Two basic characteristics of echo are as follows: The louder the echo (echo amplitude), the more annoying it is. The longer the round-trip delay (the later the echo), the more annoying it is.
Voice Network
Bob
Tx
Rx
Alice's voice
Rx
Bob's voice
Tx
Alice
56554
Voice Network
Bob
Montreal
Tx
Rx Alice's voice
Rx
Bob's voice
Echo of Bob's voice
Tx
Alice
London
56555 Locating an Echo
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Echo Analysis for Voice over IP
Table 1
shows how delay can affect the quality of a voice conversation.
Locating an Echo
In
Figure 2
, Bob experiences the echo problem, which means that a signal is leaking from his transmit
path into his receive path. The fact that Bob hears an echo illustrates one of the basic characteristics of
echo: Perceived echo most likely indicates a problem at the other end of the call. The problem that is
producing the echo that Bob hears, the leakage source, is somewhere on Alice's side of the network
(London). If Alice was experiencing echo, the problem would be on Bob's side (Montreal).
The perceived echo leak is most likely in the terminating side of the network for the following reasons: Leak-through happens only in analog circuits. Voice traffic in the digital portions of the network
does not leak from one path into another.
Analog signals can leak from one path to anotherelectrically from one wire to another (called
crosstalk), or acoustically through the air from a loudspeaker to a microphone. Also, analog signals
can be reflected in the hybrid transformer in the tail circuit. (See the section
Effect of Hybrid
Transformers on Echo
.) When these analog signals have been converted to digital bits, they do not
leak.
All digital bits are represented by analog signals at the physical layer and these analog signals are
subject to leakage. The analog signals that represent bits can tolerate a good deal of distortion before
they become too distorted to be properly decoded. If such distortion occurred in the physical layer
of the PSTN, the problem would not be echo. If you had connectivity at all, you would hear digital
noise instead of a voice echo. Echos arriving after very short delays, about 25 milliseconds (ms), are generally imperceptible
because they are masked by the physical and electrical sidetone signal.
This point is a corollary to the previous assertion that echos become increasingly annoying with
increasing mouth-to-ear delay. A certain minimum delay is needed for an echo to become
perceptible. In almost every telephone device, some of the Tx signal is fed back into the earpiece so
that you can hear yourself speaking. This feedback is known as sidetone. The delay between the
actual mouth signal and the sidetone signal is negligible, and sidetone is not perceived as an echo.
Also, your skull resonates during speech (an acoustic sidetone source) and the human auditory
system has a certain integration period that determines the minimum time difference between events
that will be perceived as separate events rather than a single one. Together, these phenomena create
a minimum mouth-to-ear delay of about 25 ms before an echo signal can be perceived.
Table 1
Effect of Delay on Voice Quality
One-Way Delay Range (ms)
Effect on Voice Quality
025
This is the expected range for national calls. There are no difficulties during
conversation.
25150
This is the expected range for international calls using a terrestrial transport
link and IP telephony, which includes only one instance of IP voice. This range
is acceptable for most users, assuming the use of echo control devices.
150400
This is the expected range for a satellite link. Delays in this range can interrupt
the normal flow of a conversation. A high-performance echo canceler must be
used and careful network planning is necessary.
Greater than 400
This is excessive delay and must be avoided by network planning. Locating an Echo
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Echo Analysis for Voice over IP
Given these two reasonsthat echos must be delayed by at least 25 ms to be audible, and that leaks occur
only in the analog portion of the networkyou can deduce much about the location of the echo source.
Figure 3
shows possible sources of echo in a simple Voice over IP (VoIP) network.
Figure 3
Potential Echo Paths in a Network with Both Analog and Digital Segments
In this typical VoIP network, the digital packet portion of the network is located between two analog
transmission segments. Bob in Montreal is connected by a 2-wire analog foreign exchange station (FXS)
circuit to a local PBX, which is connected to a local VoIP gateway by a 4-wire analog recEive and
transMit (E&M) circuit. The Montreal gateway communicates with the London gateway through an IP
network. As we will discuss later, this packet transmission segment has an end-to-end latency greater
than 30 ms. At the London end of the call, the gateway is connected in the same way to Alice's telephone
(by E&M to the PBX and by FXS to the terminal).
Tail Circuits
The analog circuit in London is known as the tail circuit. It forms the tail or termination of the call from
the perspective of the user experiencing the echoBob in this case.
A packet voice gateway is a gateway between a digital packet network and a public switched telephone
network (PSTN). It can include both digital (TDM) and analog links. The tail circuit is everything
connected to the PSTN side of a packet voice gatewayall the switches, multiplexers, cabling, and
PBXs between the voice gateway and the telephone.
Figure 4
shows that the PSTN can contain many
components and links, all of which are potential echo sources.
Figure 4
Echo Sources in the PSTN
Packet v