This document discusses ways to characterize voice quality impairments in telephone networks. It describes analog transmission impairments like attenuation and phase distortion and how to measure them. Digital transmission impairments in PSTN, packet networks and wireless networks are outlined, such as bit errors, packet loss, and jitter. Impairments from voice processing like compression are also covered. Delay, echoes, and levels are identified as key factors affecting voice quality. The document recommends tools like PSQM, PVIT, and Echo Sounder for measuring various impairments and determining overall voice clarity.

1. Characterize Voice Quality Impairments in a VoX Network
Sage Instruments
1 Introduction
This seminar presents standard as well as innovative ways of characterizing voice quality impair-
ments in a telephone network. This telephone network could be the conventional circuit-switched
PSTN, or the “new” packet-switched data network (VoIP, VoATM and VoFR etc) or the hybrid of
both. Despite its potential benefits, the convergence of data and voice communications also creates
new impairments that can severely affect voice quality. The main focus of this seminar is to help
you analyze and characterize these impairments.
2 Analog transmission impairments
Analog transmission impairments will exist as long as the last mile or yard is an analog loop.
2.1 List of analog transmission impairments
Attenuation distortion: Excessive attenuation distortion affects voice level and voice fidelity
and even voice recognition.
Phase (Envelop-delay) distortion: This does not have apparent effect on voice quality, but
it does have severe effect on data communication. More phase distortion requires better
equalization scheme.
Nonlinear distortion: This only affects data communication.
Noise: Typical sources of noise are cross-talk, inherent circuit noise and G.711 PCM quantization
noise etc. Quiet noise is particularly annoying to phone conversation.
2.2 Ways of measuring analog transmission impairments
Measuring attenuation distortion: 23-tone, 3-tone slope, tone-sweeping and single tone.
Measuring phase distortion: 23-tone and EDD.
Measuring nonlinear distortion (IMDs): 23-tone and 4-tone IMD measurements.
Noise: Quiet noise through various filters. Quantization noise by C-notch measurement. STD (or
SNR) using a single-frequency holding tone or 23-tone test.
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2. 3 Digital transmission impairments
3.1 PSTN network
In a plesiochronous PSTN digital network, the following impairments are common:
• Bit errors.
• Frame slip.
Ways of measuring the digital transmission errors:
• BER: bit-error-rate measurement.
• Frame slip indication.
• Transient impulse and phase/gain hits: bit errors or frame slips also cause measurable tran-
sient distortions in the audio signal. These transient distortions show up as impulse noise,
phase or gain hits.
3.2 Packet-switched data network
These include all the VoIP, VoATM and VoFR applications. VoATM also include VoDSL, VoCable
and VoWLL etc. A packet-switched network can introduce the following impairments that do not
exist on a PSTN network:
Packet loss: Packet loss results in bursty degradation on voice signal, which can severely impact
voice quality.
Packet jitter: The packet jitter in the digital network means the uneven inter-packet arrival time.
Excessive jitter results in packet loss if the jitter exceeds the jitter buffer size.
Voice jitter: Voice jitter means sudden delay variations in the voice signal, resulting in gapping
and jerking effects. Voice jitter is typically caused by the dynamic adjustment of jitter buffer
size.
Ways of measuring packet network impairments:
Packet loss counter: If test access to the packet network is available, one can certainly use an
instrument to monitor all packets arriving at a receiving gateway, and count the lost packets
and measure the inter-packet jitters. Drawback of this kind of measurement is that, the
end-terminal perceived voice quality remains unknown, because different gateways may have
different ways of handling lost and jittered packets.
Packet-voice-impairment-test (PVIT): this test requires only simple telephony interface (ana-
log or T1/E1), but it can measure the perceived packet loss and packet jitter. The measured
results are directly perceived by end listeners, so these measurement results are the best
indicators of the network service quality.
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3. 3.3 Wireless telephone network
Common impairments associated with a digital wireless phone network are:
• Frame erasure caused by geography-related RF fading and shadowing etc.
• Frame loss during handoff from one cell to another, or from one sector to another.
• Frame loss caused by other factors such as interference, poor modulation quality of the RF
signal and pilot pollution (CDMA) etc.
Ways of measuring these impairments:
RF measurements: These can range from simple RF power meter, to complex signal analyzer
that can perform demodulation and modulation quality measurements on the RF signal.
These measurements can uncover potential problems, but are hard to translate to direct
user-perceived voice quality.
PVIT: Again PVIT (as a drive test) presents the most direct way of measuring the voice frame
loss as an indication of poor RF coverage or incorrect RF transceiver provisioning etc.
4 Impairments caused by voice processing devices
The use of special voice processing devices are necessary in order to save the bandwidth for VoX and
wireless networks. For digital wireless phone network, for example, the use of low-bit-rate vocoders
are “mandatory” in order to stuff more voice channels into the precious limited spectrum. For VoX,
in order to reduce traffic congestion and increase service capacity, the use of voice compression and
silence suppression are also common. Typical impairments introduced in these kinds of networks
are:
Lossy compression: The use of low-bit rate lossy voice compression scheme will degrade the
intelligibility, fidelity, naturalness and even voice recognition of a conversation.
Voice clipping: The use of VAD (voice-activity-detector) can cause clipping of the leading edge
of the active voice syllables.
Background noise: The silence suppression scheme employs special technique to encode the back-
ground “silence” noise, and the receiving end replays it as comfort noise. The comfort noise
should match the original background noise in terms of power level and spectral characteris-
tics.
Ways of measuring these impairments:
PSQM for voice compression impairments: the low-bit rate vocoders defy the conventional
simple SNR or STD type of measurements using continuous mono or multi-tones. To measure
these impairments require the use of complex voice like signal and complex psychoacoustic
model to quantify the degradations of different vocoders. The ITU P.861 recommended PSQM
is one such technique. The draft P.862 PESQ claims to be an improved version of PSQM.
PVIT for voice clipping and comfort noise level: these two impairments are measured in
Sage’s PVIT.
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4. 5 Voice network specific problems: echoes and delay
Unlike most data communications, voice communication is a delay-sensitive real-time interactive
application. Furthermore, as long as the 2-wire analog loop exists, the voice network echo will
remain to be one of the most important issues for telephone service. Delay and echo are inter-
twined. Long delay not only makes conversation unnatural, it also exacerbates echo problem. Two
important attributes of echoes are echo delay and echo level.
5.1 Causes of delay and echoes
Causes of delay: major causes are voice processing delay (voice compression and expansion),
packetization/framing/interleaving delay and transmission (propagation) delay. Long delay
causes hesitation and over talk and unnatural conversation.
Causes of echoes: impedance mismatch at the 4-wire to 2-wire hybrid points, and occasionally,
the acoustic feedback. Audible echo with long delay is absolutely unbearable in a phone
conversation.
5.2 Ways of measuring round-trip delay
IEEE 743 absolute delay measurement: This technique is classic, but may not be suitable for
compressive packet-switched network.
Correlation-based technique using spread spectrum signal: This technique involves more
computations, but it is far more reliable in all kinds of environments. This technique is
employed as part of Sage’s PSQM test (PSQM delay measurement).
Echo Sounder: Sage’s Echo Sounder also presents a unique and reliable way of measuring round-
trip delay and round-trip attenuation simultaneously. This technique can even tolerate large
amount of packet loss impairments.
5.3 Measuring one-way delay
Sage’s Echo Sounder also provides a way of measuring one-way delay in a laboratory environment
where both terminals are colocated.
5.4 Ways of quantifying echo problems
Probing echoes: Echo Sounder allows users to probe a telephone network to see if there are any
measurable echoes. Results are displayed as multiple echo delays and levels.
Echo canceler performance tests: These include ITU O.22 ATME for PSTN network (4-wire
only), and Echo Sounder/Echo Generator for VoX applications (both 2-wire and 4-wire). And
G.168 test suite for echo canceler design verification test (T1/E1 interface only).
6 Conclusions, four key elements: clarity, delay, echo and level
In conclusion, the voice quality are determined by four key parameters: voice clarity, delay, echo
and voice level. All these parameters need to be measured separately, and each parameter has to
meet a specific criterion to guarantee voice quality.
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5. • For voice clarity, one can use PSQM or PESQ to obtain an overall estimation. PVIT will
then provide further diagnosis on the impairments that causes the poor voice quality.
• For delay, one can use PSQM delay, Echo Sounder or classic IEEE 743 delay measurements.
• For echo problems, one can use Echo Sounder to probe, trouble shoot and isolate echo sources.
For echo canceler test, one can use ATME, Echo Sounder/Echo Generator pair and G.168
test suite.
• For voice level, Sage’s PSQM test indicates voice level change. One of course can also use
simple sending tone and measuring tone to detect voice level change.
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