OPTICOM OPERA Broadcast, User Manual

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C H A P T E R 6 : T E L E P H O N Y B A N D V O I C E Q U A L I T Y
T E S T I N G
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accompanied cultural differences. For example, a noisy floor may be more
annoying if there are more silent intervals during a telephone conversation.
The PSQM algorithm was one out of several proposals that had been brought
before study group 12 of ITU-T in 1995 for the purpose of international
verification. Further proposals were the EPR Algorithm ("Expert Pattern
Recognition"), which consisted of measures of the "LPC Cepstrum Function",
"Information Index", and the "Coherence Function" (CHF). In a test series
conducted by the Japanese phone corporation NTT, including listening tests in
Japan and Italy, the highest correlation was achieved with PSQM results, when
compared to the subjective tests. Consequently, PSQM was recommended by
the ITU-T in 1996 for the objective quality measurement of telephone band
speech codecs. Since then, PSQM has been used intensively for R&D as well as
field applications in networks.
6.5.2 Signal Acquisition
PSQM can be used for online as well as for offline measurements. As
Figure
6.4
, shows, two kinds of signal sources be used. All OPERA™ versions can
assess files. The version including a voice board can issue test calls to acquire
the data, but the actual evaluation is also performed offline. For the data
acquisition use
OptiCall™
which is described in chapter 4.2. Online
measurements with phone lines are currently not supported. However, if there
is an audio interface option included in your OPERA™ system – in addition to or
instead of a POTS telephony board – evaluations of audio sources (e.g. VoIP
terminals) with PSQM are available.
PSQM
Files
(always)
Audio
(if audio board
included)
Sources:
Phone Line
(if voice board
included)
OptiCall.exe
Offline
Online
Figure 6.4:
Kinds of signal sources for the PSQM measurement
When using the file based version of the Telecom Version of OPERA™ (OPR-
100-xxx-x), no telephony interfaces are provided. Instead of this, use audio files
as input. Supported file formats are
WAVE files containing either plain PCM and
a-law or
µ
-law . The PSQM algorithm is defined for sampling rates of 8 kHz and
16 kHz .
As described in Chapter 4, select the files you want to assess in the first two
steps of the measurement set-up wizard.
OPERA™ systems equipped with audio interfaces may be used for online
measurements in real time too. Use our OptiCall program for the data
acquisition if the system is equipped with telephone interfaces. OptiCall may
also be used together with the audio interfaces. Please refer to chapter 4 for
details on the data acquisition.
File Based Version
Version with
Telephony and/or
Audio Interfaces