Pesq Measurement And Vad Measurement - OPTICOM OPERA - V 3.5 User Manual

C H A P T E R 6 :
6.6 PESQ Measurement and VAD
Measurement
When PSQM was standardized as P.861, the scope of the standard at that time
was state-of-the-art codecs, used primarily for mobile transmission, like GSM.
VoIP was not yet a topic. The requirements for measurement equipment have
changed dramatically since then. As a consequence, the ITU set up a working
group to revise the P.861 standard to cope with the new demands arising from
modern networks like VoIP. Within these networks, the measurement algorithm
has to deal with much higher distortions than with GSM codecs, but perhaps
the most eminent factor is that the delay between the reference and the test
signal is not constant any longer.
A first approach to overcome these problems was the development of PSQM+.
It handled the larger distortions well, as they are caused by e.g. burst errors, but
still had significant problems with the compensation of the varying delay. A
Delay Tracking Feature has been added by OPTICOM in it's OPERA™ system
that implemented an easy way to solve the varying delay issue in most cases,
without loosing the option of realtime operation. Although this feature failed for
some signals, it was the only available method to date to achieve reliable results
for the speech quality of VoIP networks.
With the new ITU standard P.862 (PESQ) this problem is finally eliminated.
PESQ combines the excellent psycho-acoustic and cognitive model of PSQM+
with a time alignment algorithm that handles varying delays perfectly. PESQ is
absolutely not designed for streaming applications, which is it's only drawback.
This is also why it cannot fully replace PSQM+. With PSQM and PESQ there
are now two standards that cover the entire problem of measuring speech
quality.
Figure 6.20
shows the new blocks that have been added to the PSQM algorithm.
Reference
signal
System
under test
Degraded
signal
= Same or similar to PSQM(+)
= New in PESQ
Figure 6.20: The structure of the PESQ algorithm
T E L E P H O N Y B A N D
T E S T I N G
gives an overview of the structure of the PESQ algorithm and
Level Input
align filter
Time
align and
equalise
Level Input
align filter
137
V O I C E Q U A L I T Y
Auditory
transform
Disturbance
processing
Auditory Identify bad
transform
Re-align bad intervals
Prediction of
perceived
Cognitive
speech
modelling
quality
intervals