signal analysis special report
Increasingly, individual services from of information are transmitted instead The ability to measure video quality
different sources are aggregated using of a continual feed. This allows the becomes critical when statistical
a digital multiplexer (MUX). The result receiver to conserve power between multiplexing is deployed. Here,
is a highly complex data stream that the bursts. content-providers are not allocated a
needs to be monitored for integrity, It is important to measure the fixed bandwidth, since bandwidth is
since any faults or errors can disable a length of the transmission burst, and it
Independent
variable in a statistical MUX. Service
service and could interfere with other is also necessary to measure any drift broadcast providers guarantee a given level of
services within the MUX. in time between transmission bursts. quality for the service, likely to be
service
For example, the MPEG transport The measured quantity, known as measured by MOS.
providers in
stream used for DVB services consists ‘delta-t jitter’, indicates whether or not One empirical measure finding
particular
of transport packets comprising a the next burst of information will be favour is ‘confidence monitoring’,
‘header’ segment and a ‘payload’. The transmitted at the right time.
will focus on
where real-time feeds allow network
header contains key information such There is also an extra layer of error-
demon-
operations centre (NOC) operators and
as transport error indicator, transport correction, known as ‘multi-protocol strating content-providers alike to confirm the
scrambling protocol - along with the encapsulation - forward error broadcast is acceptable. A step further
compliance
essential ‘packet identifiers’ (PIDs). correction’ (MPE-FEC), applied to the is ‘compliance monitoring’, where
to agreed
The PIDs are used in conjunction DVB-H signal. It is therefore necessary content streams are recorded and
with a series of index tables that are to establish whether any faults that
specific-
saved in order to demonstrate exactly
fundamental to successful DVB-T occur do so before the MPE-FEC is
ations. This
what was aired in the case of
transmission. They provide the critical applied, or whether they’re due to its places a contention, or to ensure advertising or
information that allows the application. In addition, monitoring of regulatory commitments were met.
greater
programme content to be re- the electronic service guide (ESG)
demand
assembled by the receiver. Each of transmitted to the handheld device is Maintaining integrity
than ever
these tables - along with many other particularly important for mobile TV.
tables and data, including service Files within the ESG define the source
before on
Clearly ensuring integrity of digital
descriptors that define the coding IP address and encoding information,
end-to-end
broadcast services is vastly more
structure for Dolby, MPEG-1, MPEG-2 which allows the receiver to decode verification complex than for analogue TV. Not
and similar - is critical to ensure quality the received signal. only is there an enormous number of
of the digital
of service. parameters to measure, but the
signal;
Back to the source growing need to provide analysis at
because, in
Special checks for special the service level has resulted in the
the era of
services The holistic approach to signal analysis development of advanced
extends to monitoring the source code
digital
technologies that can deliver this data.
Beyond the fundamental transport of individual broadcasts, before the This highlights the importance of a
broadcasting,
stream checks outlined above for DVB- signals are encoded and multiplexed sophisticated network operations
maintaining
T lie a host of additional checks for into the MPEG transport stream. This centre, with an advanced NMS that
signal
‘specialised’ services, such as includes monitoring content continuity can integrate all these different
multimedia home platform (MHP) to ensure that any loss of signal or
integrity -
monitoring technologies and services
interactive data carousels (or signal timing is immediately alarmed.
and the
into a common platform. It is one
infocasting) and mobile TV. For measuring source code quality, ability to thing to collect and monitor vast
MHP is a DVB specification for two parameters are commonly used: streams of data, and quite another to
take a
interactive data broadcasting, where mean opinion score (MOS) and picture act upon the information to remedy
holistic view
the content is transmitted periodically signal-to-noise ratio (PSNR). MOS has any faults that occur.
over the transport stream. Users then evolved from being a subjective
of the
Nevertheless, network operators still
interrogate their MHP-enabled set top measure of audio quality, carried out
network - is
face the challenge of establishing
box (STB), which retrieves the selected by trained groups of people, to a real-
the new
which are the critical measurements,
data from the transport stream as it time, machine-based test that is also and which are merely ideal, in order to
essential.
cycles around. The same system can used for video measurements. stay commercially viable. Independent
also be used for the downloading of Incorporating standardised numerical broadcast service providers in
objects such as software files. In add- quality indicators, the test provides a particular will focus on demonstrating
ition to monitoring the MHP carousel’s good measure of video quality, while compliance to agreed specifications.
PIDs and MPEG tables as required for being reasonably independent of the This places a greater demand than
DVB-T, it is important to monitor the actual video content. ever before on end-to-end verification
appropriate data rate and cycle time, PSNR is a power measurement, and
The highly complex
of the digital signal; because, in the
which determine how long the inform- easier to define, but does not
MPEG data stream
needs to be
era of digital broadcasting, maintaining
ation takes to download to the STB. necessarily provide a good correlation monitored for signal integrity - and the ability to take
Mobile TV technology also with viewer satisfaction. Footage may
integrity, since any
faults or errors can
a holistic view of the network - is the
introduces unique monitoring and be ‘noisy’, and result in a poor PSNR
disable a service.
new essential.
analysis issues, such as ensuring the figure, but according to human
content protection and encryption perception, the level of ‘annoyance’
systems operate correctly. Another could be quite low. Also, although
major requirement of DVB-H is to PSNR is easier to measure than MOS, it
conserve the power usage in the is less repeatable due to the
battery powered handset receivers, constantly changing nature of video
which makes signal timing critical. images. Consequently, MOS is often
DVB-H solves the issue by employing a regarded as a more reliable measure
‘time-slicing’ structure, in which bursts of picture quality.
www.ibeweb.com l march/april 2009 l ibe l 45
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