special report signal analysis
A wealth of complementary data services has
opened up a new era in digital broadcasting
and associated challenges in network
management. Doug Iles, system development
manager at Broadcast Australia, explores the
latest trends in end-to-end signal monitoring.
Signal analysis takes
a holistic view
programmes within a multiplexed stage alarm: the first when the MER
I
n the new era of digital
broadcasting, television services
signal. In an environment of shared drops below a certain value, and the
embrace a whole lot more than
digital networks, each content provider second for complete loss of signal.
the mere transmission of video
will need assurance that its MER specifications vary according to
and audio. Service innovations
programmes are going to air correctly, the purpose of the broadcast. Fixed-
such as ancillary data services,
and is likely to be concerned only with reception DVB-T services can operate
interactivity, and multichannel
an individual stream within the adequately with an MER as low as
broadcasts bring with them a
multiplexed signal. An independent 20dB, although when operating as a
multitude of new and advanced
transmission service provider will single frequency network (SFN) this
technologies that need to be
therefore need to demonstrate figure needs to be closer to 30dB. In the
supported by the broadcast network.
compliance to agreed specifications on case of mobile TV, the MER needs to
One of the most significant
individual streams. exceed 30dB due to the complex
developments in broadcast network
To support these heavy signal reception environment experienced by
operation and management is the
monitoring demands, a sophisticated mobile handheld devices.
need for vigilant monitoring of digital
network management system (NMS) Networks operating in single
services. Opportunities for error exist
is essential. The NMS aggregates and frequency network (SFN) mode
throughout the entire ‘life’ of a digital
analyses individual alarms and test demand a host of additional checks,
signal - from the content source base
results, enabling support staff to trace since the data, bitrate and frequency
code through the encoding and
the root cause of faults with improved must be identical for every site, plus
multiplexing of the transport stream,
speed and accuracy. accurate time-offsets are applied. It is
and the transmission and reception of
therefore essential to monitor both the
the RF signal itself. End-to-end End-to-end
The quality of RF timing and frequency of the transmitted
monitoring of services is essential to
monitoring of signals at SFN sites. Monitoring
digital broadcast
ensure errors are detected and recti-
Owing to its very nature, digital transmitted power is equally important
services is essential
fied before they reach a critical level.
to ensure any
transmission permits the monitoring for SFN networks, since a small drop in
Moreover, from a diagnostic point of
errors are detected
and assessment of a host of different signal strength can potentially affect a
and rectified before
view, there is increased emphasis on they reach a critical
parameters. However, it is important to large part of the audience.
the ability to isolate individual
level. find the balance between what’s One of the fundamental tests for SFN
possible and what’s practical and network timing is related to the channel
economic. impulse response (CIR) measurement.
Consider first the RF signal. Basic CIR characterises the performance of
parameters, such as bit error rate (BER), the channel, providing a graphical
signal level and carrier-to-noise ratio, indication of signal strength versus time
are routinely measured. However, delay for signals received at the given
perhaps the best indication of signal location.
quality is the modulation error ratio
(MER). This measures the amount of Mastering the MPEG stream
noise and distortion transmitted, and is
used as a ‘figure of merit’ for service Upstream of the transmitter is the
providers. It is often used as a two- DVB-T MPEG transport stream.
44 l ibe l march/april 2009 l www.ibeweb.com
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