Alarming on drift in DVB-T SFN networks This technical note details

Alarming on drift in DVB-T SFN networks This technical note details
Alarming on drift in DVB-T SFN networks
This technical note details a novel mechanism for detecting absolute timing offsets in Single Frequency
Networks as found in DVB-T architectures. Timing offsets down to +/- 0.1 microseconds are registered. The
functionality described is available in the BRIDGETECH VideoBRIDGE monitoring product family.
The mechanism has applications into SFN networks where a practical problem often is to ensure that the
transmitters and modulators are kept in absolute and continuous synchronization across the network. Any drift
between the transmitters will cause reception degradation in fringe areas covered by two or more SFN DVB-T
transmitters.
The SFN drift alarming functionality is available on both the ASI input and COFDM input simultaneously.
Overview
The products of relevance are the VB120 and VB220 IP probe with one or two VB252 COFDM demodulator
front-end modules.
Specifications of relevance are:
 ETSI TR 101 190 Digital Video Broadcasting (DVB) “Implementation guidelines for DVB terrestrial
services”; Transmission aspects.
 ETSI TR 101 191 Digital Video Broadcasting (DVB) “DVB mega-frame for Single Frequency Network
(SFN) synchronization”
The picture below shows the BRIDGETECH 1RU chassis equipped with one VB120 Broadcast IP probe and two
VB252 COFDM front-end input modules.
The connectors on the VB120 module are from left to right: USB Type-A connector for initial setup, 10/100/1G
RJ-45 for video segment, alternative SFN port for video segment, ASI SMB connector for ASI INPUT, 10/100T
RJ-45 for management and ASI SMB connector for ASI OUTPUT.
The connectors on the VB252 module are from left to right: RF input (F-connector), 1pps 50 ohm TTL GPS
reference input, ASI output.
Bridge Technologies Co AS – Bentsebrugata 20, NO-0476 Oslo, Norway – tel: +47 22 38 51 00 – www.bridgetech.tv
Applications
Application for verifying correct timing of ASI signal after SFN adaption at head-end:
The operator is alerted in case the SFN adapter at the head-end starts to drift outside user-configurable time
window. Window is defined in units of 100 ns with an accuracy of +/- 100 ns. Drift can happen if the SFN adapter
looses its local GPS reference and must default to its built-in timing reference instead.
Bridge Technologies Co AS – Bentsebrugata 20, NO-0476 Oslo, Norway – tel: +47 22 38 51 00 – www.bridgetech.tv
Verifying that the transmitters stay locked to GPS timing reference at transmitter towers on both ASI and COFDM
level:
Advantages to this solution include:
− Alerts the operator in case the transmitted COFDM signal starts to drift outside user-configurable time
window. Window is defined in units of 100 ns with an accuracy of +/- 100 ns. This can happen in cases
where the transmitter defaults to its local oscillator reference instead of the incoming GPS reference. It can
also happen as the transmitter power stage changes characteristics slowly over time.
− Alerts the operator in case the ASI signal prior to COFDM modulation is outside pre-configured time window.
This can happen if distribution network has varying 'memory'. That is, the amount of bytes coming out of the
network is varying slightly compared to the number of bytes going into it over time.
Bridge Technologies Co AS – Bentsebrugata 20, NO-0476 Oslo, Norway – tel: +47 22 38 51 00 – www.bridgetech.tv
The GUI screenshots show maximum timing difference between the ideal and actual mega-frame transmission
point since last readout. Values are measured for both the ASI and COFDM input interfaces simultaneously. The
readout resolution is in 0.1 microseconds (=100 ns) with an accuracy of +/- 0.1 microseconds. Negative values
signify negative delay. Negative values equal to the maximum_delay configuration setting are measured - and
expected - on the ASI prior to modulation and transmission since the transmitter delays the signal by the same
maximum_delay value.
Bridge Technologies Co AS – Bentsebrugata 20, NO-0476 Oslo, Norway – tel: +47 22 38 51 00 – www.bridgetech.tv
The operator configures the TR 101 290 alarm template for the ASI and the COFDM transponder to measure on.
The Drift Zero Adjust parameter is used to zero calibrate the drift reading and is subtracted from the SFN drift
timing difference measured before presenting the result in the ASI or COFDM GUI screens described earlier. In
this manner it is possible to calibrate away constant timing offsets introduced by over-air transmission delay and
COFDM demodulator delay.
The parameter called Max Drift sets the criteria for SFN drift alarming. If the numerical value of the offsetcompensated drift value - negative or positive - is larger than the value set in the template then an alarm is
generated. Units for both offset and drift are in 100 ns, or 0.1 microseconds.
Algorithm used - theory
Define T=0 as when 1 pps pulse goes high and reference all timestamps relative to this.
Use existing 27MHz clock to timestamp. This gives an accuracy of +/- 37 ns, which is better than the 10MHz
reference normally used in GPS (+/- 100 ns).
Receive all MIP packets. Use the pointer field from the MIP to identify which packet is the start of the next (M+1)
mega-frame.
Timestamp - using local 27Mhz oscillator - the reception of the first (M+1) mega-frame packet. The timestamp is
relative to the T=0 point in time occurring as the 1pps pulse goes high. Call this value
(synchronisation_time_stamp+maximum_delay)_ACTUAL
Extract the corresponding fields from the MIP packet M. Call the sum:
(synchronisation_time_stamp+maximum_delay)_IDEAL.
This is a calculated value and represents the time from when the 1pps pulse goes high to when the start of the
(M+1) mega-frame should be transmitted out of the antenna of each transmitter tower.
ACTUAL is larger than IDEAL in a real-life system due to the transmission delay from the transmitter antenna
output to the COFDM demodulator antenna input. A second delay factor is the delay in the COFDM demodulator
stage of the VB252 COFDM demodulator input card.
If the GPS sync of the transmitter is failing then the transmitter will slowly start to drift out. We will see this as a
gradual change in (ACTUAL-IDEAL).
This is what is alarmed against.
Generate an alarm if the (ACTUAL – IDEAL) value is starting to drift. User specifies alarm criteria as a number
(units: time) that the absolute value of (ACTUAL – IDEAL) difference should stay within.
For example: Generate alarm if ABS(ACTUAL – IDEAL) > 10 us.
The alarms are logged in the normal fashion inside the VB120 unit. The alarms can be forwarded as SNMP
TRAPs using the existing VB120 framework or polled using XML over HTTP. Please refer to VB120 users
manual for details on this framework.
Bridge Technologies Co AS – Bentsebrugata 20, NO-0476 Oslo, Norway – tel: +47 22 38 51 00 – www.bridgetech.tv
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