Specifications

5-56 888-2857-001 5/9/13
WARNING: Disconnect primary power prior to servicing.
Section 5 Maintenance and Alignments
Platinum VAX-C Series
5.16 DAB Single Frequency Network (SFN) Setup
A popular option for DAB/ T-DMB implementation is the single frequency network
(SFN). In a single frequency network, all transmitters in a geographic region are
synchronized in frequency and modulation content (symbol). For such a system to
work, two parameters must be tightly controlled at the transmitter site: the RF center-
channel frequency and the program modulation (symbol) delay.
For SFN operation, the RF center-channel frequency in the VAX-C is locked to an
internal 10 MHz reference oscillator. The 10 MHz oscillator is, in turn, locked to a 1 pps
reference signal from an internal or external GPS receiver. This holds the frequency to
within the 10Hz recommended precision level for SFN operation in DAB mode 1, Band
III.
The symbol delay is controlled by three different parameters in the LPU: dynamic
delay, static delay, and offset delay.
5.16.1 Dynamic Delay
Dynamic Delay is an automatic system addressing delays in the ETI distribution
network from the multiplexer to the transmitter input. The exciter compares timing
information embedded in the incoming ETI stream against a locally received GPS - 1
pps reference. Each individual transmitter site adds additional delay to synchronize its
ETI input signal to a target “maximum network delay” value added to the ETI stream at
the multiplexer. The maximum network delay set to be equal to the ETI transit delay to
the furthest site, plus a safety margin.
This forces all sites to be in synchrony at the dynamic delay control point at the input to
the COFDM encoder. Site-to-site differences in signal processing delays occurring after
the dynamic delay circuit inside the COFDM encoder (i.e. outside dynamic delay
control zone) are addressed by the static delay setting.
Dynamic delay is sometimes known as “Network Padding Delay"
5.16.2 Static Delay
Static Delay is a fixed setting to compensate for different processing delays in DAB
transmitters from different manufacturers arising after the dynamic delay circuit (i.e.
outside dynamic delay control zone). The static delay for each site may be set via the
web remote interface. Each individual transmitter site adds additional static delay to
make its output synchronized with the output of the most-delayed site in the network.