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SFN
FM
Single Frequency
Networks
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SFN
Agenda
• Why SFNs?
• FM SFN Theory / Challenges
• Live Demo
• Customer experience
• HD Radio SFN
• Questions
Chuck Kelly
Director of Sales
Philipp Schmid
Research Engineer
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SFN
Your questions please?
(if you don’t see the control panel,
click on the orange arrow icon to
expand it)
Please enter your questions in the
text box of the webinar control panel
(remember to press send)
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SFN
42 Years of Nautel Radio Transmitters
1972 2011
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SFN
NXSeries MW
NX25
Nautel Major Product Families
3.5kW – 88kW300W – 2.5kW
NVSeries FMVSSeries FM
25kW – 2.0MW
Advanced User Interface (AUI)
NX200 NX300 NX400 NX800NX100NX50
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SFN
SFN: Why?
• Two or more FM transmitters:
– same frequency
– same programming
– overlapping coverage areas
• More efficient use of spectrum
• Allows listeners to drive long distances and
not change the tuning on the radio
• Allows stations to link station identity to
frequency
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SFN
SFN: Applications?
• Applications:
– Fix “holes” in coverage
• terrain blocked from the main transmitter.
– Create a long unbroken coverage area
• for instance a highway
– single station can be heard without changing frequency.
– Create a large coverage area from many smaller
transmitters
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SFN
Main Transmitter
Interference Areas
Booster Transmitter
SFN: A simple example
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SFN
SFN: Coverage analysis of road
Transmitter 2
Road
Transmitter 1
Interference
Zone
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SFN
• True, regional stations may be created by synchronizing high power stations.
• Station identity can be created – the frequency is the same across the whole region.
• Frequency allocation is easier – the same frequency can be used for adjacent stations.
SFN: Regional Coverage
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SFN
SFN: Basic Concept
To optimize performance – synchronize everything:
– RF Carrier Frequency
– Pilot Frequency & Phase
– Audio content:
• Amplitude
• Phase
– Subcarriers:
• RDS
• SCA’s
Timing is everything
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SFN
1st Tx
2nd Tx
Interference
Zone
Where the
coverage areas
overlap, and the
ratios of the
signal strengths
approach unity,
the signal quality
is affected.
The Problem: Interference Zones
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SFN
• If the RF carriers are not frequency synchronized
– terrible distortion and noise will result.
• If the audio levels are not exactly the same
– the noise floor increases dramatically with a “white noise”
which varies with the level of the audio.
• If the pilots are not synchronized
– the pilot detector in the receiver will switch back and forth and
this will be audible in the stereo signal.
• If the audio phase is not synchronized
– distortion results.
• If everything – audio, pilot & carrier are all synchronized,
– the signal will sound like a multipath condition.
Interference Zones
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SFN
1st Tx
If this
was a
REALLY
big mirrorWe’d have
multipath
If everything is
perfectly sync’d,
we’ll have
multipath where
the primary and
reflection have
equal signal
strength.
When everything is sync’d - Multipath
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SFN
Defining SFN Interference
• Equal power levels is worst case
– requires near perfect time alignment
• Where time alignment is not achieved we require one signal to be stronger
– Signals ratios within 14 dB at and a 10 us difference can cause interference
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SFN
Constant Delay Lines
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SFN
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SFN
Synchronize Everything
• The entire system is digital.
• Audio is synchronized using an uncompressed Digital
AES/EBU STL.
– transmitted audio levels are locked
– Nautel uses the AES/EBU inputs on the Nautel digital exciter.
• Carriers and the pilot are synchronized using GPS.
– In the Nautel digital exciter the pilot and stereo generator are
synchronized to the same frequency standard as the carrier.
• The deviation is locked digitally.
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SFN
The link between transmitter sites
• Must be digital AES/EBU
• Must be uncompressed
• Must have stable propagation time
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SFN
Typical Block Diagram
Uncompressed
AES/EBU STL
Uncompressed
AES/EBU STL
Nautel
Digital ExciterRF To
Transmitter
RF To
Transmitter
GPS Receiver with 10MHz
and 1pps outputs
Transmission Site A Transmission Site B
AES/EBU
Audio from
Studio or STL
STL Path
1PPS
10MHz
10MHz 1PPS Henry Digimax 2x6
Nautel
Digital Exciter
GPS Receiver with 10MHz
and 1pps outputs
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SFN
What is needed?
• 2 X Nautel FM exciters
• 1 X Uncompressed Digital STL with AES/EBU input and output. – We have used the Moseley Starlink and DTS Link
– The STL must be digital, uncompressed, stable propagation time.
• 2 X Rack mounted GPS receivers – 10MHz and 1pps TTL level outputs
– an outdoor antenna.
– We have used the ESE Model 110.
• 1 X AES/EBU Splitter –– i.e. the Henry Digimax 2x6
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SFN
No-cost SFN functionality in all NV and VS Series:
• Phase locked pilot
• 10MHz and 1pps TTL level outputs
• Built-in micro adjust time delay– 1 usec increments
• But you will require a synchronous link– Synchronous link = uncompressed digital link
Nautel NV and VS: What’s built-in?
NV Series 3.5kW-88kW FM
VS Series300W-2.5kW
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SFN
SFN Demonstration
Audio Processor
VS300 VS300
Coupler
Spectrum Analyzer Sangean FM Receiver
split AES feed
1 pps10 MHz
50 ohm
50 ohm50 ohm
RF monitor RF monitor
1 pps10 MHz
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SFN
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SFN
System Timing
Main TX
In-fill Tx
A
B A – B = km of differential delay
Example:
A = 16
B = 10
A - B = 6
6 x 3.34 = 20 usec delay
Location of interference
zone
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SFN
Installation Process
• Set both exciters to lock to the external 10MHz reference frequency
• Adjust the exciter at transmitter site #1 for proper modulation
– as fed from the studio audio processor.
• Set the exciter at transmitter site #2 for the exact same AES input gain
level as recorded from transmitter site #1.
• Enter the calculated differential delay in the AES/EBU delay unit.
• Using cellular phones, or other means of communication
– monitor the audio in the interference zone, and
– fine adjustments in the delay time to minimize artifacts.
– when properly adjusted, the audio performance in the interference zone should
be similar to normal multi-path distortion.
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SFN
Getting the timing right
Studio
Tx A Tx B
Interference
Zone
ScopeFunction
Generator
Trig
RPU
FM
Rcvr
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SFN
Nautel Synchronous FM – What it isn’t
• Not a “system”
– rather it is a portfolio of solutions
– use to create reliable and high performance
system.
• RF design to minimize the size and ideally
locate the interference zones is still critical.
• Nautel does not provide RF consulting
services.
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SFN
Real world example
John IoannidisDTS Broadcast
Thessaloniki, Greece
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Conclusions
• Synchronous FM is now practical – and interference zone artifacts may be minimized.
• Interference zone distortion is similar to multipath.
• Key factors include– precise synchronization of modulation levels.
• Single frequency networking– can provide significant benefits to FM broadcasters
– as well as to spectrum regulators.
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SFN
Extra Credit: SFNs for HD Radio
• Relaxed timing margins– 40us to 75 us
• Relaxed DU ratios– IBOC works with 4 dB on channel DU (30 dB for FM)
• Require hybrid boosters– Receivers designed with 6dB 1st adjacent D/U
– Ensure at least 0 dB FM/IBOC everywhere
• Must now synchronize IBOC, FM audio, FM pilot, FM RDS, FM SCAs– FM timing criteria still hold
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SFN
Modulation Control
)))()((()(
)sin()()cos()())(sin()()(
tj
ccc
cetjQtIts
ttQttItttAts
IQ over IP: one FM+IBOC modulator, synchronize output
• Digital IQ over IP delivered across RF link
• Complex in-phase and quadrature valued is digitized
– Mathematically exact signal copy on all exciters
– Identical FM modulation
• No pilot tone synchronization
• Automatic sub-carrier synchronization
• Channel modulation remains at exciter
• Method is modulation agnostic
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SFN
IQ over IP
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SFN
IBOC Timing DU Requirements
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SFN
Your questions please?
(if you don’t see the control panel,
click on the orange arrow icon to
expand it)
Please enter your questions in the
text box of the webinar control panel
(remember to press send)
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SFN
Learn More / Stay in touch
• Nautel Waves Newsletter
http://www.nautel.com/newsletter/
• Webinars
http://www.nautel.com/webinars/
• YouTube
http://www.youtube.com/user/NautelLtd
• Nautel Store
http://store.nautel.com/
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SFN
Questions? We’re ready to help.
• www.nautel.com
Gary Manteuffel US Corporate Accounts & Canada
Ellis Terry
Jeff Welton
Western US
Central US
Gary Liebisch
Steve Schmitt
Eastern US
Sales Engineer
John Abdnour
Wendell Lonergan
Hal Kneller
Gerardo Vargas
Asia / Pacific
Middle East & Northern Africa
Europe & Southern Africa
Latin America / Caribbean
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SFN
Thank You