1. FM
Single Frequency
Networks

2. 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

3. 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

4. SFN: A simple example Main Transmitter Interference Areas
Booster Transmitter

5. SFN: Coverage analysis of road
Transmitter 2
Interference
Zone
Road
Transmitter 1

6. SFN: Regional Coverage
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.

7. 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

8. The Problem: Interference Zones
2nd Tx
Where the coverage areas overlap, and the ratios of the signal strengths approach unity, the signal quality is affected.
Interference Zone
1st Tx

9. Interference Zones 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.

10. When everything is sync’d - Multipath
If this was a REALLY big mirror
If everything is perfectly sync’d, we’ll have multipath where the primary and reflection have equal signal strength.
We’d have multipath
1st Tx

11. 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

12. Constant Delay Lines

14. 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.

15. The link between transmitter sites
Must be digital AES/EBU
Must be uncompressed
Must have stable propagation time

16. Typical Block Diagram

17. 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

18. Nautel NV and VS: What’s built-in?
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
VS Series
300W-2.5kW
NV Series 3.5kW-88kW FM

19. SFN Demonstration Audio Processor 10 MHz 1 pps 10 MHz 1 pps
split AES feed
VS300
VS300
50 ohm
50 ohm
RF monitor
Coupler
RF monitor
50 ohm
Spectrum Analyzer
Sangean FM Receiver

21. System Timing Location of interference zone Main TX A B
A – B = km of differential delay
Example:
A = 16
B = 10
A - B = 6
6 x 3.34 = 20 usec delay
In-fill Tx

22. 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.

23. Getting the timing right
Interference Zone
Tx A
Tx B FM
Rcvr
Scope
Function
Generator
Trig
RPU
Studio

24. 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.

25. John Ioannidis DTS Broadcast Thessaloniki, Greece
Real world example
John Ioannidis DTS Broadcast Thessaloniki, Greece

26. 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.

27. 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

28. Modulation Control
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

29. IQ over IP

30. IBOC Timing DU Requirements

31. Thank You