SFN

FM

Single Frequency

Networks

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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Your questions please?

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42 Years of Nautel Radio Transmitters

1972 2011

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

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

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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Main Transmitter

Interference Areas

Booster Transmitter

SFN: A simple example

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SFN: Coverage analysis of road

Transmitter 2

Road

Transmitter 1

Interference

Zone

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• 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

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

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

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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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

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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Constant Delay Lines

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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.

SFN

The link between transmitter sites

• Must be digital AES/EBU

• Must be uncompressed

• Must have stable propagation time

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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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

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 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

SFN

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

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.

SFN

Getting the timing right

Studio

Tx A Tx B

Interference

Zone

ScopeFunction

Generator

Trig

RPU

FM

Rcvr

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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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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.

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

SFN

Modulation Control

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)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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IQ over IP

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IBOC Timing DU Requirements

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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)

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/

SFN

Questions? We’re ready to help.

• www.nautel.com

• sales@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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Thank You