135 replies to this topic

[alanh]

All,
Single Frequency Networks (SFN) is where the same TV channel is used for two or more transmitters within range of each other. This system is used to conserve channel allocations and can only be used by Digital TV and Radio.

Normally the interference would be intolerable except close to one of the transmitters.

DVB-T is used for Digital TV in all areas except North America. It uses 1705 carriers (2k). The North American system called ATSC uses one carrier, this is like serial transmission.  In our system we can slow the data rate by 1/1705th for each carrier, thus is using parallel data transmission. The receiver will detect the start of the pulse on that carrier and ignore any following pulses until the original pulse is finished. The following pulse may be from a reflected signal or from another transmitter further away. This makes portable and mobile reception more achievable than the ATSC system.

When SFNs are used, the number of carriers is increased to 6817 carriers (8k mode) This increases the pulse length by 4 times, giving a greater tolerance to longer delayed signals.

The gap between pulses (Guard Interval) is selectable by the broadcaster, so can be increased with a drop of data rate. Options are 1/4, 1/8, 1/16, 1/32.

To make the system work two things need to be done
1. All the transmitters on the same channel must be at exactly the same frequency. This is done by using a satellite frequency as a common reference through GPS.

2. Delay the signals so that where the signal strength is equal the signals arrive a the same time from both transmitters. This is done by electronically delaying the main transmitter so that the time it takes for the signal to go from the main transmitter to the repeater and back to the receiver is identical.

If the signals are not arriving within the time window allowed, the only option available to the viewer is to use a very directional antenna to exclude the signal from one source. To make this easier some repeaters are on the opposite polarisation to the main station. For example Mt Tamourine is on Horizontal and Currumbin is on Vertical.

AlanH

[John_Barber]

alanh, on Jun 13 2005, 09:12 PM, said:

All,
Single Frequency Networks (SFN) is where the same TV channel is used for two or more transmitters within range of each other. This system is used to conserve channel allocations and can only be used by Digital TV and Radio.
....
....

Interesting, Alan.
Keep 'em coming, more detail where available!

Cheers
JB

[Timmy Downawell]

typo alert:  DVT-T should be DVB-T

[COFDM MAN]

Currumbin (Farrel Lokout) has always used Vertical polarisation. The fact that this is the opposite of Mt Tamborine is more chance then design.

If polarisation was changed at child sites when digital was installed every viewer would need to change antenna orientation!

I know of no cases where this has been done in australia. Good theory though Alan!

[alanh]

COFDM MAN,
SFNs are also used in Sunshine Coast, Central Coast NSW, Mornington Peninsula, Dandenong Ranges in Victoria to name a few. If you look at  Click on Geographical Viewers' Forums. Subforums Select your geographic subforum and then open Get the best reception. Which transmitter and which antenna for these areas, they all have mixtures of horizontal and vertical polarisations.

AlanH

[COFDM MAN]

Point Taken Alan. However It is by chance not design my friend.

[EvanOnTheGC]

There is also a new member of the Gold Coast SFN being the transmitter at Springbrook.

Same frequencies and relatively close in location to both Tamborine and Currumbin.

Not sure what the polarity of Springbrook is, but I'm sure it wouldn't be diagonal (given Alan H's theory and that horizontal and vertical are already used).

Newcastle also has a 3 member SFN (Sugarloaf (H), Kotara (V), Merewether(H)).

The Sunshine Coast has a 5 member SFN...... 4 x H, 1 x V.

The 2 transmitters serving Gympie are both Horizontal.



Then there is the whopping big SBS SFN in SE QLD. 9 transmitters + 2 spares.


So yeah, the use of vertical or horizonal polarity appears to be purely due to historical reasons (probably to do with analogue/analogue interference when the transmitters were constructed) rather than to comply with SFN technicalities.


Evan :->

[alanh]

All,
The ABA tries to use the same polarisation as existing analog services so as to save the viewers' money.

However if you look at the locations of the changes in polarity it is usual that they go alternately. The ABA has known for a long time about SFNs.

The reason for using alternate polarisations is that it makes it easier to get one transmitter or the other and not both.

AlanH

[Deaf]

alanh, on Jun 20 2005, 08:13 PM, said:

All,
The ABA tries to use the same polarisation as existing analog services so as to save the viewers' money.

However if you look at the locations of the changes in polarity it is usual that they go alternately. The ABA has known for a long time about SFNs.

The reason for using alternate polarisations is that it makes it easier to get one transmitter or the other and not both.

AlanH

<{POST_SNAPBACK}>

As Alanh says  the reason is obvious, think of 2 transmitters close to one another
with different  freqencies, an antenna would be able to pick them both up, although
one would be of poor reception which would interfere with the good signal
and that is the reason for Polorisation and the only reason.     Deaf

[COFDM MAN]

Initial design for Farrel Lookout may have used the opposite polarisation to exclude interferers, however, who 47 years ago when Mt Coot-Tha started broadcasting Television would have predicted how busy the spectrum now is! The beauty of digital is that guard bands are now basically obsolete. This means interference when analogue eventually ends will never be a problem.  (Except for electromagnetic affects on some STB's)

[James T Kirk]

Some observations
Alanh:
"Delay the signals so that where the signal strength is equal the signals arrive a the same time from both transmitters. This is done by electronically delaying the main transmitter so that the time it takes for the signal to go from the main transmitter to the repeater and back to the receiver is identical."

You don't have repeaters in an SFN, you have multiple transmitters that are individually fed programme. In fact, the main transmitter is not delayed, the gap fillers are delayed.

"When SFNs are used, the number of carriers is increased to 6817 carriers (8k mode)"

6817 carriers are used in Australia whether its an SFN or not.

I agree in any DVB-T SFN system use the highest gain/ highest front to back ratio antenna you can afford.

COFDM MAN:
Spot on!!  The ACA/ABA/ACMA would be nuts to have DTV on a different polarity to ATV. You'd hear the broadcasters screaming and complaining from miles away.

GoldCoastGuy:
Spot On!! except there's only one spare.

Alanh:
"The ABA has known for a long time about SFNs."
mmmmmmmm....No longer than the rest of us and the polarities were decided long long ago, back when they knew what they were doing at ABA and before they heard of SFN's.

Deaf:
"As Alanh says the reason is obvious, think of 2 transmitters close to one another
with different freqencies, an antenna would be able to pick them both up, although
one would be of poor reception which would interfere with the good signal
and that is the reason for Polorisation and the only reason."

I've read this one ten times. Once I understand it I'll reply.

[alanh]

Deaf,
If one signal is of a lower strength it will be ignored by the receiver. The main problem is when both signals are of equal strength.

AlanH

[COFDM MAN]

alanh, on Aug 12 2005, 10:03 AM, said:

Deaf,
If one signal is of a lower strength it will be ignored by the receiver. The main problem is when both signals are of equal strength.

AlanH

<{POST_SNAPBACK}>

Alan,
        With FM the receiver will always lock to the strongest. With DTV in a SFN the signals from two TX's in one SFN can  combine to negate each other. The result can be no signal at all. Transmitter ERP, GPS timing and locality of the viewer all come into play.

[alanh]

COFDM MAN,
Both FM and COFDM use limiters to remove amplitude variations in the signal prior to demodulation although with COFDM this effect is less.

You assume the signals will be out of phase. They could also be inphase, strengthing the signal. This is why the carrier frequencies of all transmitters must be very accurately controlled.

AlanH

[James T Kirk]

Limiters in FM systems remove amplitude variation and reshape to ensure the only information left in the signal is its frequency and rate of change of frequency therefore any amplitude variation in the carrier is eliminated.

In COFDM, the information is contained in the amplitude and phase of each carrier.
Would anyone care to suggest what a limiter would do to such a signal?

[alanh]

James,
You are talking about QAM using different levels. However, the end result of COFDM is thousands of carriers, which are just a series of pulses on each carrier.  After all in FM it consists of a series of sidebands whose amplitude is controlled by the deviation and the modulating frequency. You still limit that signal!

AlanH

[James T Kirk]

Alan

Yes there are 6817 carriers, all the time, as I stated earlier, but in what form is the information held in each carrier? Consider why then would amplitude limiting be undesirable?

"After all in FM it consists of a series of sidebands whose amplitude is controlled by the deviation and the modulating frequency. You still limit that signal!"

Again there's lots of talk about FM, are you suggesting these 6817 carriers are frequency modulated?

[alanh]

James,
I was not suggesting that COFDM is FM.
I was trying to make a comparison. Limiting is used in all FM receivers. The total signal is cut off to be a constant amplitude. Even under these conditions the frequencies used to make the FM signal vary in amplitude and phase. When they are all added together you end up with a constant amplitude signal which varies in frequency, according to the signal to be transmitted.

If you look at the waveform of the FM signal from a modulator it is a constant peak to peak amplitude even when the modulating signal is strongest.

AlanH

[James T Kirk]

Alan: Why choose to make a comparison when there is no correlation at all to FM. Why did you say " Both FM and COFDM use limiters to remove amplitude variations in the signal prior to demodulation although with COFDM this effect is less." ? The intelligence is in the amplitude and phase domain. Amplitude limiting would render the signal unuseable!

[alanh]

James,
If you look at a spectragram of an FM signal you will find many frequencies of different amplitudes as sidebands, their phase can also change. When all of these sidebands are added together you get a constant amplitude sine wave which can vary in frequency.

In COFDM You have a large range of frequencies of different amplitudes. If you add together frequencies which are very slightly different it will appear to have phase differences particularly if present for a short time. So what is the effect of adding all these carriers together? A waveform which will vary in instantaneous frequency, depending on which carriers are present. Do you see a similarity??
AlanH

[bellotv]

AlanH

I thought that a constallation display was showing the PHASE and AMPLITUDE variations of the QAM signal.16 QAM has 4 phase and 4 Amplitude possibilities per symbol .64 QAM has 8 phase and 8 Amplitude possibilities per symbol.

Surely if you limit the signal prior to demodulation you would get the phase differences but badly corrupted Amplitude levels.

Secondly I thought that COFDM deliberately has piolot carriers dispersed across the channel that carry no modulation whos amplitudes are monitored and used as part of the viterbi decoding process to predict reliability of other nearby carriers that may be affected by reflected signals .

Surely limiting signal prior to demodulation would render this part of COFDM useless too

I may have this all cocked up but I think its pretty close.

[James T Kirk]

Alanh
In no way, such a statement indicates a less than rudimentary understanding of what's actually happening.

[alanh]

All,
Does anyone have a spectrum analyser capable of solving this one. It will need high resolution not only in the amplitude direction but also in the frequency axis as well.

AlanH

[M'bozo]

In circuits of DVB-T receivers I see no limiting function being applied in the recovery of baseband signals.

[bellotv]

alanh, on Aug 18 2005, 09:56 PM, said:

All,
Does anyone have a spectrum analyser capable of solving this one. It will need high resolution not only in the amplitude direction but also in the frequency axis as well.

AlanH

<{POST_SNAPBACK}>

6817 carriers in a 7Mhz channel is close to 1026hz spacing between carriers.
I doubt anyone that owns a spectrum analyser that could resolve such a signal would want to comment to a bunch like us that are only postulating on what goes on inside a chip thats complexity is way beyond our grasp.

In the days of analog transmission it was easy for technicians to understand the technology down to component level but technology has changed to the point where we understand the principals that occur but how each chip functions is left in the hands of those who design and manufacture.A sad reality for us techo types that enjoyed hands on approach for years.

Most techs understand how DVD,s work but the reality is that they are not worth repairing.Digital STB's fall into the same category .