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James T Kirk

SFN's From James T Kirk As Posted 20/8/2005

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Posted August 20, 2005 · Report post


Digital Television and the SFN

SFN

A Single Frequency Network as applied to Digital Television Broadcasts in Australia is a technique that allows multiple transmitters with overlapping coverage to operate on the same channel. Steps are taken to minimise the risk of interference between transmitters in these overlapping areas by careful control of output power and timing. It is not a perfect solution and particular care needs to be taken by antenna installers to achieve a reliable service in these overlapping areas.

Note on DVB-T

In Australia, all DVB-T broadcast channels occupy 6.656250MHz of their 7 MHz bandwidth with a Coded Orthogonal Frequency Division Multiplex (COFDM) signal comprising 6817 carriers spaced at 976.563Hz apart. Most of these 6817 carriers are quadrature amplitude modulated (QAM) and every carrier is capable of carrying different information. A few carriers are fixed to a known amplitude and phase for the purposes of synchronisation and referencing and the majority carry the sound ,vision and text etc i.e. the payload is 6048 carriers. Generating and modulating each of these 6817 carriers individually was unthinkable until recent times. Now a process called Inverse Fast Fourier Transformation (IFFT) allows all 6817 carriers to be generated at once in a manner that spaces them specifically to minimise interference between each carrier (the coded orthogonal part of the process).

The 7MHz system as used here with error correction capabilities versus desired payload allows for data rates around 19 Mbit/s (Seven and SBS) and 23 Mbit/s (ABC, Nine and Ten).

Unfortunately we cannot insert pictures on this forum so I’ll try to describe a constellation pattern and how we can use it to observe if the QAM signal is working well or not and how received noise affects this display. The version of QAM used here is 64QAM, this means for the 6048 payload carriers there is 8 discrete phase changes of a carrier and 8 discrete levels of amplitude change, from this we have 64 combinations of amplitude and phase. If you imagine an 8 by 8 matrix of squares on a page and place a small dot in the centre of each of the 64 squares this would represent an excellent received signal. Keep in mind that a small dot here means the target is being hit in the same spot time after time after time. The position of the dots away from the centre of each target gives the operator of the transmitter information on the health of the transmitter. For those at the other end receiving the signal, the dots should be always found to be in the centre of the target. Now if these small dots represent a target being hit at the same point continuously and noise is introduced, the dots being fired at the target are shifted and what was a defined small spot grows larger with increasing noise and its edges become fuzzy. Eventually the whole 8 by 8 matrix becomes a blur, but the system keeps going. All is still well so long as it still lands somewhere in its own target. Should it hit an adjacent target or worse, an error is registered and then the bit error ratio (before viterbi) starts to worsen. If things continue to deteriorate the programme is lost very quickly.

If you have noticed that you get sound blips or picture freezing occasionally when the fridge or dishwasher etc starts, then you probably have a marginal signal in your area, a poor installation, you’re in a mush zone or a bit of all. However I only seek to mention it here because it also indicates a noteworthy characteristic of DVB-T and that is as the received signal becomes weaker, reception becomes more susceptible to impulse noise. This is because the data in the payload carriers is amplitude and phase modulated (QAM) and any amplitude modulated signal can be affected. Anyone who has listened to AM radio has heard interference from car ignition systems, kitchen appliances and lightning etc. These common place interferers that cause noise on AM radio, may cause horizontal interference lines on analogue television and corrupted data in DVB-T causing intermittent freezing and audio blips or complete loss because the targets in the constellation as describe above have been missed.

Simultaneous amplitude and phase modulation has been around and in use by us all for a long time. Some recent examples are Stereo AM radio and colour television. When AM radio became AM stereo, phase modulation had been used to allow left-right information to be transmitted and in 1975 when colour television started the colour sub-carrier was added and its amplitude and phase determined the colour and its intensity.

Despite its vulnerability to impulse noise, the extremely complex error correction techniques manage in the majority of cases to cope with such impulse noise issues. It will not fix everything though, the receiver can’t recover the data if the signal is too low compared with the impulse noise.

One of the nice things about digital television is that there is no ghosting of the picture, in fact ghosting isn’t possible. The receiver is able to disregard, up to a point, reflected signals from other sources such as buildings, water towers and hills. Built into the transmitted signal is a guard interval, 1/8 for (Seven and SBS) and 1/16 (ABC, Nine and Ten). This allows for reception of the main signal and echoes successfully so long as the echo delay does not exceed 128uS for (Seven and SBS) and 64uS (ABC, Nine and Ten). This level of tolerance to reflected signals or echoes allows the digital signal to be successfully received whereas the analogue could be rendered unwatchable. Furthermore it is this tolerance to echoes that allows the possibility of a SFN.

The SFN

As any good television antenna installer knows, the choice of receive antenna is only a portion of making a reliable service for the consumer. Cable lengths, the location of the antenna and its height on the home is all-important and makes a substantial difference to the result. The reason for taking such care is the surrounding topography and buildings can play havoc with predicted received signal levels. The very same topography issues are considered when decisions are made for locations of television transmitters. Once the transmitter location is decided upon, it will be a compromise, designed to get the best coverage to most of the people, however some will miss out. In the past with analogue television, smaller translators (transmitters that receive the main transmitted signal and retransmit it on another channel) were used to help cover those who missed out. You can do the same with digital television however there is not enough spare channels available while analogue is still to air. The solution was to have them all on the same frequency. As mentioned above, digital television receivers can cope well with echoes, so if you can time a second and third transmitter’s output so it appears as an echo within the guard band of that channel at the receiver, it will be treated as an echo and stable pictures and sound are produced. To support this, all transmitters in the SFN are fed with the same programme at around the same time, usually delivered by microwave link or optical fibre. There is timing information contained within the data stream and while the main transmitter is not delayed, the others are delayed to ensure their signals fall within the guard band of the main transmitter in its area influence. All are referenced to the GPS system to ensure each transmitter is on frequency and data rate is correct.

Where does it go wrong (Mush Zones)

Whether the topography is flat or mountainous there are areas where it all goes wrong. There will be areas where more than one transmitter within the SFN is received and their timing will be outside the guard band. By design these are preferred to be placed over water or in unpopulated areas as the topography permits. There will also be areas where they do fit within the guard band but are of similar levels and the receiver struggles to cope with deciding which is the main signal and which is the echo. These can be very difficult to sort. If you are installing an antenna in these areas, suddenly, rather than just knowing the gain and front to back ratio of a receive antenna, you also need to know the shape of its pattern and if it has any considerable acceptance on side lobes. The reason here is this antenna needs to be able to discriminate between the two or three signals ahead of the receiver thereby providing the receiver a signal it can cope with. In such difficult locations, knowing from what directions signals need to be avoided becomes as important as knowing the direction of the one you need.

I hope this goes some way in helping to understand an DVB-T system operating as an SFN

James

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Easy to follow and informative. Thanks for reproducing this, James Tiberius.

[How anyone could seriously suggest you lack knowledge about SFNs (as recently occurred in another thread), is beyond me.]

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11 hours ago, MLXXX said:

Easy to follow and informative. Thanks for reproducing this, James Tiberius.

[How anyone could seriously suggest you lack knowledge about SFNs (as recently occurred in another thread), is beyond me.]

You're welcome MLXXX

On reading this post of 12 years ago, there's a few items in there that could be updated like changes to Seven and SBS, clarifications on constellations and more detail on timing in SFN's, but overall considering this was an off the cuff response to some woefully inaccurate posting by the originator, it's not bad.

James

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On 6/21/2017 at 0:33 AM, MLXXX said:

Easy to follow and informative. Thanks for reproducing this, James Tiberius.

[How anyone could seriously suggest you lack knowledge about SFNs (as recently occurred in another thread), is beyond me.]

MLXXX - I can't find the post you're referring to, can you post the which discussion it was in so I can have a read (and laugh)?

James - I haven't previously read your post about SFN - very informative.

 

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13 minutes ago, Colin Matten said:

MLXXX - I can't find the post you're referring to, 

This is the post I was referring to: http://www.dtvforum.info/topic/226127-1-million-norwegan-cars-dab-total-cars-registered-2-6-million/?do=findComment&comment=2096967\

 

Of course to view James T. Kirk's original posting of his off the cuff comments about SFN you can simply click on the first hyperlink in  post #1 of this thread. 

 

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29 minutes ago, MLXXX said:

This is the post I was referring to: http://www.dtvforum.info/topic/226127-1-million-norwegan-cars-dab-total-cars-registered-2-6-million/?do=findComment&comment=2096967\

 

Of course to view James T. Kirk's original posting of his off the cuff comments about SFN you can simply click on the first hyperlink in  post #1 of this thread. 

 

It was the first part I wasn't able to find - thanks for the link.

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