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alanh

Single Frequency Networks (SFN)

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Thanks all of you!

I can sure about our SFN network is satisfied all condition of SFN, include GPS, same TS with MIP, same frequency, TX with SFN mode.

And we using Horizontal polarised for all TX stations. About manufactures, they only provide equipment and it is difficult to ask them about problems like this.

Additionaly, the distance between Tx1 & Tx2 station about 15 km. So, i don't think it is effected by delay.

Sory for inconvenience, i have not really understood our problem. In your country, this problem does not happen ?

Thanks & regard

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Thanks all of you!

I can sure about our SFN network is satisfied all condition of SFN, include GPS, same TS with MIP, same frequency, TX with SFN mode.

And we using Horizontal polarised for all TX stations. About manufactures, they only provide equipment and it is difficult to ask them about problems like this.

Additionaly, the distance between Tx1 & Tx2 station about 15 km. So, i don't think it is effected by delay.

Sory for inconvenience, i have not really understood our problem. In your country, this problem does not happen ?

Thanks & regard

Hi Lunar_man,

What sort of link is used to get the Transport Stream to the two transmitters? I assume that the stream is either ASI or IP based but regardless of that, the most important thing is to make sure that no rate adaptation takes place that destoys the timing constraint of the stream. Are they dedicated point-to-point (dark fibre) links or are they carried as a 'service' by a Telco?

I would expect the modulators to alarm if there were any discontinuities in the incoming stream.

Finally, what type of receiver do you test with, a domestic STB or a professional IRD?

Cheers

Rusty

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Hi Lunar_man,

What sort of link is used to get the Transport Stream to the two transmitters? I assume that the stream is either ASI or IP based but regardless of that, the most important thing is to make sure that no rate adaptation takes place that destoys the timing constraint of the stream. Are they dedicated point-to-point (dark fibre) links or are they carried as a 'service' by a Telco?

I would expect the modulators to alarm if there were any discontinuities in the incoming stream.

Finally, what type of receiver do you test with, a domestic STB or a professional IRD?

Cheers

Rusty

Hi Rusty,

Our transmission line using fibre point-to-point with IP. Then, IP signal is converted to ASI by IRD before put into trasmitter.

If input signal is not good ( no input, no MIP, datarate, SFN delay...), the modulators will alarm. And we using Promax product to test signal.

thanks & regards

Lunar

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Hi Rusty,

Our transmission line using fibre point-to-point with IP. Then, IP signal is converted to ASI by IRD before put into trasmitter.

If input signal is not good ( no input, no MIP, datarate, SFN delay...), the modulators will alarm. And we using Promax product to test signal.

thanks & regards

Lunar

Well, if we can trust the IRD's to recover the stream without shuffling packets (what brand are they?) then we should trust that the input to both tx's is correct.

As for the GPS timing, do you use an external GPS clock or has the R&S modulator a built in one? If they are external, are they the same make / model?

I have seen a case where two different GPS devices were used with different duty-cycles, one was 50%, the other's 1pps impulse was more a needle shape, say 10%. Depending on whether the R&S modulator uses the rising or falling edge of the impulse the outcome may be quite different.

I don't know the R&S product too well and I don't know whether you can select whether to use the rising or falling edge of the 1pps impuls, but it may be worth checking.

Cheers

Rusty

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Well, if we can trust the IRD's to recover the stream without shuffling packets (what brand are they?) then we should trust that the input to both tx's is correct.

As for the GPS timing, do you use an external GPS clock or has the R&S modulator a built in one? If they are external, are they the same make / model?

I have seen a case where two different GPS devices were used with different duty-cycles, one was 50%, the other's 1pps impulse was more a needle shape, say 10%. Depending on whether the R&S modulator uses the rising or falling edge of the impulse the outcome may be quite different.

I don't know the R&S product too well and I don't know whether you can select whether to use the rising or falling edge of the 1pps impuls, but it may be worth checking.

Cheers

Rusty

Hi Rusty,

thanks for your information.

Sory for inconvenience, but maybe you are understanding wrong about our problem.

Please note the, our SFN network run good almost places.

ONLY!!! some places happen this problem as i said.

thanks & regards

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Hi Rusty,

thanks for your information.

Sory for inconvenience, but maybe you are understanding wrong about our problem.

Please note the, our SFN network run good almost places.

ONLY!!! some places happen this problem as i said.

thanks & regards

OK,

If only some area's are affected, where are these area's in relation to the transmitters, are they 'in between' the tx's or are they more in the fringe area's (away from the transmitters)?

I'm not a network planning expert but adjustment of individual delay offsets, or adjusting the guard interval may be required.

There will always be situations where the issues have to be addressed at the receive end by optimizing the directivity of the antenna.

Cheers

Rusty

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CWulf,

If the ACMA doesn't have a policy of alternating polarisations to minimise problems it has a funny way of showing it.

Check the transmitter lists links in the "Get the best reception posts for Regional Qld for Cairns area, Sunshine Coast, Gold Coast, Newcastle, Central Coast, Melbourne (Dandenongs, Eastern Port Phillip Bay).

The only exception is in Sydney between Kings Cross and Manly Sewage Works, because nearly all of the distance between them is over the waters of Sydney Harbour. There isn't many viewers there!

AlanH

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The only exception is in Sydney between Kings Cross and Manly Sewage Works, because nearly all of the distance between them is over the waters of Sydney Harbour. There isn't many viewers there!

I gave the example of SBS (Channel 34) from Artarmon, Kings Cross and North Head that are all horizontally polarized and overlapping. Following your logic, no one at the lower north shore can watch SBS digital.

As CWulf correctly points out, the question whether any digital transmission (MFN or SFN) is H or V polarized rests on the fact which antenna set-up was available on the TX site and, of course, they were virtually all built with an analogue network in mind.

Please try to distinguish between opinions and facts, I don't think it helps Lunar Man (or anyone else) in any way if one present 'facts' that are just based on ones 'opinion'.

I have disclosed to Lunar Man that I'm not a network planning expert, so he should take my opinions just as opinions. If he would ask anything related to Transport Streams I would be able to answer them with some authority.

Cheers

Rusty

Edited by Rusty Juggler

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Additionally is the signal delay feeding the transmitters adjusted to make the signal arrive at halfway between the transmitters simultaneously. The relative delay may be adjusted to make the "centre point" over a less densely populated area.

AlanH

All ACMA allocations use alternate polarisations moving in a particular direction. For example Bouddi and Wyong are vertically polarised and Gosford which is in between is norizontally polarised along with the high powered Newcastle transmitters.

The reason for this is to make the area where similar strength but delayed signals as small as possible.

AlanH

All the above statements by AlanH are wrong and may have been addressed earlier in this same thread or related one's where in previous years it became necessary to refute his misinformation.

Specifically:

(1) "Additionally is the signal delay feeding the transmitters adjusted to make the signal arrive at halfway between the transmitters simultaneously."

This is utter fantasy and demonstrates no knowledge of what is actually done, it's not even close to the implemented technique.

(2) "All ACMA allocations use alternate polarisations moving in a particular direction. For example Bouddi and Wyong are vertically polarised and Gosford which is in between is norizontally polarised along with the high powered Newcastle transmitters.

The reason for this is to make the area where similar strength but delayed signals as small as possible."

Mercifully, this is one of truly silly statements by AlanH that is easily refuted and my explanation will be understood by any reader here, both the technically savvy and those whose skills reside in other fields. The reasoning is that the population have existing antennas for their analogue television services prior to arrival of digital television in 2001. If you wanted to introduce new digital television to the masses it makes sense to give them that new service without causing the public the need to change their antenna system on their homes. So this is exactly what happened, digital television was given the same polarity as analogue.

For anyone interested in the operation of single frequency networks they would be wise to source their information from just about anyone other than AlanH .

James

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James Kirk,

The DAB+ repeaters in the CBD of Melbourne are still on air despite your protestations. Eventually the Moderator had to stop the strand due parttly to your posts.

Alanh

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James Kirk,

The DAB+ repeaters in the CBD of Melbourne are still on air despite your protestations. Eventually the Moderator had to stop the strand due parttly to your posts.

Alanh

AlanH

I would consider that most people reading this thread would not be conversant with the mess you made of describing what happened for the Melbourne DAB+ repeaters, let alone your decision to bring the subject up again here, I suspect most would be quite perplexed.

All that aside, your irrelevance in any informed SFN discussion is unquestioned.

James

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If one re-reads the DAB+ repeater thread it becomes abundantly clear why the thread was closed and it was not at all due to posts made by James T Kirk, nor ChaosMaster, nor MLXXX, nor ....

The thread appears to have been closed because when faced with reality alanh, in typical fashion, started misquoting documents claiming that they proved his point and essentially kept digging his hole deeper and deeper and deeper until even the overly tollerant moderators of this forum couldn't stomach it any longer.

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OK,

If only some area's are affected, where are these area's in relation to the transmitters, are they 'in between' the tx's or are they more in the fringe area's (away from the transmitters)?

I'm not a network planning expert but adjustment of individual delay offsets, or adjusting the guard interval may be required.

There will always be situations where the issues have to be addressed at the receive end by optimizing the directivity of the antenna.

Cheers

Rusty

Hi Rusty,

They are are more in the fringe area's (away from the transmitters) as you said.

Some our configurations for SFN network, maximum delay 900,000 us, guard interval 224 us, network delay 514 ms ( adjustment in IRD's delay buffer).

As i knew, with this guard interval ( 224 us) and the distances from these area to transmitters less than 10 km, it will have no SFN interference.

Please advise me any thing in Transport stream, adjustment delay, measurement method....

thanks & regards

lunar_man

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Hi Rusty,

They are are more in the fringe area's (away from the transmitters) as you said.

Some our configurations for SFN network, maximum delay 900,000 us, guard interval 224 us, network delay 514 ms ( adjustment in IRD's delay buffer).

As i knew, with this guard interval ( 224 us) and the distances from these area to transmitters less than 10 km, it will have no SFN interference.

Please advise me any thing in Transport stream, adjustment delay, measurement method....

thanks & regards

lunar_man

Hi Lunar_man,

How was the maximum delay value in the MIP determined? It should take into account your worst case actual delay, but also has to cater for any processing delay i.e. a modulator has to be able to process the entire megaframe befor the next 1PPS pulse arrives.

Why do you add 514 ms delay in the IRD?

If one (or some) transmitter(s) are delayed too much, they may end up being sync'd to the next 1PPS pulse and no alarm will be raised. Remeber that the SFN timing is relative, not absolute. A few broadcasters here in Australia use satellite distribution for their SFN, their delay budgets are much longer than the maximum delay value set in the MIP but because the transmitters don't know this, they just sync to the next pulse regardless. This has been working very well for many years but needs a lot of carefull planning.

Again, without having detailed knowledge of your system this remains a guessing game.

Regards

Rusty

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Hi Lunar_man,

How was the maximum delay value in the MIP determined? It should take into account your worst case actual delay, but also has to cater for any processing delay i.e. a modulator has to be able to process the entire megaframe befor the next 1PPS pulse arrives.

Why do you add 514 ms delay in the IRD?

If one (or some) transmitter(s) are delayed too much, they may end up being sync'd to the next 1PPS pulse and no alarm will be raised. Remeber that the SFN timing is relative, not absolute. A few broadcasters here in Australia use satellite distribution for their SFN, their delay budgets are much longer than the maximum delay value set in the MIP but because the transmitters don't know this, they just sync to the next pulse regardless. This has been working very well for many years but needs a lot of carefull planning.

Again, without having detailed knowledge of your system this remains a guessing game.

Regards

Rusty

Hi Rusty,

At the moment, the maximum delay in our system is 900ms. We add 514 ms delay in the IRD to remove jitter incident in IP network.

And i think network delay ( example 514 ms) will not affect to transmitter when it is less than maximum delay (900 ms).

The transmitter will stop radiated RF when network delay of input signal is larger maximum delay.

thanks & regards

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The transmitter will stop radiated RF when network delay of input signal is larger maximum delay.

Not entirely true, as I said before, if the sum of the actual network delay and processing delay exeeds the 'maximum delay' by a fair margin, the TX will time the megaframe against another 1PPS pulse. The example I gave with Satellite distribution uses such an 'offset', but as all tx's are in the same time domain it doesn't matter. You could delay a transport stream by minutes or even hours, it would still play out against the current SFN timing as the timing is not absolute but relative to the next 1PPS impulse.

What are the chances, in your network, that one or a two transmitters are 1 second out of step because the sum of the actual network delay and the IRD delay exceeds 900ms by enough margin to be in the next 1s window?

Have all network links been actually measured?

Regards

Rusty

Edited by Rusty Juggler

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It has recently been suggested in another thread that this pinned thread could be a candidate for being unpinned:

...Not to mention the ones like Single Frequency Networks, which is both embarassingly wrong and largely irrelevant now.

They should probably all be unpinned and allowed to die gracefully, rather than taking pride of place at the top of each subforum.

I myself think this particular thread still serves a purpose, though not quite the one intended by the opening poster. Numerous errors of the opening poster are addressed in the course of this thread. Thus, this pinned thread has been serving (and could continue to serve) as a caution for new members of DTV Forum, to take the views of the opening poster with a grain of salt.

In my opinion, a particularly helpful contribution in this thread is post #26 by James T Kirk, which provides an easy to digest and informative overview of how DVB-T can be operated as a Single Frequency Network.

One of the more surprising claims made in this thread by the opening poster is that the COFDM transmission is put through a limiter in the receiver prior to demodulation:

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.

I thought bellotv gave an easy to follow explanation as to why using a limiter would be an odd thing to do:

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.

And as M'bozo stated:

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

Why alanh thought that limiting prior to demodulation would be used in a receiver when the overall power envelope of a DVB-T signal varies markedly as a function of time, and when the multiplicity of individual carriers contributing to the power envelope are modulated both in relation to amplitude and phase, is not immediately clear. I note that a spectrum analyser testing voltages or currents at any particular sub-carrier frequency would not be a particularly helpful tool for determining whether the sub-carrier were being amplitude modulated.1

I note that DVB-T transmitters are required to be linear in order to comply with the requirements of the broadcast authority as regards spurious emissions, and in order to provide a clean signal with low MER for receivers. The ratio between the peak power and the RMS power (the crest factor) is a statistical function, and the average value varies depending on the modulation constellation in use. However, rarely, extremely high peaks would occur when the subcarriers, by chance, instantaneously reinforced each other. In practical transmitters these extremely high peaks are clipped, as mentioned in this 700 KB pdf about issues that arise when combining signals from different DVB-T transmitters: http://cdn.rohde-sch...02/7TS02_2E.pdf. A technique for reducing the basic crest factor of a DVB-T transmission (thus reducing the need for clipping) is mentioned in this 987KB pdf: http://www.rohde-sch...rty_Flyer_e.pdf.

Intentional clipping in DVB-T transmissions affects such a small percentage of the transmission that the MER in the receiver can be maintained at an acceptable level. A slight drop in the level at which clipping commenced would affect a higher proportion of the transmission, and if the clipping level dropped even further (taking the transmission MER below permitted specifications), a point would be reached of massive, uncorrectable, error in receivers.

Alanh,

here is an (admittedly very late) opportunity for you to provide a reference source to back up your claim that a DVB-T receiver uses uses a limiter "to remove amplitude variations in the signal prior to demodulation"; or, alternatively, you may wish now to withdraw that claim.

____________________

Edit 1/8/13: This claim now appears as entry 006 in the alanh "facts".

1 In a different context, using a spectrum analyser for the output of a broadcast band AM transmitter on a carrier frequency of say 1MHz with a 1kHz test tone at 99% modulation, the spectrum analyser would read a constant amplitude of RF energy at 1MHz, whether the amplitude modulation were on or off. The spectrum analyser averages over time, and in any event its effective Q is high in order to be able to distinguish between 1.000MHz. and a sideband at 1.001MHz.. For readings of cyclic variations in radio frequency amplitude of a broadcast band AM transmitter, an oscilloscope is the traditional, and more appropriate, tool. The suggestion of using a spectrum analyser for the extremely closely spaced sub-carriers of a DVB-T transmission is not helpful. It would however be a simple matter to view the overall envelope of the transmission; and it is that overall envelope that a pre-demodulation limiter in a DVB-T receiver would actually limit. If a practical demonstration were desired of the effects of limiting, a low power DVB-T exciter could be programmed to perform limiting to a progressively lower than usual threshold, and the progressively increasing MER in a nearby receiver monitoring the exciter output could be observed. Also, the point at which failure in audible output of the radio occurred could be noted.

Edited by MLXXX

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Hi MLXXX,

I pretty much agree with what you've said; my main thought was about the other threads which have had their initial post removed. The whole reason they were stickied in the first place was so the information could be continually updated and easily found in the first post - now that's gone, they're just another {thread about the subject}. I didn't check all of them, but the ones I did check (some of the shorter ones) don't even have much additional or current info in the comments.

This one I mentioned only as a standout, because the initial post is pretty much completely wrong (e.g. "DVB-T is used for Digital TV in all areas except North America", the whole 'it normally uses 2k mode, but 8k is used for SFNs' [paraphrased], etc) or completely confused (e.g. the stuff about 'slowing down the data rate', etc). The corrections and explanations by JTK, bellotv, and M'bozo are excellent - but it's a bit stupid for such a completely broken post to be given prominence at the top of the sub-forum.

My idea would be to have anyone who's interested write up new posts for inclusion as stickies, remove the sticky status from the existing broken / wrong ones, and let them disappear into the archives. Ideally the old ones would still be linked where there's relevant info, but much of it could be summarised and developed into new updated and accurate stickies.

Anyone interested? I intend to go through all the stickied threads in the next day or so and sort them into

  • Ones that have had their main post removed
  • Ones that have overwhelmingly incorrect or outdated info in their main post
  • Ones that are out of date and would benefit from being re-written and updated

When I do, I'll post the list in the Suggestions sub-forum, see if anyone wants to take particular topics over, and see if the mods will un-sticky at least the broken threads.

Edited by Malich

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Anyone interested? I intend to go through all the stickied threads in the next day or so and sort them into

  • Ones that have had their main post removed
  • Ones that have overwhelmingly incorrect or outdated info in their main post
  • Ones that are out of date and would benefit from being re-written and updated

When I do, I'll post the list in the Suggestions sub-forum, see if anyone wants to take particular topics over, and see if the mods will un-sticky at least the broken threads.

One aspect to consider, of course, is whether a stickied topic is needed at all! Many times I think it is a case of a thread growing merely by virtue of others seeking to correct or clarify the OP's claims. (This might have made it appear to the mods that the topic was of relevance, interest, and/or importance. )

If redoing some of the DTV Forum stickied topics, I wonder whether it might in some cases be enough to provide a quick introduction and then link(s) to Wikipedia. I've observed that Wikipedia pages on technical aspects of TV and radio invariably contain footnotes linking to reference material, and typically are concisely and helpfully presented, and with good accuracy. [it is only very rarely that I have thought a Wikipedia entry so seriously misleading that I have been moved to correct it, and of course provide substantiation of my correction (visible in the Wikipedia "view history" tab).]

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One aspect to consider, of course, is whether a stickied topic is needed at all! Many times I think it is a case of a thread growing merely by virtue of others seeking to correct or clarify the OP's claims.

Aside from that, my personal opinion - and, to be absolutely clear to everyone, it really is just my opinion - is that in some places there are probably too many stickied topics, and some of those shouldn't really be sticked (although the information in them should be stickied somewhere). An example would be the PVR sub-forum, where there's six stickies - most of which overlap somewhat - including some where the main post isn't really sticky-worthy in its own right e.g. the 'Alternative Topfield Website' sticky.

(Not to say that particular example shouldn't be a sticky in some form - 350+ posts in 4 months pretty much indicates it should be! - but it should probably be a traditional 'stickied' post with the current info from the whole thread summarised in the opening post. There's plenty of senior members helpful & active in that thread who could argue amongst themselves who's best suited to adopt the subject, start a new thread to be stickied, and keep the opening post updated.)

If redoing some of the DTV Forum stickied topics, I wonder whether it might in some cases be enough to provide a quick introduction and then link(s) to Wikipedia. I've observed that Wikipedia pages on technical aspects of TV and radio invariably contain footnotes linking to reference material, and typically are concisely and helpfully presented, and with good accuracy.

I'd generally agree, but I think the 'quick introduction', with added local relevance, is the important bit. My personal take on the Wikipedia entries on many of these subjects is that they're somewhat confused in structure and don't do a very good job of leading from intro to intermediate to detailed level. Many seem to be semi-random cut-and-paste jobs from a variety of sources aimed at different levels - and the end result is that to find what you want to know, at a level you understand, you basically have to read through the whole thing.

Then there's things like the following sentence from the second paragraph of Wikipedia's DAB entry:

"DAB is more robust with regard to noise and multipath fading for mobile listening, since DAB reception quality first degrades rapidly when the signal strength falls below a critical threshold, whereas FM reception quality degrades slowly with the decreasing signal."

Which, yeah, OK, technically is right - but it only makes sense if you either (a) just accept it as true, or (B) already know enough to understand what they're getting at. At a 'thinking intro' level, it makes no sense - you'll end up wondering "But how can DAB be more robust if it degrades rapidly?" and things like that...

But yeah, enough relevant info to cover the subject for the likely audience with links to supporting sources, extended info, and additional resources like Wikipedia, would be ideal.

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