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alanh

ABC now promoting DAB+

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The ABC management has been kicked by the politicians and others because they switched off all receivable radio to most of the NT and surrounding areas. The money is to be used instead to pay for DAB+ transmitters in Darwin, Gold Coast, Canberra and Hobart. It has been pointed out that the reason they think that there was no listeners for NT short wave was that they do not promote it or even acknowledge its existence except for a few lines on their Reception line site. Only a few know it existed. Their location information recommends AM and FM transmitters thousands of km away!

Similarly they are constantly advertising listen on the phone via the app, which costs the listener to listen, where as with DAB+ it is free after you buy a receiver. I pointed out to the parliamentary enquiry that they don't advertise DAB+ limiting the number of listeners. The reason they are doing this is because in the 1990s the ABC in the Eastern States were converted to high powered FM except for the capital cities because they were scared of loosing listeners. If they had bitten the bullet, now mobile phones' FM tuner would work on their programs as it is now, no phone receives AM so they want you to use an app instead.

http://www.abc.net.au/radio/perth/dab+-radio-upgrade/8622144

They also have been playing an "advertisement" for DAB+ in the breakfast program weekdays.

Alanh

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Hi Alan.  Haven't you heard the ABC say "On the radio, on mobile and on digital"? or "On the radio, online and on digital?" or similar?

 

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All a bit pointless promoting it when DAB coverage is still quite flakey and the ABC's digital coverage is the worst.

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My issue with ABC advertising is that often their information is wrong.

They promote a radio program, without pointing out that it is only available in State Capitals.

They also promote a radio program as available on "digital radio" when it is not available on DAB+.

To their credit, they remove incorrect information from their websites, when I send them an e-mail. However, they never reply to my e-mails.

 

 

 

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And they have gone back to their old habits in Perth of continuing after the breakfast program with their Saturday morning Sports Talk program on the local ABC Perth digital service instead of breaking away as they should to the nation wide alternate program. They only do this in the non daylight saving period.
The sport program is also broadcast on the Grandstand digital service as well as the normal statewide AM and FM local radio stations so there is absolutely no need for it to be on the local Perth ABC radio digital service too. I had correspondence with the station manager two years ago when they started to do it and was told that it was to get more coverage via the digital service. (Edit: It was also suggested that I could listen to a rebroadcast of Glynns' program "It's Just Not Cricket" on Sunday morning at 4:00 am or some such ridiculous time!!! Or I could listen on line - but I didn't get a digital radio to be reliant on being tied to the computer) I did go on to point out that there was already the coverage on digital through the Grandstand channel. However a few weeks later as I recall it did change, but in hindsight I think that was because daylight saving started again. Last year they got it right but as I said they have got it wrong again this year ever since daylight saving finished.

Hey Alan, maybe with your alleged connections you can get it sorted. After all, you have said that Glynn Greensmith's program is worth listening to, so surely you too are not happy about their nonbroadcasting of the first hour here in Perth? And Glynn often promotes the alternate program availability on digital of his and other programs when sport is on, so for Perth to NOT broadcast it is surely incorrect by the programmers.

Edited by hrh

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Ron,

In the ABC's terms digital means on line either via the internet or by phone not by DAB+ transmitters.

Liz,

Where I am the coverage areas compared to the commercials is identical and I am talking of the edge of the coverage area where both ABC and commercial signals break up.

DAB+ repeaters are still required on TV translator sites which do not have substitutes such as Bilgola replaces Bouddi where there is to be no coverage in the adjacent licence areas.

After all the DAB+ transmitter power is the same as the TV power from the main transmitter sites and the channels used surround the DAB+ channel giving similar coverage areas.

What are you using for a receiver and what is the antenna. If it is inside, is there a metal roof or a unit above you? Have you tried listening to DAB+ in one of the new cars so equipped?

Hrh, I am a big fan of Glen's program but he is a contractor who has another job during the week outside of the ABC.

The ABC seems to be unaware that all telcos sell 1 GB of prepaid data for $10 per month. Their programs are sent to the phone at 64 kbit/s.

I wrote to the producer of the Drive program who keeps recommending that listeners use their phone between 5:30 - 6pm on Fridays and some Thursdays when the footie is on. I pointed out to her that if a listener listens to her program every weekday for a month and a half the listener would have paid for a DAB+ radio from a chain store.

Remember that the ABC rejected an offer in the 1990s for metro local radio stations to go FM. They thought they would loose listeners. The high powered FM local radio network was rolled out down the eastern seaboard an is on air today.

 

Alanh

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El Saif,

The ABC can and does split their networks between capital cities and country areas. For example the advertising I mentioned in my initial post was in the breakfast program is local. For example Dan Bourchier does breakfast, Genevieve Jacobs mornings and Adam Shirley Drive where as their Sydney equivalents are Robbie Buck, Wendy Harmer and Richard Glover.

The only DAB+ promos I have heard have been in the local breakfast program.

Alanh

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11 hours ago, alanh said:

Hrh, I am a big fan of Glen's program but he is a contractor who has another job during the week outside of the ABC

And that has what to do with them not broadcasting the first hour of his program in Perth as well as the two programs preceding it after the breakfast program which is available to rest of the DAB+ audiences in the other cities? By the same thinking then, they may as well not be broadcasting part of Tracey Vo's programs!!!

As per usual you are telling me what I already know, (not that I have said so as such) that being I am fully aware that Glynn has a job at one of the tertiary institutions, because he is who I first corresponded with regard to the non-broadcasting of his program two years ago.

Edited by hrh

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Alan, I'm using the DAB radio in my 2016 Camary Atara as a reference point. In home dab reception west of Blacktown is very hit and miss and has been ever since DAB+ launched. Mobile reception via a car radio is too patchy to be useful. Large trucks moving by me on the highway is enough to see the audio disappear for a minute or two usually during the most interesting part of an interview. So even though I've got a DAB equipped car we use AM/FM only because it reliably covers the Sydney area something DAB is a long way from yet.

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1 hour ago, lizclinton said:

So even though I've got a DAB equipped car we use AM/FM only because it reliably covers the Sydney area something DAB is a long way from yet.

Yeah,roll out and acceptance is very slow,probably a decade or more before its a feasible alternate to FM.

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Liz,

When DTV started there were quite a few posts about unreliable reception in Penrith and surrounding areas. There was a few causes for this, one is the level of interference due to industrial activities including high voltage power lines, the distance from Artarmon and the terrain.

When television started in 1956 there was only a very small population in Western Sydney and with the deep valleys along the coastline, it was decided to put the TV transmitters in Gore Hill, Willoughby and later Artarmon. If this decision was to be made today I suspect that another high point further west would have been used. Since all fixed outdoor antennas are directional it is unlikely that the main Sydney transmitter site will change. The solution has been to create a network of UHF translators to fill in the shadow areas. Considering that on the main transmitter sites DAB+ transmissions are between TV channel "9" and TV channel "10" and all transmitters including digital radio are at the same power. 

The coverage area for TV is defined as reliable reception from antennas up to 10 m above the ground. This is unlikely to be the case for DAB+ for headphone portable, tabletop and car radios.

The current TV translators in the Sydney licence area on the left and the DAB+ repeaters are in italics

far Northern Beaches (Bouddi)  Sydney Northern Beaches (Bilgola)

Sydney North West (Kurrajong Heights). Penrith (Hawkesbury)

Manly/Mosman, Collaroy Plateau (Collaroy) which should reach Manly

Kings Cross for Sydney City, Neutral Bay and the Eastern coastal suburbs  Sydney City 100 Market St for Sydney City, Neutral Bay and possibly Mosman

Sydney South (Redfern)

Sydney South West (Razorback), Campbelltown (Gregory Hills)

Picton,

Woronora

What is needed is to produce a second http://myswitch.digitalready.gov.au using the same program but inserting an antenna height of 1 metre instead of 10 m and the height of the DAB+ antenna on the Artarmon transmitter and the sensitivity of DAB+ receivers. A new colour should be added to indicate indoor reception to include and allowance typical absorption of building materials. and also including the repeater sites. Hopefully it will discover the areas of insufficient signal strength in your area and other.

How reliable Is the FM radio reception (does the stereo indicator stay on) where the DAB+ disappears?

I ask this question because the main FM transmitters are 160 kW and commercial 150 kW but the DAB+ is 50 kW.

If stations use the alternate frequency system where on DAB+ and FM RDS a table of alternative sources of the same program will cause the receiver to automatically switch to the best signal. This is up to the broadcasters to transmit it, I don't know how many DAB+ receivers will respond and lastly there is a delay in encoding and decoding DAB+ signals which can cause either part of the program to be repeated or be lost on the switch.

 

Lastly I blame some car manufacturers for poor DAB+ reception because of the sharkfin antenna. is nowhere near 1/4 wavelength long (370 mm) https://aerpro.com/ct27uv57 as a better example. The sharfin are more designed for the frequencies used for GPS.

 

Alanh

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Alan, I'm not sure what all the DTV references have to do with DAB but yes we do have a bit of multi path interference around the area on FM but some flutter is easy to live with when compared to all audio vanishing at the most inconvenient moment like DAB does. Most of my listening is to 702 ABC Sydney and it's on good old reliable AM so coverage is excellent everywhere I go but I must say it sounds fantastic on DAB when I'm east of Blacktown. If only DAB coverage wasn't so poor and I find it totally irresponsible of the ABC to promote DAB as an "upgrade" to its AM broadcast when many will find themselves outside of its coverage area.

Edited by lizclinton

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Liz,

The DTV is relevant because the transmitted power is the same as DAB+ as is the transmission frequency is very similar and their transmitting antennas are on the same towers.  Some FM transmitters are on the same towers but their power is higher and their frequency is roughly half meaning that it will go round obstructions better. DTV use the same modulation system as DAB+.

The effect of multipath reception is distortion on FM if the DAB+ is subjected to the same multipath reception will cause errors. If the errors are sufficient then cannot be corrected and the receiver will mute.

High levels of reflections occur in the CBDs because of the tall buildings which also block the original signal at ground level. This is why the first DAB+ repeater was installed in Collins St Melbourne CBD. Now there is repeaters in Sydney and Perth CBD for this reason. Parramatta has tall buildings so there may be a case for another repeater on the top of one of those buildings.

You should put a complaint into http://www.abc.net.au/reception/contact/ and let us know their reply.

This is what the ABC claims to be the coverage area https://www.google.com/maps/d/viewer?ll=-33.79740899999998%2C150.99609399999997&spn=0.798849%2C0.961304&hl=en&msa=0&z=9&source=embed&ie=UTF8&mid=1kNyJ8aqkP_HS4-IuHhTgrnl2JeA

and the Commercials version of the coverage area

http://www.digitalradioplus.com.au/DigitalRadioPlus/media/DRP_Media/Coverage-Maps_Sydney.pdf

Neither of these maps have the repeater coverage and the level of detail used by the 3D computed coverage area maps on www.myswitch.digitalready.gov.au for DTV.

What is your reception like at home?

I particularly like the high frequencies and the stereo sound. It is a pity they don't use stereo sound during interviewing a guest.

Alanh

 

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While I understand that DAB operates on similar frequencies to DTV and here in Sydney the main transmitters are co located with the DTV services. As a non technical person I would have thought a 50Kw 1.5Mhz wide channel would propagate differently to a 50Kw 7Mhz wide channel radiated from the same antenna not to mention I'd expect the error correction to be far more robust on the DAB signal when compared to DTV. Looking at the coverage maps the ABC one looks exactly right from my experience and the commercial one is incredibly optimistic or just completely incorrect. We have DAB in home coverage but it's only in certain spots through the house and nothing like what anyone would call acceptable for a home well inside the licence area. How broadcasters have been allowed to have no coverage in some parts of their licence area is quite odd. I was always of the opinion that if a broadcaster chose not to serve parts of their licence area those parts were permanently surrendered from the licence area but obviously that's just not the case.

Edited by lizclinton

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Liz,

4 DAB+ channels fit into a TV channel. The total power of the channel is being measured so one would expect that the total power of the 1.5 MHz of the DAB+ channel would be 4 times higher than for a 1.5 MHz portion of a TV channel. This is an increase of 6 dB. However this increase is drop in the ocean when compared to the losses introduced by buildings, blocking terrain etc.

The question about the maps is why are they different? The pair of commercial transmitters and the repeaters are identical to the ABC/SBS transmitters and repeaters. The operating frequencies are very similar so the maps should be identical, however how were they produced?

Was it driving selected routes measuring the signal strength, which means lots of areas are unmeasured.

The myswitch website map is likely to be the most accurate because it is computed using a 3D map of the earth and well researched formulas which calculate the path losses.  It does however assume the antenna is outside and up to 10 m above the ground.  The map has no details about buildings and vegetation blocking paths. At 10 m above the ground it is less of a problem than 1 m above the ground as is the situation in cars.

Are you using the telescopic antenna fully extended and vertical? What is the roof made of? I ask this because my next door neighbour used to have a concrete tile roof and her mobile phone was fine. The roof was replaced with a steel roof. Now she has to use the phone outside. Radio waves do not travel through metal. The reason why the AM radio has a wavelength around 300 m compared to DAB+ which is around 1.4 m.

Metal blockages which are a wavelength or longer in size it will drastically reduce the signal strength. This is why you are having less problem with AM. In addition the path that your AM signal is from Prestons which is different to the one from Artarmon.

http://www.acma.gov.au/licplan/defmaps/documents/maps/la_542.pdf is the licence area for commercial/community radio stations. The Licence area is to keep other competitors out.

Please complain to the ABC reception site.

Alanh

 

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11 hours ago, lizclinton said:

While I understand that DAB operates on similar frequencies to DTV and here in Sydney the main transmitters are co located with the DTV services. As a non technical person I would have thought a 50Kw 1.5Mhz wide channel would propagate differently to a 50Kw 7Mhz wide channel radiated from the same antenna not to mention I'd expect the error correction to be far more robust on the DAB signal when compared to DTV. Looking at the coverage maps the ABC one looks exactly right from my experience and the commercial one is incredibly optimistic or just completely incorrect. We have DAB in home coverage but it's only in certain spots through the house and nothing like what anyone would call acceptable for a home well inside the licence area. How broadcasters have been allowed to have no coverage in some parts of their licence area is quite odd. I was always of the opinion that if a broadcaster chose not to serve parts of their licence area those parts were permanently surrendered from the licence area but obviously that's just not the case.

Hello Liz

Although you state you are a non technical person you are in fact much closer to the mark than those responding have chosen to acknowledge.

"As a non technical person I would have thought a 50Kw 1.5Mhz wide channel would propagate differently to a 50Kw 7Mhz wide channel radiated from the same antenna not to mention I'd expect the error correction to be far more robust on the DAB signal when compared to DTV." You are so spot on, others noting the same 50kW ERP for both DTV and DAB is an utter irrelevance as compared to the error correction capability afforded DAB+ due the adopted modulation method as you correctly note.

Responses such as this are inept and seriously flawed " 4 DAB+ channels fit into a TV channel. The total power of the channel is being measured so one would expect that the total power of the 1.5 MHz of the DAB+ channel would be 4 times higher than for a 1.5 MHz portion of a TV channel." This is an increase of 6 dB."  Possible differences in horizontal radiation pattern ie antenna system gain and differences in licensed ERP leave no doubt this is a ridiculous statement to make.

Going back to your "robust" comment, here's some technical stuff. What DAB+ does is use a different modulation scheme to DVB-T, DAB+ uses DQPSK whereas DVB-T presently uses 64QAM. (Slower data rate permits weaker signals to be recovered) In essence the difference between how much the received DVB-T signal needs to be above the noise floor over how much is needed for DAB+ is around 12dB, ie DAB+ can operate from a much weaker signal.

All three DAB+ ensembles are  being transmitted from Artarmon tonight and once you get to Blacktown, Artarmon reception is terrain dependent and the on channel repeater at Hawkesbury Heights becomes line of sight for you, again transmitting all three DAB+ ensembles.  This may the issue you are facing, your DAB+ receivers with no directional antenna are struggling with multiple sources combined with terrain reflections. If your television at home from Artarmon is stable and reliable, this would tend to support this theory.  Contacting the Commercial and National Broadcaster with some details of your reception issues could prompt an investigation.

James

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I have cleaned this thread up - it had turned less into an informative discussion or debate, but more of a "who knows more than who". This is not in the spirit of the DTV forum and community.

Please, as per the warning above, keep it civil or the thread simply cannot continue.

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Alan I have used the link you have provided to ask the question about ABC DAB coverage west of Blacktown. Your average punter expects to be able to buy a portable radio and take it home, switch it on and start using it with a minimum of fuss. The reality of DAB right now is that there is hundreds of thousands of listeners living inside capital city radio licence areas who have little or no DAB signal at home. For a respected broadcaster like the ABC to encourage its listeners on air to "upgrade" to DAB without any warning that digital coverage does not go anywhere near providing similar reception to the current analogue outlets is in my opinion a little irresponsible. Personally I have a tuner connected to our home TV antenna so it's not a big issue for me. We live on a high point of our suburb in Sydney's NW with a concrete tile roof and indoor DAB reception is limited to a few hotspots throughout the house for both the commercial and ABC stations. James thanks for the transmission info I really struggle to get my head around just how multiple transmitters operate in the same area on the same frequency without interfering with each other. Again as a non technical person I'm imagining that the DAB transmission is kind of like a machine gun with the gap between bullets being the guard interval. This interval allows for the situation where if the bullet from gun A doesn't reach the receiver in good condition or at all then there is the opportunity for a bullet from gun B or C to be received instead a fraction later. I'm also assuming that each transmission site fires its bullets at precisely the same time perhaps coordinated by GPS timing?  I'm sure I have you knowledgeable gentlemen rolling on the floor with laughter with my machine gun analogy but it is the only way I could think to put it into words. Considering we have a local transmitter at Hawkesbury heights would anyone like to venture an opinion as to why coverage is so poor out this way?

Edited by lizclinton

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Liz,

Hawkesbury repeater of a tiny 300 W is the same distance as the Main 50,000 W transmitter at Artarmon. As a result the power of the Hawkesbury transmitter is so low by comparison it will not even be detected by any receiver in the Blacktown area. So the Hawkesbury transmitter is 22 dB less than the Artarmon. The type of modulation is irrelevant because the signal from both transmitters are identical except for a much lower power and a delay from the Hawkesbury transmitter.

There is 3.8 million DAB+ listeners in total covering Brisbane, Sydney, Melbourne, Adelaide and Perth and rising. Since 1/3 of all new cars sold in the last 12 months contain DAB+ radios, many more listeners will hear the variety of programs increasing the number of listeners even more.

I have checked the profile from Artarmon to Blacktown and there is a shadow valley east of Blacktown, and the profile does not include buildings, semi-trailers etc. As I have already stated there is probably a case for a repeater East of Blacktown, however if the ACMA/ABC/SBS/ Commercial Radio Australia are not alerted to holes in the coverage then nothing will be done. My complaint about the current coverage area maps is that they do not show any holes in coverage which you have demonstrated exist.

Alanh

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Guest Malich
8 hours ago, lizclinton said:

Again as a non technical person I'm imagining that the DAB transmission is kind of like a machine gun with the gap between bullets being the guard interval. This interval allows for the situation where if the bullet from gun A doesn't reach the receiver in good condition or at all then there is the opportunity for a bullet from gun B or C to be received instead a fraction later. I'm also assuming that each transmission site fires its bullets at precisely the same time perhaps coordinated by GPS timing?  I'm sure I have you knowledgeable gentlemen rolling on the floor with laughter with my machine gun analogy but it is the only way I could think to put it into words.

 

It’s actually not a bad analogy at all, and with a couple of tweaks it's a fair overall explanation:

  • In the case of a single gun (transmitter), by the time the bullet (packet of data, aka ’symbol’) gets to a distant target it’s too weak to register a hit (reception).
  • Add in another gun, lower-powered but synchronised to the first (SFN repeater) so that the combined mass of the 2 bullets (signals) is enough to register at the target. That’s what a DVB-T/DAB+ SFN tries to achieve.
  • Whether they actually register as a single hit depends partly on the timing as they hit the target:
    • if one lags the other too much they’ll either interfere with each other hitting the target, or simply appear to be two weak hits that both fail to register.
    • On the other hand, if they’re close enough together in time - in the case of a DVB-T/DAB+ SFN, within the symbol time + guard interval - the 2 bullets effectively ‘add’ together & register as a hit.

All that neglects or skims over a few important things, but it’s a fair illustration. One of the important things it neglects is that the guard interval isn’t “empty” but contains some information to help the receiver detect/decode symbols from the 2 (or more) sources as 1 single one.

(Another important thing it skips over is that each transmitter is less “gun” than “over a thousand machine-guns firing their bullets at the same time, each one aimed at a slightly different part of the target” :blink:.)

Something else to remember with the DAB+ repeaters in Australia: the ABC doesn’t participate, so the ABC signal in repeater coverage areas will be much lower than the commercial signals. (For various technical reasons the ABC signal still has to be broadcast by each repeater though - just at a much lower level than the commercials, since they’re not paying for it). The upshot of that is that the ABC’s coverage will be basically the same as the main DAB+ transmitter, while the commercial coverage area will include the area covered by repeaters as well.

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I guess all this leads to the obvious question was Hawkesbury heights power level a mistake or are there technical restrictions to the power levels that can be used for these outer area repeaters?

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Guest Malich
1 hour ago, lizclinton said:

I guess all this leads to the obvious question was Hawkesbury heights power level a mistake or are there technical restrictions to the power levels that can be used for these outer area repeaters?

From Hawkesbury Heights, towards Blacktown? Technical limits.

The more power, the further the signal travels (& travelling takes time). Because the main signal is coming from pretty much the opposite direction, at some point between the two the signal timing will be so far out of whack that they'll destructively interfere with each other. One of the tricks with planning SFNs is to limit the radiation pattern & power level so that the signals in the overlap area add rather than interfere - and, hopefully, by the time they get so far out of sync that they combined effect would be destructive, one is much stronger than the other and will predominate at the receiver.

As Alan has previously said, Hawkesbury Hts is much lower power so its effective coverage area is much further W & NW than Blacktown for this exact reason.

Edited by Malich
Add bit about H Hts coverage area (and fix many minor typos/mistakes - it's late!)

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It looks like we may well be living with this level of coverage for some time then with the funny situation that people living further out west may well have better coverage than we do. With the news that the ABC does not participate in the DAB repeater program you really have to wonder why they are the only broadcaster I've heard promoting DAB directly to their listeners. 

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Malich,

Please quote the paragraph number from ETSI standard ES300401 what these signals in the guard band are and describe what the receiver does with them.

The ABC/SBS transmitter in Hawkesbury is 10 W more than the commercial repeaters, and the main transmitter in Artarmon is the same power. See the latest ACMA transmitter list.

If you wish to prove that the ABC/SBS transmitter in Hawkesbury is operating you need to do the experiment I did at Mindarie. You need a portable battery operated DAB+ receiver drive to a location close to the repeater, but has a near by hill between you and the main transmitter. When I did this the signal strength was maximum, but as you drive away from the site back towards the main transmitter the signal level drops back to the general strength for the region. This forum had said that the ABC repeaters when they first started were not operating on a number of sites, so I did the above test and proved that the Mindarie one was on air.

Alanh

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Guest Malich
2 hours ago, alanh said:

Please quote the paragraph number from ETSI standard ES300401 what these signals in the guard band are and describe what the receiver does with them.

Why would I go looking in EN 300 401 (I presume you meant N, not S)? That's all about data-level framing & transport structure of DAB, with only cursory mention of the details of COFDM broadcast and barely mentioning guard intervals except for their existance.

I'd look somewhere like any decent book on (C)OFDM (e.g. Fischer's "Digital Video and Audio Broadcasting Technology: A Practical Engineering Guide" *), or some of the BBC R&D papers (e.g. "A Technical Overview of Digital Radio" **).

If you specifically want a ETSI document that refers to them in DAB, then TR 101 496-3, "Digital Audio Broadcasting (DAB); Guidelines and rules for implementation and operation; Part 3: Broadcast network" *** is one off the top of my head, although its now considered "historical" - you're expected to refer to the other ETSI standards for (C)OFDM transmission which don't specifically mention the broadcast type.

The purpose? Many-fold. They reduce the dynamic range required in both the transmitter and receiver; they help the receiver synchronise in the presence of multipath signals (both incidental reflections & deliberate SFNs); since their structure is knowable they can be used to improve channel estimation & correction in the receiver; etc, etc.

If you've believed for all these years that the guard interval is simply no carrier or empty of signal, then I am afraid you've been mistaken...

-----

* p360-363: "However, the guard intervals are not simply set to zero. Usually, the end of the next symbol is keyed precisely into this time interval and the guard intervals can thus not be seen in any oscillogram. Purely from the point of view of signal processing, these guard intervals can be generated quite easily. The signals produced after the IFFT are first written into a memory in any case and are then read out alternately in accordance with the pipeline principle. The guard interval is then simply created by first reading out the end of the respective complex memory content in corresponding guard interval length.

But why not simply leave the guard interval empty instead of filling it up with the end of the next symbol as is usually done? The reason is based on the way in which a COFDM receiver locks onto the COFDM symbols. If the guard interval (also often called CP = cyclic prefix) were not occupied with payload information, the receiver would have to hit the COFDM symbols exactly at the right spot which, however, is no longer possible in practice due to their rounding off due to multiple echoes during the transmission" ... etc.

** p2: "In DAB the guard interval is generated by inserting it before the active symbol, and using data identical to that at the end of the active symbol, which avoids a discontinuity at the boundary between ‘active symbol’ and guard interval"- see also fig 3, p6.

*** sect 5.3.1, p28: "The guard interval is constructed by a cyclic continuation of each symbol"

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