What Is DAB+ and How Does It Work?
March 27, 2017 / Broadcast Technology/
Digital Radio in Australia started in 2005 with legislation passing to allow for the first DAB+ transmitters. DAB+ uses shared multiplexing facilities in each city, meaning all stations in a city are broadcast from one, two or three transmitters (instead of one-per-station as is the case with analog transmission).
Digital Radio has been a big political issue within the industry, and there’s much more to it than just the technical operations. My aim in this article is to cover both the legal/legislative background into how licenses are issued, and also the technical aspects of how it actually works.
Let’s start with the basics. How do listeners experience DAB+?
- There is a special radio receiver.
- The receiver lists the names of the stations, not the frequency. This is used for ‘tuning’.
- The receiver automatically scans for every available multiplex.
- DAB+ is only available in capital cities in Australia, with trials starting soon in some major regional areas.
- Radio receivers can display text messages of up to 128 characters long. These are set by the broadcasters, and can change regularly. It’s called DLS (Dynamic Label Segment). It’s similar to RDS’s RadioText.
- Some receivers also receive image slideshows, called SLS.
- There are other ‘applications’ such as a an electronic programme guide (EPG) and traffic data, although I haven’t seen these implemented myself.
The Legal Basics – Licensing & Ownership
The transmitters in each city are owned and run by a Joint Venture Company (JVC). JVCs are formed in each city, with share capital distributed between all participating stations. ACMA can issue DAB+ multiplex licenses to these JVCs. There are three categories of licenses:
- Category 1: Commercial & Community Radio Stations Only
- Category 2: Commercial, Community & National (Government) Stations
- Category 3: National (Government) Stations Only
ACMA issues Foundation Licenses in each area, providing standard access arrangements to incumbent broadcasters. These standard arrangements are as follows:
- Each existing commercial broadcaster is entitled to 1/9ths of Category 1 & 2 capacity.
- All community broadcasters are entitled to share 2/9ths of Category 1 & 2 capacity.
- Each national broadcaster is entitled to 1/9ths of Category 2 capacity.
So far, only Category 1 & 3 licenses have been issued. There are no provisions in the legislation to issue non-foundational licenses. Incumbent broadcasters are effectively protected by these provisions.
All broadcasters own shares in these JVCs, except Community Radio. There are costs associated with being a shareholder, and at the time of establishment of DAB+, the community sector was unable to commit the funding required to become a shareholder. Thus, Community stations are just access seekers (paying a fee for use of the bandwidth).
The Technical Basics – Multiplex Capacity & Bandwidth
DAB+ employs digital transmission technology. Instead of modulating audio onto a carrier (as with FM), we instead modulate bits (one’s and zero’s) onto a carrier. This means you can multiplex (‘combine’) multiple bitstreams into one transmitter. In practice, this means we can send multiple audio ‘streams’, ‘channels’, or ‘stations’ on one DAB+ carrier. Each one of these stations shows up as a separate station on a DAB+ receiver.
A single DAB+ transmitter/multiplex uses 1.536Mhz of bandwidth, and sits in the VHF band. In comparison, DVB-T (Digital TV) in Australia uses 7Mhz of bandwidth per licensee (each TV network is a licensee, and each licensee can run multiple channels on their one multiplex).
In Sydney, we use VHF channels 9A (202.928Mhz), 9B (204.64Mhz) and 9C (206.352Mhz) for DAB+. These three frequencies are used across all the Australian capital city DAB+ multiplexes (but not all cities use all three – some only have one or two multiplexes).
This 1.536Mhz of spectrum gives us 1152Kbps of usable data, assuming a 3A 1/2 Forward Error Correction (FEC) rate (more on that later). Assuming all stations use 3A FEC, we can fit in 18x 64Kbps stations.
DAB+ digitally encodes audio using the AAC+ audio codec (HE-AAC). This is a fairly common lossy digitally coded algorithm, standardised by MPEG, and licensed to software and hardware companies by Via Licensing Corporation. DAB+ receivers need to be licensed, as do the encoders used in the transmission chain.
The older DAB standard (used in areas such as the UK) employs the original AAC codec. The main difference between DAB and DAB+ is the audio codec. This basic chart shows perceptive quality between the various version of AAC codecs:
As mentioned earlier, the standard error correction rate is 3A 1/2. The error rate is configurable per-station. This table shows how many 64Kbps services can be provided at each FEC rate.
You can mix-and-match error correction rates on each multiplex. An interesting analysis by Gough Lui shows the FEC rates for each station in Sydney in November 2016:
You can see that most stations use the 3A FEC rate, although community stations are using both 3B and 4A. I believe this is done mainly for capacity purposes – trying to squeeze in additional services. In Sydney, the CBAA occasionally allows stations to run a popup service. This bandwidth is available because of the error correction.
However, lowering the error correction effectively lowers the coverage and reliability. Receivers have a basic error rate meter, and if it drops below a certain level the station becomes un-receivable.
The Cost of Leasing Or Operating A DAB+ Multiplex
One would think the cost of running a DAB+ station is a closely guarded secret. Most companies don’t publish these sort of details about their operations. However, thanks to Section 118PN of the Radiocommunications Act and the ACCC, these Joint Venture Companies need to publicly publish their annual reports and disclose certain details.
According to the 2015-2016 Annual Report (Pages 9-11), access seekers paid between $5,460 and $7,453 per 32Kbps per quarter. This fee is the same across all access seekers, but varies based on the costs of operating transmission facilities in each city.
This one annual report covers all the capital city Foundational License JVC operation companies. Government multiplex operators aren’t subject to the same reporting requirements. In Sydney, this means we have data for the two commercial/community multiplexes.
These costs don’t necessarily include the charges of program linkage and encoding. I think it’s just the actual ‘transmission’ (getting the content to the multiplex is your responsibility). The In-Band Repeaters would be included in these costs, as there’s no way for specific stations to opt-out of a repeater.
As Australia currently only has Foundation DAB+ licenses allocated at the moment, you can’t just rock up and demand access. There have been instances where third-parties have leased DAB+ bandwidth, but this is done via existing access seekers – not directly with the JVC. Some companies have excess bandwidth, which they are welcome to lease out to other companies as they see fit (they can’t sell it per-se).
Using the access fee for the most expensive city (Perth), and assuming we have 32x 32Kbps slots on a multiplex, it costs $268,308 per quarter ($1.073 million) to run a single DAB+ multiplex.
In-Band Repeaters & Single Frequency Networks
DAB+ coverage wasn’t too crash hot when it was first deployed in Australia, but it’s been slowly improving. How is that possible?
The answer is: Single Frequency Networks.
In simple terms, there are multiple transmitters in one license area all set to broadcast the same signal on the same frequency. Each additional transmitter receives the fully multiplexed signal from the main multiplex, and locks to a GPS signal. It then transmits in perfect alignment with the main transmitter. This is a bandwidth-efficient transmission method, as you don’t need to license additional frequencies in one area.
If you look up the ACMA RadComm database, you can see the locations of all the additional DAB+ transmitters. Here’s the list of additional locations for the Sydney Multiplex on channel 9B (Artarmon TXA is the main site):
Low Powered DAB+?
I always find it interesting to look at what is happening around the world with various broadcast technologies. There have been some trials over the last few years, particularly in Europe, of low-powered DAB+ transmission.
Ofcom (UK) have been trialling low-power DAB in ten towns since June 2015. In September 2016, they published a report and announced the trial licenses would be renewed until at least 2018. The report for this trial is very interesting.
Ofcom provided a standardised setup, consisting of open source encoders and multiplexes, VHF power amps, mini-PCs, and SDR hardware. All up, the setup cost roughly £9,000 for a simple transmitter setup. They also trialled SFNs and on-channel repeaters, which cost as much as double.
Ofcom even included a picture of their standard setup in their report:
If you take a look at the Open Digital Radio Facebook Page, you can see photos of people all over Europe installing DAB+ systems – some at low power, and others with considerable output. Apparently there are at least 12 muxes operating in Switzerland with OMB tools.
As far as I’m concerned, this is very cool – and could probably be implemented in Australia for sub-metro community stations. For what it’s worth, ACMA have a form on their website to apply for spectrum for ‘scientific trial broadcasts’. If anyone in Australia was serious about trialling low-powered DAB+, it’d be very interesting to see what happened if you completed this form.
Other Digital Radio Technologies
DAB+ has been very popular across Europe, and increasingly Asia, due to the high population densities. This makes DAB+ a highly efficient technology. In other countries, different technologies have been implemented.
Of particular note are two in-band technologies – HD Radio and DRM. These two technologies are fundamentally different to DAB, in that they broadcast within a broadcaster’s existing AM or FM spectrum. They require no shared transmission facilities.
Everyone has their own opinion about which Digital Radio technology is best suited for Australia, so I won’t go into that here, only to say that HD Radio and DRM are preferred because it’s much easier for individual stations to setup and offer digital radio facilities. In most circumstances, you’re under no obligation to offer it – it’s merely a value add. If you can see value in the service, you can go ahead and invest in the technology.
HD Radio is popular in the United States, and DRM is seeing growth particularly for shortwave services across Asia and Europe.
Conclusion & Further Reading
DAB+ isn’t a particularly well understood technology, primarily because it’s limited at the moment to out capital cities and managed by Broadcast Australia and Agile on behalf of all broadcasters. There is a very good presentation on the World DAB website, created about the Australian DAB+ model, which goes into a lot more detail on transmission models.
It’s important to understand how these technologies work. Technology is a critical area for all broadcasters, and we can’t risk loosing operational knowledge at a station level.
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