Low Audio Quality is inevitable on DAB in Europe unless the AAC+ codec is adopted

The two major problems with the current DAB system are that:

• Transmission costs are extremely expensive
• There is insufficient spectrum available to carry everything that needs to be carried on the DAB system

Both of the above two problems make it inevitable that radio stations will be transmitted at low audio quality if the current outdated DAB system is used, because radio stations will be forced to transmit at low bit rates. 

However, as I will explain below, if DAB adopts the AAC+ audio codec then both of the major problems with the DAB system will be solved. 

Australia has already committed to using AAC+ on DAB (dubbed 'DAB+'), so the DAB system is definitely going to adopt the AAC+ codec anyway. The crucial issue is that European countries adopt DAB with the AAC+ codec BEFORE they commercially launch digital radio in their country.

DAB's Very High Transmission Costs

This section discusses the transmission costs on DAB using the UK as an example, but all of the issues discussed are just as relevant to other countries as they are to the UK.

The following table shows transmission costs for radio stations on DAB and FM. These figures were provided to me by someone in the UK DAB industry, and a report last year verified these figures (it is actually rumoured that the transmission costs on the national Digital One multiplex have gone up to £1.2m per annum recently).

1 - Transmission costs are charged on a pro rata (propotional to) basis with bit rate, so a radio station using 224 kbps costs 224/128 times as much as a 128 kbps station

The problem with DAB's extremely expensive transmission costs was even admitted in a quote made last year at the International Broadcasting Conference (IBC) by the current President of WorldDAB, Quentin Howard, who is also the chief executive of the UK national DAB multiplex operator Digital One:

"Now, when DAB was proposed, people were talking about using 192 kbits or maybe 256 kbits for audio broadcasting. If one was to use 192 kbits in a multiplex you would only get 6 audio services. Economically, and particularly for commercial broadcasters, that was difficult."

And the following quote is from a BBC R&D report (page 34 of the pdf file, paragraph 4), which is referring to the bit rate levels required for good audio quality that Quentin Howard is referring to above:

"A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal [4]. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of `joint stereo' encoding (i.e. some sounds appearing at the
centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material."

But instead of providing the accepted bit rate levels required to provide good audio quality on DAB, the UK broadcasters simply decided to use low bit rates in order to save money on transmission costs, and now 98% of all stereo stations on DAB in the UK are transmitting at a bit rate of 128 kbps, which provides very poor audio quality. 

Furthermore, there is also a large and increasing number of music radio stations that are transmitting in mono on DAB in the UK, which is again to save on transmission costs. For example, the Kerrang and Heat radio stations are each available on many local DAB multiplexes, but they are transmitted in mono on each multiplex.

And while the largest radio stations could afford to transmit at higher bit rates if they desired, the medium and smaller-sized radio stations (most of which have budgets which are a tiny fraction of those of the big stations) would find it impossible to spend £168,000 on transmission costs. 

A good example of this issue occurred recently when Passion Radio started transmitting on the Sussex Coast local DAB multiplex, but being a relatively small radio station it could only afford to transmit in mono despite the fact that it is a music radio station. 

Indeed, Peter Davies, Ofcom's Director of Radio & Multimedia, said made the following comment last year on the subject of DAB's transmission costs:

"the economics of DAB for many smaller stations just don’t work." 

and Ofcom estimates that around 90 of the existing 326 analogue radio stations will never be able to transmit on DAB either due to being unable to afford the high transmission costs or because the local DAB multiplex in their area being already full. 

And if all of the above wasn't bad enough, DAB multiplexes were originally planned for outdoor mobile reception, and the Regional Radio Conference 2006 (RRC-06) has changed the planning to indoor portable reception, which translates to an increase in the transmitter powers of around 6 - 8 dB. And drawing on an assumption put forward by a German DAB proponent in an EBU Technical Review article that transmission costs are roughly propotional to transmitter powers, an increase in power by 6 dB would translate into a quadrupling of the transmission costs listed above! In reality, increased transmission powers likely wouldn't increase by a factor of 4, but they would increase substantially, which makes the likelihood of good audio quality even more remote than it is at the moment, and makes DAB unaffordable to even more small and medium-sized radio stations.

The only conclusion that you can come to is that DAB's very high transmission costs make it effectively inevitable that stations will transmit at low audio quality if the current DAB system is used -- if they can even afford to transmit on DAB at all.

Lack of spectrum

The other problem with using the current DAB system in Europe is the lack of spectrum. 

The Regional Radio Conference 2006 (RRC-06) recently concluded in Geneva, which determined the allocations of Band III spectrum that can be used for DAB or digital TV.

The outcome of RRC-06 is that European countries have typically been allocated very similar amounts of spectrum that can be used for DAB as the UK has been allocated (European countries have typically been allocated a Band III TV channel plus 2 DAB channels, and the TV channel can be converted to 4 DAB channels if DAB is used in that country).

But due to the high bit rates that need to be used to provide FM-quality using the MP2 audio codec, if there's only 6 DAB multiplexes in a given area, very few radio stations can be transmitted:

Not only is 30 radio stations hardly an attractive proposition, but in many European countries there are far more than 30 radio stations already transmitting on FM:

and there are a similarly high number of FM stations transmitting in other cities in Italy and France, and other countries also have a much higher number of FM stations transmitting than is usual in the UK.

And yet the available DAB spectrum needs to carry all of the following:

• existing FM and AM stations
• new digital-only stations launched to encourage listeners to buy digital radios
• T-DMB mobile TV services (T-DMB uses the same spectrum as DAB) -- a T-DMB mobile TV channel typically consumes 1/3rd of the capacity of a DAB multiplex on its own
• new data services, such as EPG and music downloads

Again, the only conclusion that you can draw is that if the current DAB system is used it will be inevitable that radio stations will have to be transmitted at low audio quality, because there simply isn't anywhere near enough spectrum to allow radio stations to transmit at good audio quality. 

The Solution: DAB must adopt the AAC+ codec

There are two main MPEG-4 AAC audio 'profiles' used for broadcasting applications:

• LC-AAC (Low Complexity Advanced Audio Coding), which is the 'ordinary' AAC codec, and is used at higher bit rate levels, and 
• HE-AAC (HIgh Efficiency AAC), which is used at lower bit rate levels

So in order to avoid confusion, I refer to the combination of these two codecs as simply AAC+.

The benefit of using AAC+ is that it requires far lower bit rate levels to provide a certain level of audio quality compared to the MP2 codec used on the current DAB system:

As you can see from the above table, the AAC codec is twice as efficient as MP2, and HE-AAC is around three times as efficient as MP2.

Furthermore, when AAC+ is adopted on DAB, much stronger error correction coding will be used, called Reed-Solomon coding, and this will increase the capacity of DAB multiplexes.

The following table shows how many stations can be carried on a DAB multiplex using the bit rate levels in the above table, and assuming Reed-Solomon coding is used with AAC+ in conjunction with the PL4A DAB protection level.

And because more stations can be carried on a multiplex, that means the transmission costs come down in proportion:

The reason I've included values for the 'Slightly worse than FM-quality' and 'Significantly worse than FM-quality' is simply to show that even small radio stations could afford to transmit on DAB if AAC+ were used, whereas at present many smaller stations cannot afford to transmit on DAB at all in the UK, while the few that have decided to transmit on DAB are transmitting in mono, because they cannot afford to transmit in stereo. 

Conclusions

DAB adopting the AAC+ codec would therefore solve both of the major problems with the DAB system:

• The problem with DAB's extremely high transmission costs is solved
• The problem of there being insufficient spectrum is solved¹

1 - assuming that mobile TV on T-DMB does not consume excessive amounts of spectrum

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