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DAB

(from Hi-Fi World, June 2006 issue)

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Lost In

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Digital Audio Broadcasting was meant to usher in a new age for radio, with the promise of CD-quality sound, rock-solid reception and a wide array of stations catering for niches that had previously been impossible to serve on analogue. Steve Green explains what went wrong...

 

Although most people think DAB is a new digital radio system, it actually dates back to 1986 when the European research project ‘Eureka 147’ was formed by a consortium of French, German, Dutch and UK companies.

 

The breakthrough that led to the formation of the Eureka 147 DAB project was the emergence of so-called ‘perceptual’ audio codecs, which were able to compress the 1,411kbps (1,000 bits per second) bitrate of CD audio down to around 250 kbps with relatively little deterioration in sound quality. Prior to this, only crude audio compression formats existed, which could reduce the CD bitstream down to around 1,000kbps. Perceptual audio codecs made it possible for the first time to transmit radio stations digitally using a similar amount of bandwidth to an FM station.

 

The main original requirements of the Eureka 147 project were to design a digital radio system that transmitted radio stations with “audio quality comparable to that of a CD” and to provide robust reception even when on the move (more of which later). Other requirements included easy-to-use receivers and the transmission of data services. On the transmission side, the system was required to use spectrum efficiently and that the transmitter powers should be low.

 

By 1990, all the main technologies that make up the DAB system had been chosen—the audio codec, modulation scheme and error correction scheme. And the system in use today in the UK still uses exactly the same technologies as those chosen sixteen years ago.

 

Of the technologies chosen for the DAB system, the modulation scheme, called OFDM, or Orthogonal Frequency Division Multiplexing, was without doubt the best of the decisions made. DAB was the first system to adopt OFDM, and it is now the favoured modulation scheme for terrestrial broadcasting systems and wireless computer networks (e.g. Wi-Fi), and it is expected that the 4G mobile phone system(s) that will be developed over the next decade will also use OFDM. The main benefits are that transmissions are relatively immune to multipath reception, and that it allows single-frequency networks (SFNs) to be employed.


marantz-dd-82-path

Philips Digital Compact Cassette was the first commercial use of ‘Perceptual

Coding’, aka ‘data reduction’...


Multipath reception occurs when the transmitted signal travels over many paths from the transmitter to the receiver (for example signals can bounce off hills or buildings), and multiple versions of the transmitted signal, or echoes, arrive at the receiver with different relative delays. The sum of all the received echoes can lead to ‘frequency dependent fading’, where a band of frequencies is heavily attenuated due to the phase angles of the sine waves - that the signal is comprised of - cancelling one-another.

 

OFDM mitigates multipath reception by splitting the high-speed data signal into hundreds or thousands of low-speed sub-channels, called subcarriers—DAB transmitted in the UK uses 1,536 subcarriers. All of these subcarriers are prone to fading, but by using forward error correction (FEC) coding, data received in error on faded subcarriers can be corrected to a certain extent.

 

The error correction scheme is thus vital to the robustness of the signal, and it also dictates how much data can be transmitted on a multiplex. Unfortunately, the DAB system designers chose to use a relatively weak error correction scheme, which explains to a large extent why so many people suffer from poor DAB reception, although this can be solved by significantly increasing transmitter powers.

 

The second advantage of OFDM is that it allows single- frequency networks (SFNs). This is advantageous because instead of having to transmit a national FM radio station such as BBC Radio 2 on around thirty different frequencies between 89-91MHz, SFNs, as the name suggests, use just one frequency for all transmitters. This makes national SFNs, such as the BBC’s, much more spectrum efficient than FM. Local FM stations, on the other hand, are only transmitted on one frequency, so local DAB multiplexes are no more spectrum-efficient than FM in this case.

LOSS IS MORE

The audio codec used in the DAB system was chosen after a listening test in 1990 that compared the performance of various proposed perceptual audio codecs. The two clear winners were the codecs that went on to become MPEG audio codecs Layer 2 and Layer 3, nowadays better known as MP2 and MP3.

 

The main characteristics of MP2 and MP3 are that MP2 is simpler to decode than MP3, and hence MP2 consumes less power—although the difference is very small when compared to the total power consumed by a DAB portable radio. But MP2’s lower power consumption comes at the price of it needing to use 192 kbps to achieve the same level of quality as MP3 provides at 128 kbps.

 

The efficiency of the audio codec is crucial for a digital radio system, because in the case of using MP2 versus MP3, a system using MP2 requires 50% more spectrum than if MP3 were used. And the efficiency of the codec determines how many radio stations can be transmitted in a multiplex, which means that if a less efficient codec is used the transmission costs per radio station are higher because the total transmission costs can only be shared between a smaller number of stations.


tivoli-model-dab-02

Thanks to heavy promotion, DAB portables are now selling very strongly...


What it really boils down to is that if you use an inefficient digital radio system it is far more likely that the audio quality will be traded-off because there is insufficient available spectrum, and because radio stations will not want to pay high transmission costs. Which begs the question: why didn’t the UK broadcasters and regulators spot that the current DAB system was not up to the job?

 

For instance, the BBC’s plans to launch new digital services on both TV and radio can be traced all the way back to 1992, when John Birt, the then Director-General of the BBC, published a document called “Extending Choice in the Digital Age”.  And to quote Jenny Abramsky, the current Director of Radio at the BBC: “[the BBC] always planned to launch new services”.

 

The BBC started test transmissions of DAB in January 1990, and its national DAB multiplex went on air in 1995. The BBC DAB multiplex carried Radios 1 to 4 at a bit rate of 192kbps per station, plus the mono services Radio 5 and the World Service using bit rates of 96 kbps and 80 kbps, respectively. At these bit rate levels the audio quality was good, albeit not as good as on FM. The BBC’s DAB multiplex has a maximum capacity of 1,184 kbps, which meant there was enough room to add one more stereo station without degrading the audio quality. Instead, they added five new stations, and the audio quality of the stations plummeted...

 

It’s interesting to think that the BBC went from 1992 to 2002 either without noticing that there was a glaringly obvious problem with their plans to launch new services, or that they noticed the problem but did nothing.

 

The other organisation behind the poor audio quality on DAB is the Radio Authority, which regulated commercial radio before it was subsumed into Ofcom. The number of commercial radio stations increased rapidly throughout the 1990s, which meant that by the time the Radio Authority began awarding DAB multiplex licences in 1998 it should have been obvious that DAB would require a huge amount of spectrum if all the existing and envisaged radio stations were going to transmit at good audio quality levels on DAB. But instead of taking some decisive action to ensure that the audio quality standards would be kept high, the Radio Authority made the fateful decision to set the minimum bit rate for stereo radio stations at 128kbps, which 98% of all stereo radio stations on DAB in the UK now use. This is why DAB sounds so poor in the UK.

 

Quite simply, the MP2 audio codec should have been replaced and stronger error correction used. MP3 was available from the very beginning, but by 1993 development of the AAC (Advanced Audio Coding) codec had begun, with its main intent being to provide better performance than MP3 at low bitrates—which made it perfectly suited to what DAB needed. If AAC and stronger error correction had been adopted, then I doubt there would be a problem with the audio quality at all in the UK, or at the very least the BBC would be providing very good audio quality on all of its stations.

 

Ironically, it now looks virtually certain that the AAC and AAC+ audio codecs and stronger error correction will be incorporated into the DAB system in the very near future. Unfortunately, though, too many DAB radios have already been sold in the UK for us to change to the new version of the DAB standard now, so it looks like we’re stuck with the MP2 codec with its dull and muffled sound, either indefinitely, or for a number of years at the very least.

NEW CONTENT

It’s certainly fair to deride DAB – as we know it in this country at least – for its ropey sound, but to be fair it does have one abiding benefit over FM, which is the number of stations you can receive. In London you can receive over fifty stations and in most of the larger towns and cities you can typically receive about thirty five stations.


perstel-dr101


DAB portables are a great way to enjoy all those new digital networks,

but reception and battery life are poor...


The best known of the new digital stations - if for no other reason than the amount of TV advertising they receive - are the new BBC stations. The new speech station BBC7 has been the most popular addition, with its mix of old and new comedy, drama and children’s programmes. The second most-listened-to of the new BBC stations is Radio 5 Sports Extra, which broadcasts around thirty five hours of sports commentary per week, including football, rugby, cricket and tennis. Controversially, however, whenever Radio 5 Sports Extra goes on air, either Radio 4 has to be reduced to mono in the evening or Radio 3’s audio quality has to be reduced in the daytime...

 

BBC 6 Music’s remit was originally to play the best in popular music over the last forty years, and although it plays some good stuff, it seems to have drifted towards an NME-approved indie/alternative style of music, which isn’t actually a particularly good representation of what has been popular over the last forty years, in my humble opinion. Given the ongoing controversy over Radio 2 targeting an ever-younger age-group, it would seem that 6 Music was the ideal station to fit in-between Radio 1 and Radio 2, but the presentation style and choice of music combined with the dubious audio quality makes it sound like Radio 1 on Valium much of the time.

 

The remaining two new BBC stations, Asian Network and 1Xtra, were launched as a result of the BBC “under serving ethnic minorities”. Translated from BBC-speak, what this actually meant was that a lot of Asian and young black people were simply not using any BBC services - Asian people were increasingly watching the Asian-language TV channels on satellite, and young black people preferred pirate radio and dedicated urban radio stations to BBC services - which is bad news for a corporation that relies on a universal compulsory TV licence.  And if you thought the audio quality of the BBC’s main radio stations on DAB was bad, spare a thought for listeners to the Asian Network: 32% of its output is music, yet on DAB it is broadcast in mono, and to my ears sounds little better than MW.

 

The commercial radio groups have also launched numerous new digital stations, although they have predominantly targeted the same genres of music, which has led to a lot of stations overlapping one another. The most favoured genre has been rock music, both in classic and contemporary forms, but inevitably a number of pop music stations have been launched as well.

 

One of the most significant phenomena has been the rise of the so-called ‘quasi-national’ radio stations. These stations consist of either new digital services, or well-established larger local stations wanting to expand, that are transmitted on numerous local and regional DAB multiplexes across the country, as well as transmitting on the digital TV platforms. The rise of the quasi-national stations - typified by the likes of Kiss 100, Galaxy, Heart, Kerrang, Smash Hits, Classic Gold and Xfm - was a reaction to the Radio Authority’s decision to only licence one national commercial DAB multiplex, which, in typically short-sighted manner, they advertised as being the only one there would ever be! Ofcom has recently overturned that decision, however, much to the disgust of Digital One, who had threatened Ofcom with legal action if any more national multiplexes were licensed.

 

The negative effect of these quasi-national stations is that they consume such a large amount of capacity on the local and regional DAB multiplexes. Also, as mentioned earlier, local DAB multiplexes represent the least spectrum-efficient way to transmit DAB stations (whereas national SFN multiplexes are efficient). So starting from a position where there was a serious lack of spectrum, the Radio Authority allowing these quasi-national stations onto the local DAB multiplexes actually exacerbated the situation substantially.

GOING FORWARD

Reception quality is currently very hit-and-miss, with some people getting very good reception while others are plagued with the dreaded ‘bubbling mud’ sound that occurs when the signal strength is too low. But reception quality should improve after the Regional Radio Conference (RRC) in Geneva concludes this summer, which should allow broadcasters to increase the transmitter powers they use.

 

Another method to improve reception quality, that has actually always been an option open to the broadcasters, is to add low-power ‘gap-filler’ transmitters to the single-frequency networks, in locations that suffer from low signal strength. The only thing stopping the broadcasters from doing this is cost, but as DAB ownership increases they will be more likely to invest to improve coverage this way.

 

Ofcom also announced last year that they will be licensing a new national commercial DAB multiplex and local DAB multiplexes for areas that don’t already have one. The new national multiplex, which will be licensed later this year, is likely to add between seven and ten new stations, depending on whether and how much capacity will be used for mobile TV channels (i.e. TV on mobile phones).


arcam-dt91-final

Arcam’s DT91 is one of best new DAB tuners, but still features an

FM option for serious listening...


Virgin Mobile is planning to launch a mobile TV service later this year, which will be transmitted on the Digital One multiplex. Interestingly, the service is only made possible by using the stronger error correction and modern audio (and video) codecs that are denied to the radio stations. The casualty to make way for the new mobile TV service is the Primetime radio station, which has been forced to withdraw from the Digital One multiplex.

 

Ofcom is also trying to acquire spectrum for a second national DAB multiplex, but judging by the language it used it doesn’t expect to be successful. If it does acquire this additional channel then the BBC might be able to lease a small amount of it, but if only one new national multiplex is launched the BBC doesn’t stand any realistic chance of acquiring any capacity on it. All will be revealed following the RRC this summer... But despite the new spectrum being made available for DAB, Ofcom admitted last year that even after the expansion there will be around 90 analogue radio stations—out of a total of 326—that are either unable to get on DAB because the local multiplexes are full or they can’t afford the carriage fees. Ofcom is also planning to issue a further thirty FM station licences, which will undoubtedly push that figure higher still.


marantz-tuners


The golden age of wireless – classic FM tuners run song rings

around modern DAB designs.

 

To end on a note that typifies the way DAB has been regulated in the UK, Ofcom announced last year that it will allow the broadcasters to reduce the bitrates of their stereo stations from 128 kbps to 112 kbps if they invest in slightly improved MP2 audio encoders. The BBC already uses these encoders, and – as anyone can hear with their own ears - their stations sound terrible at 128 kbps, so Ofcom’s decision doesn’t exactly bode well for the medium’s future...

 

The sad thing about DAB is that it’s a missed opportunity; flawed from the conceptualisation stage onwards, the various parties responsible for ‘Digital Radio’ in this country have repeatedly missed chances to improve it. If anything, they have made it worse. It’s another case of a great idea, ruined by poor execution.


 

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