Anatomy Of A Digital Broadcast Radio System

What does a Hackaday writer do when a couple of days after Christmas she’s having a beer or two with a long-term friend from her university days who’s made a career in the technical side of digital broadcasting? Pick his brains about the transmission scheme and write it all down of course, for behind the consumer’s shiny digital radio lies a wealth of interesting technology to try to squeeze the most from the available resources.

In the UK, our digital broadcast radio uses a system called DAB, for Digital Audio Broadcasting. There are a variety of standards used around the world for digital radio, and it’s fair to say that DAB as one of the older ones is not necessarily the best in today’s marketplace. This aside there is still a lot to be learned from its transmission scheme, and from how some of its shortcomings were addressed in later standards.

Channels and Capacities

The spectrum of a wideband FM broadcast transmission, on 93.9 MHz.
The spectrum of a wideband FM broadcast transmission, on 93.9 MHz.

You will all be used to analogue broadcasting on AM and FM, in which each station has its own transmitter and occupies its own frequency. With a digital system like DAB each transmitter does not restrict itself to only one station, instead it transmits several at once in a multiplex. Each multiplex has a data rate of just under 1.2 Mbits/s, which in practice allows it to carry around ten MP2-compressed stations depending on the data rates of each individual station. It’s difficult to state a hard and fast figure for the channel capacity of a multiplex, because not only can different sample rates be used for each channel, those rates can be changed on the fly.

dab_logo-svgThe British multiplexes are transmitted in the spectrum once occupied by the upper set of the old British 405-line TV channels around 200 MHz. However they are not modulated onto an RF carrier in the same way as a traditional analogue radio or TV station is. To understand why this is the case, imagine for a minute that you had a serial port with a 1.2Mbit/s data stream on it. If you were to feed the stream to a traditional modulator on an analogue transmitter, you’d have a transmitted bandwidth of just over 1.5 MHz. In an idealised free space environment that would make a passably good broadcast system, but to see why it would not work in the real world just think for a moment about watching analogue TV with an inadequate antenna.

Multipath

Our ultra-high-budget simulation of analogue TV ghosting.
Our ultra-high-budget simulation of analogue TV ghosting.

Sometimes on your TV in the analogue days you would see a second “ghost” image, a faint clone of the main image overlaid to the right of it. This was the result of the transmitted signal taking multiple paths to your receiving antenna, the main image being via the direct path and the “ghost” image being a path via a reflection from an object such as a tall building or a passing aircraft. The distance on the screen between the real image and its ghost represented the time difference between those two radio paths.

Now imagine that high-speed digital data stream again, only instead of in idealised free space put it in a real-world situation with passing aircraft, and all that ghosting. The time difference between the real stream and its ghost is now very significant compared to the length of an individual data bit, and thus overlaying the ghost on the original stream has the effect of causing huge errors in the received data stream. Clearly some means of combatting this problem is required.

Many Little Channels

The answer comes in the form of increasing the length of the data stream bits such that the ghost time difference is no longer significant in relation to it. Simply lengthening the data bits of the stream would reduce the data rate to the point of uselessness, so they instead split the one single high data rate stream into many individual low data rate streams with much longer bit lengths.

Part of the spectrum of a DAB transmission, at 210.9 MHz. The individual carriers can clearly be seen.
Part of the spectrum of a DAB transmission, at 210.9 MHz. The individual carriers can clearly be seen.

That single carrier with an over 1.5 MHz bandwidth then becomes over 1500 individual carriers each with a 1 KHz bandwidth, and each of those carriers has a low enough data rate for the ghosting to no longer be a problem. The overall data rate is the same, as is the overall spectrum bandwidth. but the resistance to ghosting has been improved enormously. It also has the handy effect of improving the resistance to typical narrow-band RF interference, because a certain number of the individual carriers can be lost without exceeding the ability of the error correction to compensate for it.

 

Splitting the stream into multiple carriers in this way is referred to as COFDM, or Coded Orthogonal Frequency Domain Multiplexing, and since each carrier is phase modulated by the four 90-degree-apart quadrature vectors the modulation scheme is referred to as DQPSK, for Differential Quadrature Phase Shift Keying. Yes, the linguistic influence of [Samuel Morse]’s key finds its way into digital broadcasting.

Sounds Like Mud

Of course, the nature of the RF side of DAB and other similar transmissions is only half the story There is the compression algorithm and the error correction algorithm, which define the real-world characteristics of the standard. DAB in particular is notorious for poor performance under low signal conditions, in which the signal can dissolve into a sound that is colloquially described as “like boiling mud”. Other countries have either abandoned their DAB rollout or gone straight for a more recent standard such as DAB+. That’s the price Brits pay for their country being an early adopter.

So why does DAB have this poor performance compared to its successor? According to my friend as we cracked open another couple of San Miguels cooled by the frosty night outside the window, the secret isn’t in its use of MP2 rather than AAC, but in the error coding scheme. The designers of DAB tried to shape the standard so that the components they considered most important to the intelligibility of the received audio were protected. They thus put a weighting in the error coding scheme towards certain frequencies, and it seems this is responsible for the flaw because it left it more vulnerable at the other frequencies. The resulting degradation in quality becomes much steeper as the  percentage of the stream that is lost rises, to the extent that the system is quickly rendered unusable.

We all pick up in-depth knowledge of the systems and technologies we work on during our careers. I knew my friend worked in this line, and this was a fascinating opportunity to gain some understanding of a system about which I had a basic grasp but didn’t really know what made it tick. It’s this kind of information-sharing that’s so valuable, while little may come of my new-found understanding of DAB there is a lot to be said for accruing technical knowledge for its own sake. If you find yourself hanging out with a friend from way back, make sure you ask them about their specialities, you might learn something interesting.

DAB radio header image: Yisris (CC BY-SA 2.0) via Wikipedia Commons.

32 thoughts on “Anatomy Of A Digital Broadcast Radio System

  1. I’ve met the Opendigitalradio guys at a camp once, suuuper friendly people. You (yes, you!) can broadcast DAB (and the much-improved, but even less-abundant) DAB+ using their tools:

    http://www.opendigitalradio.org/mmbtools

    There’s a DAB receiver implementation in GNU Radio, too, so you can RTL-SDR the hell out of that (instead of buying a new radio for your kitchen)

    https://github.com/andrmuel/gr-dab

    Advantage: there’s a (seriously lacking attention) Android port of GNU Radio, and there’s an android port of librtlsdr… you do the math of what’s possible (because I haven’t come across a single smart phone that’d do DAB yet).

  2. DAB and DAB+ too have the same problem as DVB – there’s a very sharp transition between “works beautifully” and “Fucking hell what is this shit!?”

    Back at my parents place, can’t watch certain channels on the TV if the wind is shaking the antenna because the picture stops repeatedly until the reciever crashes and you have to reboot the TV. It’s just horrible.

  3. Here in the US we have HD which is the most Heinous Deception ever put into an advert. High Distortion! It’s proprietary and not worth hacking even with a free hack. Streams of 128k and higher are becoming the norm, bypassing this shoehorned tech.

    1. Come again?

      HD is a series of specifications for “better” video, not some proprietary locked extra $$ format. Sounds like your cable provider has put one over on you. We get several HD signals over the air, using a homemade antenna in the attic. Be free, ditch the cable. (unless it’s also your internet)

      1. Almost a decade ago our AM public station did the HD thing, it was 10k hi-fi but then useless. I tried a friends HD radio and could barely listen to local jazz. Even the stereo mix would cut to mono on loud solos. Anything from off the network, even speech was awful. Last fall we got FM at low power repeater, so now the jazz is in FM stereo. They know we are not satisfied with HD and know AM is becoming the band that “has noise and noise”, and is hard to hear cause it’s lo-fi. HD will go the way of AM stereo.

    2. What amazes me is that governments around the world have handed over control, for free, to what has essentially been public communication system to one or two companies who control all access. RIAA love this because no one, on paper, can record.
      DAB patents in Europe expired on the 18th January 2013.
      DAB+ patents (HE-AACv2 codec [paid to VIA Licensing]) will expire mid 2020 in Europe.
      HD-Radio plan to upgrade their codecs to keep that money flowing into iBiquity.

      (You have to love VIA Licensing, they could grab real estate of hundreds of licenses, group them together, and they will charge full whack until all have fully expired, great closed system to keep new people out of their garden).

      So a public system where anyone could legally design a radio for reception has become a walled garden with all access and control (in the case of the US system) handed over to one company.

      Why a public system is not using a public modulation (DAB) and public codec (opus codec) is a bit beyond me.

  4. In my opinion DAB was the biggest con. trick ever pulled on the great British wireless-loving public. We were promised “near CD quality” in the blurb. I was an early adopter with a Technics tuner and the initial impression of the sound was quite good – but what do we eventually end up with? More and more stations squeezed in with ever-decreasing bit-rates (one in glorious MONO, for goodness’ sake!), not to mention the ever-present audio compression and processing (Optimod?) that, at a stroke, completely wrecks any pretensions to half-decent sound quality, the result being that DAB sounds just as foul as current FM with absolutely no dynamics and quite audible “pumping” on some stations. Everyone seems to be trying to make their particular station sound more “punchy” than the next. Almost painful to listen to. Incidentally, I’m fairly sure that I read somewhere (or did I dream it?) that there was going to be user-adjustable audio compression built-in to the DAB spec. so folk who wanted to ruin the sound could do just that, leaving an un-doctored signal for those us who actually appreciate decent audio. Either way (putting it bluntly) I think that DAB has been a dismal failure.

    1. Again, same thing with DVB. “Better image quality”, and then they cram 12 channels to a mux and the picture turns to shit.

      There’s tons of completely utterly pointless channels that run repeats of the same programs two times a day six days a week – you can watch TV months apart and it’s still the same four of five episodes of Wheeler Dealers running on two different channels!

      There’s so many of them that they can’t even get advertisers for all the slots, so they run five minutes of the channel logo and some dude comes on air live to rant about the coming programs – and sometimes he doesn’t even do that so it’s just the logo for the whole break.

      I remember several months before the digital broadcasts went national the TV suddenly went a bit blurry and remained that way – turns out they deliberately reduced the analog channels’ quality by switching the already downsampled and compressed digital picture through the analog network, instead of the original uncompressed feed, so the people would think the digital was actually better. That was sneaky.

    2. Totally agree. The system was advertised on the basis that it would be able to let you receive more stations and in better quality. The reality is that coverage and quality is significantly worse than plain old FM….

  5. One thing that I have always wondered … and if anybody has an answer, please let me know….

    Do you remember back when a VHS VCR was king of home theater? Back then, you could even get Dolby “surround sound” out of specially-encoded VHS tapes. This includes 5 channels. Does anybody have any idea at all how you can get 5 channels out from only two channels in, especially considering that the 2 channels was still limited to approximately 20 KHz (no frequency multiplexing)?

    I have tried my Google-Fu to find the answer, but no luck.

    1. They sell it in the UK. Whether it’s actually Spanish or comes out of another tap on the gigantic vats owned by the 2 or 3 companies that make all our beer, I dunno. But yes.

  6. The spectrum efficiencies we can gain by moving beyond am fm and atsc make me laugh… how many times has the fcc come to public service or military to ask for more bandwidth? I wonder if anyone in Harrisburg has any technical background anymore. Good thing siriusxm killed the hd radio fm and drm am market before it even started…. oye.

    1. That ,multipath ghosted, Hackaday logo shows a pretty good example of what “Monocular Diplopia” does to your eyesight.
      Now IF I could just find a Doctor to clean up my brains visual processing signal path.

  7. The US would need to place the DAB in the now mostly abandoned TV channels 2 to 6. The FCC is already planning on taking 20 more UHF channels, leaving us with the highest channel of 31.
    So, does this mean that the FM dial will be vacant (88 to 108 MHz)? How many pirate stations will pop up? How many will use SPARK https://www.drm-sender.de and broadcast with a digital signal? Or even the AM broadcast band.
    Someone needs to create a cheap DRM receiver, maybe using a Pi.

    1. Are digital signals immune to interference that plagued analog signals in the low band VHF spectrum? In the event they are not that could be that band seems to be abandoned, and may never be useful for digital signals.

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