Hello Habr.
In recent years, the introduction of the DAB+ digital radio standard has been discussed in Russia, Ukraine and Belarus. And if the process has not yet progressed in Russia, then in Ukraine and Belarus it seems that they have already switched to test broadcasting.
How does it work, what are the pros and cons, and is it even necessary? Details under the cut.
Technology
The idea of digital radio began to emerge in the late 80s, when it became clear that there were not enough “places” in the regular FM band for everyone - in large cities, the free spectrum in the 88-108 MHz range was exhausted. In this regard, DAB was considered a good alternative - it is a digital standard in which, due to more efficient coding, more stations can be placed. The first version of DAB used the MP2 codec, the second version (DAB+) used the newer HE-AAC. The standard itself is very old by modern standards - the first DAB station was launched in 1995, and the DAB + station in 2007. Moreover, the “age” of the standard in this case is even more of a plus than a minus - now there is no problem buying a radio receiver for every taste and budget.
There are quite a few differences between DAB and the usual FM. And the point is not even that one is a “digit”, and the other is an “analog”. The principle of content transfer differs. In FM, each station broadcasts independently, while in DAB+, all stations are combined into a "multiplex", each of which can have up to 16 stations. Different frequency channels are provided, so that different countries can choose those that are free of other services.
From a business point of view, this difference causes a number of disputes among broadcasters regarding how to broadcast in multiplex. Previously, broadcasters themselves obtained a license for a frequency, bought an antenna and a transmitter, now the license will be issued to the multiplex operator, and he will already lease channels to radio stations. It’s hard to say whether it’s better or worse, it’s more convenient for someone to have everything of their own, for someone it’s more convenient to rent.
By the way, in this regard, DAB has a huge and fat minus for the listener - multiplex rental price depends on bitrate. And if you choose between 192 and 64kbps ... I think everyone understands what will be chosen. If it is rather difficult to broadcast with poor quality in FM, then in DAB it is even economically encouraged (it is clear that this is not the fault of the standard developers, but nevertheless). Russian prices, of course, are still unknown, but for example, you can see English prices .
From a technical point of view, the DAB+ multiplex is a wideband signal with a spectrum width of about 1.5 MHz, which is clearly visible with an RTL-SDR receiver.
A more detailed description in PDF can be found .
Competing standards
In general, there are not so many of them. DAB+ is used in Europe, standard is popular in the USA , in India experiments were carried out with the standard but how they ended is hard to say.
The card is a little outdated (DRM was also tested in Russia, but abandoned), but the general idea can be understood:
(a source )
Unlike DAB, the creators of the HD Radio standard have taken a different route by placing the digital signal directly next to the analog signal, allowing broadcasters to use their own antennas and masts.
However, this does not solve the problem that started it all - the problem of the lack of free seats in the spectrum. Yes, and purely geographically (and probably politically), in the ex-CIS countries, the adoption of the European standard looks more logical than the use of the American standard - the choice of European goods is still larger and it is easier to buy receivers. In 2011 there were still mentions , but everything died out (and thank God, because its own digital standard is incompatible with anything, this is the worst thing that could be invented for radio listeners).
The test is
Finally, let's move on to the practical part, i.e. to testing. DAB is not working in Russia yet, so we will use SDR recordings from the Dutch multiplex. Those who wish from other countries can also join and send me records in IQ format, I will process them and make a pivot table.
How can you listen to DAB? Because digital standard, then it can be decoded using a computer and an rtl-sdr receiver. There are two programs - и , both can work with rtl-sdr.
looks like a student's term paper, and the author obviously didn't bother with the design - fonts don't fit into controls, windows don't scale. But for us, the most important thing is that it allows you to read and write IQ files.
still in beta, but it works much better and decodes better. It is also possible to output quite a lot of additional debugging information:
But welle.io does not yet know how to work with iq files, so we will use Qt-dab.
For testing, I uploaded 3 files to cloud.mail.ru, each contains a one-minute DAB multiplex record, the file size is about 500MB (this is the size of IQ records for SDR with a bandwidth of 2.4MHz). You can open the files in Qt-dab, the download link of which is given above.
File-1:DAB-8A.sdr- . Multiplex 8A operates at a frequency of 195.136 MHz and contains 16 stations. The bitrate of all stations is 64Kbps.
File-2:DAB-11A.sdr- . Multiplex 11A at a frequency of 216.928 MHz. It contains 6 stations, with bitrates of 48, 48, 48, 48, 64 and 48KBps respectively.
File-3: DAB-11C.sdr - . Multiplex 11C at a frequency of 220.352 MHz, also contains 16 stations. The bit rates of all stations are respectively: 80, 80, 80, 80, 56, 96, 80, 64, 56, 48, 64, 64, 64, 96, 80 and 64Kbps.
As you can see, there are no problems with the number of stations, but the main problem is the low bitrate. As for the content itself, tastes are different and I will not discuss it, those who wish can download the files and listen on their own. Not all multiplexes are listed in the entries, but the general idea is, I hope, clear.
Conclusions
If we talk about the prospects for digital broadcasting, then, alas, they are rather sad. The main advantage of DAB is the more efficient use of the spectrum, which allows more stations to be on the air. In this regard, DAB + makes sense only for those cities where there is no free space in FM. For Russia, this is probably only Moscow and St. Petersburg, in all other cities there are no such problems.
As far as sound quality is concerned, DAB+ can technically provide bit rates up to 192Kbps, which will give you almost HiFi sound. In practice, as we see above, broadcasters save money and do not go over the bar even at 100Kbps. Of the three multiplexes, only one (!) station was found broadcasting at 96Kbps (and I can’t call broadcasting music from 48kbps anything other than blasphemy - such broadcasters should be deprived of their licenses;). So, alas, we can say with 99% certainty that when switching from FM to DAB, the sound quality will be worse than it was. Of course, the situation may be better in other countries, but for example, an English review on youtube with an eloquent title . Technically, DAB is good and there are no complaints about it, but economically, "the loot defeated evil."
Returning to Russia, is it worth bothering to start broadcasting in DAB at all? From the point of view of international prestige, probably yes, so as not to look like a backward third world country in the eyes of neighbors, and as a bonus, so that cars and radios bought in Europe can fully receive all stations. But from the point of view of listeners and sound quality, most likely, users will not receive any advantages either in sound quality or in quality of content.
If you think about long-term prospects, then probably in the future the radio will be a device with an integrated e-sim card and a subscription to Yandex music Spotify or Apple Music upon purchase. The future is clearly in streaming services and personalized content. How soon this will happen, we'll see, time will tell.
Source: habr.com