Norway says goodbye to the analogue radio in favor of digital: the Oslo government has announced that January 11, 2017 will start the switch off of the FM (frequency modulation) transmission technology that is most used in the world since the second half of last century.
The country will be the first in the world to switch to the Digital Audio Broadcasting (DAB) system characterized by a sound quality equal to that of the compact disc.
This technology, in Norway, is available now for 20 years. According to a survey conducted by TNS Gallup in Norway, 56% of the listeners uses the standard digital daily and 55% already has a DAB device and 20% of the car is equipped with DAB systems.
Features Digital Audio Broadcasting (DAB)
Benefits of DAB
Current AM and FM terrestrial broadcast technology is well established, compatible, and cheap to manufacture.
- Improved features for usersDAB radios automatically tune to all the available stations, offering a list for the user to select from. DAB can carry “radiotext” (in DAB terminology, Dynamic Label Segment, or DLS) from the station giving real-time information such as song titles, music type and news or traffic updates.DAB receivers can display time of day as encoded into transmissions, so is automatically corrected when travelling between time zones and when changing to or from Daylight Saving. Some radios offer a pause facility on live broadcasts, caching the broadcast stream on local flash memory, although this function is limited.
- More stationsDAB is not more bandwidth efficient than analogue measured in programmes per MHz of a specific transmitter (the so-called link spectral efficiency). It is less susceptible to co-channel interference (cross talk), which makes it possible to reduce the reuse distance, i.e. use the same radio frequency channel more densely. The system spectral efficiency (the average number of radio programmes per MHz and transmitter) is a factor three more efficient than analogue FM for local radio stations, as can be seen in the above numerical example. For national and regional radio networks, the efficiency is improved by more than an order of magnitude due to the use of SFNs. In that case, adjacent transmitters use the same frequency.In certain areas – particularly rural areas – the introduction of DAB gives radio listeners a greater choice of radio stations. For instance, in South Norway, radio listeners experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.
- Reception qualityThe DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analogue systems.Also, as DAB transmits digital audio, there is no hiss with a weak signal, which can happen on FM. However, radios in the fringe of a DAB signal, can experience a “bubbling mud” sound interrupting the audio and/or the audio cutting out altogether.
- Less undocumented station interferenceThe specialised nature and cost of DAB broadcasting equipment provide barriers to undocumented stations broadcasting on DAB. In cities such as London with large numbers of undocumented radio stations broadcasting on FM, this means that some stations can be reliably received via DAB in areas where they are regularly difficult or impossible to receive on FM due to undocumented radio interference.
- Variable bandwidthMono talk radio, news and weather channels and other non-music programs need significantly less bandwidth than a typical music radio station, which allows DAB to carry these programmes at lower bit rates, leaving more bandwidth to be used for other programs.
- Transmission costsIt is common belief that DAB is more expensive to transmit than FM. It is true that DAB uses higher frequencies than FM and therefore there is a need to compensate with more transmitters, higher radiated powers, or a combination, to achieve the same coverage. However, the last couple of years has seen significant improvement in power efficiency for DAB-transmitters.This efficiency originates from the ability a DAB network has in broadcasting more channels per network. One network can broadcast 6-10 channels (with MPEG audio codec) or 10-16 channels (with HE AAC codec). Hence, it is thought that the replacement of FM-radios and FM-transmitters with new DAB-radios and DAB-transmitters will not cost any more as opposed to newer FM facilities.
Disadvantages of DAB
- Reception qualityThe reception quality on DAB can be poor even for people who live well within the coverage area. The reason for this is that the old version of DAB uses weak error correction coding, so that when there are a lot of errors with the received data not enough of the errors can be corrected and a “bubbling mud” sound occurs. In some cases a complete loss of signal can happen. This situation will be improved upon in the new DAB standard DAB that uses stronger error correction coding and as additional transmitters are built.
- Audio QualityBroadcasters have been criticized for ‘squeezing in’ more stations per ensemble than recommended, by:
- Minimizing the bit-rate, to the lowest level of sound-quality that listeners are willing to tolerate, such as 112 kbit/s for stereo and even 48 kbit/s for mono speech radio such as LBC 1152 and the Voice of Russia.
- Having few digital channels broadcasting in stereo.
- Signal delayThe nature of a SFN is such that the transmitters in a network must broadcast the same signal at the same time. To achieve synchronization, the broadcaster must counter any differences in propagation time incurred by the different methods and distances involved in carrying the signal from the multiplexer to the different transmitters. This is done by applying a delay to the incoming signal at the transmitter based on a timestamp generated at the multiplexer, created taking into account the maximum likely propagation time, with a generous added margin for safety. Delays in the receiver due to digital processing (e.g. deinterleaving) add to the overall delay perceived by the listener. The signal is delayed by 2–4 seconds depending on the decoding circuitry used.
Time signals, on the contrary, are not a problem in a well-defined network with a fixed delay. The DAB multiplexer adds the proper offset to the distributed time information. The time information is also independent from the (possibly varying) audio decoding delay in receivers since the time is not embedded inside the audio frames. This means that built in clocks in receivers will be spot on.
- CoverageAs DAB is at a relatively early stage of deployment, DAB coverage is poor in nearly all countries in comparison to the high population coverage provided by FM. An exception is Norway, as the country will have 99.5% coverage by the end of 2014.
- CompatibilityIn 2006 tests began using the much improved HE-AAC codec for DAB+. Virtually none of the receivers made before 2008 support the new codec, however, thus making them partially obsolete once DAB+ broadcasts begin and completely obsolete once the old MPEG-1 Layer 2 stations are switched off. New receivers are both DAB and DAB+ compatible; however, the issue is exacerbated by some manufacturers disabling the DAB+ features on otherwise compatible radios to save on licensing fees when sold in countries without current DAB+ broadcasts.
- Power requirementsAs DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is higher. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk). This means that they use more energy than analogue Band II VHF receivers.
- Use of Licensed CodecsThe use of MPEG previously and later AAC has prompted criticism of the fact that a (large) public system is financially supporting a private company. In general, an open system will permit equipment to be bought from various sources in competition with each other but by selecting a single vendor of codec, with which all equipment must be compatible, this is not possible.