The technology behind radio: from AM to DAB+

AM and FM: two ways to hide sound in a wave

A radio station sends out a wave with a fixed frequency, the carrier wave. The sound is cleverly hidden in it, and your radio takes it out again. There are two main methods for this.

AM (amplitude modulation): the strength of the wave follows the sound. This is simple and the waves travel far, especially on long wave, medium wave and shortwave. The drawback is that every disturbance, from thunderstorms to a light switch, is heard as crackle or hiss. The sound is also narrow, roughly like a telephone with slightly better sound.

FM (frequency modulation): not the strength but the frequency of the wave follows the sound. Disturbances that affect the strength then matter little. The sound is clear, and stereo is possible. The drawback is the range: FM waves travel in straight lines, so you need many transmitters. In Europe the FM band lies between 87.5 and 108 megahertz.

Background photo: John Reinartz with a shortwave receiver (1925), Wide World Photos, public domain. Resized and tinted.

Long wave, medium wave and shortwave: the world in your living room

2. 1927
   Philips in Eindhoven broadcasts across the sea for the first time, experimentally on shortwave, around 30 metres. The station is called PCJJ.
3. November 1928
   Eddie Startz starts the Happy Station on PCJJ, a programme for Dutch people all over the world.
4. 1933-1940
   PCJJ broadcasts under a licence and moves to Huizen. In November 1937 the PCJ in Huizen is demonstrated, with a rotatable directional antenna that could reach any point on earth with a sharp beam, intended to reach the Dutch colonies, especially the Dutch East Indies.
5. May 1940
   After the German invasion the shortwave service is shut down. The transmitter is destroyed and later used by the Germans for propaganda. In October 1945 PCJJ is back on the air.
6. 15 April 1947
   The Stichting Radio Nederland Wereldomroep is founded. In 1949 the station has 40 kilowatts; by the late 1950s it is three transmitters of 100 kilowatts. In 1960 new studios open on the Witte Kruislaan in Hilversum, and in the 1960s relay stations come to Bonaire and Madagascar.
7. 1938 or 1941
   A medium wave transmitter is built at Lopik. Sources give 1938 or 1941 for the start (it replaced the transmitter at Jaarsveld from 1937). On 21 August 2004 the second mast (165 metres, 1332 kilohertz) is taken down.
8. September 2015
   Radio Maria ends its last medium wave broadcast from Lopik (675 kilohertz) on 1 September. On 4 September 2015 the 196-metre mast is taken down: the end of 75 years of medium wave from this spot.

Background photo: Announcer with gramophone records in the studio (1940s), archive of the Finnish broadcaster Yle, public domain. Resized and tinted.

FM in the Netherlands: the network of 1954

2. June 1954
   NOZEMA's ten-year plan gets under way: the Netherlands is to get a national FM network for better reception. The first FM transmitter is at Hulsbergen near Maastricht (97.5 megahertz) and is officially in use on 15 June 1954, at first with Hilversum 1. So the transmitter plan begins in the south-east, not in the big cities.
3. 26 October 1954
   Hengelo (Overijssel) starts test broadcasts on 97.2 megahertz, with a range of about 50 kilometres. Markelo, Diever, Hoogezand and Irnsum follow, among others, and in a later phase Goes, Breda, Eindhoven, Alkmaar and Lopik. The aim was a field strength of at least 1 millivolt per metre, in the countryside at least 0.5.
4. 10 December 1960
   The Gerbrandy Tower near IJsselstein, also called the Lopik transmitter mast, broadcasts the first two 50-kilowatt FM transmitters for the public broadcaster. The tower is 366 metres high and was opened by Queen Juliana on 9 May 1961.
5. 31 March 1969
   The Gerbrandy Tower starts broadcasting FM stereo.
6. March 2004
   Digital radio also starts from the Gerbrandy Tower, then still T-DAB.
7. 15 July 2011
   A fire in the Gerbrandy Tower temporarily switches off the radio stations and large parts of the Netherlands lose FM reception. KPN's FM transmitters stay on, though. Only on 11 April 2012 is everything fully back in operation. In 2017 the tower becomes a national monument.

The plan of 1954 was to switch off the AM support transmitters as soon as FM coverage was complete. Medium wave would remain for foreign listeners and for the Dutch-language service. As seen above, that switching off took until well into the 21st century.

Background photo: The Gerbrandy Tower in IJsselstein, Gouwenaar, CC0. Resized and tinted. On Wikimedia Commons there is also a photo by Steven Lek (CC BY-SA 4.0).

Internet radio: from a talk show in 1993 to the NPO Radio 2 stream

2. 1993
   Carl Malamud starts Internet Talk Radio with his weekly programme Geek of the Week, streamed via multicast: one stream for everyone on the network, not a separate stream per listener. On 24 June the band Severe Tire Damage gives the first live internet concert.
3. 1994
   On 7 November the American WXYC announces that it will become the first ordinary radio station to broadcast on the internet. On 3 December KJHK starts one of the first live and continuous internet radio streams.
4. 1998-1999
   Nullsoft, the maker of the music player Winamp, releases the streaming server SHOUTcast in 1998. The protocol works over ordinary HTTP and sends the title and artist along in the stream; the codes begin with ICY. AOL buys Nullsoft on 1 June 1999.
5. 2009
   Apple releases HTTP Live Streaming (HLS): short pieces of audio or video via ordinary web servers, with a playlist that the player follows. The BBC uses HLS for live internet radio from 2014-2015.
6. Now
   Most stations stream between 64 and 128 kilobits per second, with codecs such as MP3, AAC and Opus. You listen on your phone, smart speaker, car or TV, and through apps such as TuneIn.

What does it take to broadcast radio over the internet?

The studio with mixing desk and playout software delivers the sound to an encoder. That squeezes the sound into a codec such as MP3 or AAC. A streaming server, for example Icecast, SHOUTcast, Wowza or nginx with a streaming module, passes the stream on to listeners, often via a CDN, a network of intermediate servers that spreads the load. Your player or app fetches the stream and plays it.

Background photo: Server racks at NOIRLab in Tucson, NOIRLab/NSF/AURA/T. Slovinský, CC BY 4.0. Resized and tinted. You can see a modern radio studio in the photo of the 3FM studio by Dulve (CC BY-SA 4.0).

DAB+: digital radio over the air

DAB+ broadcasts radio digitally via transmitter masts, without the internet. You receive several stations per frequency block, a multiplex, and usually have less trouble with noise than on FM. A set with a screen can also show the artist and title.

2. 2013
   The commercial national stations have broadcast on DAB+ since 1 September 2013, public radio since 14 October 2013. In 2014 the public broadcaster's coverage goes from 70 to 95 percent.
3. 2020
   In the first quarter ten local ensembles start with more than 40 stations in total. There are also several regional multiplexes. In early March 2021 the WorldDAB page counts 2 national and 5 regional multiplexes and 121 DAB+ stations, of which 68 are also on AM or FM and 53 digital only.
4. 2024
   A consultation is under way for local DAB+ with 57 areas (allotments); more than 200 local public broadcasters apply for a licence, with 815 licences in total.
5. November 2025
   A study by Statistics Netherlands (CBS) shows that 34.1 percent of the Dutch have DAB+ at home or in the car, against 32 percent a year earlier. Almost every new car has DAB+ as standard: 95 percent of the cars newly sold in the second half of 2021.

And FM? In the past the Dutch government has talked about switching off FM if DAB+ were successful enough, with 2023 as a possible date in an old text. We found no fixed end date for FM in the sources used. If you want to know the current state of affairs, check WorldDAB or the Dutch national government.

Background photo: Sangean DPR-215, a DAB receiver, Solaris2006, public domain. Resized and tinted.