What radio platform is best?

This is a full overview of broadcast radio platforms, with benefits, drawbacks and rough costs for each; and, roughly, which countries are using these technologies. Comments are always welcome.

AM and FM

The original way of broadcasting radio, 'Amplitude Modulation', was supplanted by a higher quality transmission method in the late 1950s, called Frequency Modulation. Unlike AM, FM transmissions can include a stereo effect, as well as, because of their higher transmission frequencies and different modulation format, a greater resistance to interference.

AM/FM is used everywhere in the world; though the US and Canada use different frequency spacing to Europe, and some parts of the world, notably Japan, uses different frequencies. FM also has subtle differences between different continents in terms of "pre-emphasis", which essentially means that a European-bought radio won't sound as good in the US as a US-bought one.

Benefits: Everyone's got a radio. Robust reception for many uses.
Drawbacks: AM can sound awful for music, particularly at night. FM can be subject to interference from pirate radio stations and occasionally atmospheric effects; as well as poor signal strength. Finite frequency resorce can mean a lack of choice.
Potential listeners: Transmissions can be targeted at a small local area, or much larger. Every household has on average 4.5 radios.
Current listeners: 61.1% of all radio listening in the UK is to AM or FM. (RAJAR Q2 2012)
Sound quality: AM is highly variable and mono. A good FM signal can sound excellent.
Transmission cost: Around £7,500 for a single local FM transmitter. National networks are far higher.
Reception cost: Receivers are anything from £1 upwards.
Coverage: From a few miles to hundreds of miles each.
Pay-radio capable: No
Now playing capability: limited information via an RDS-equipped FM set
Schedule capability: None
Display: station name and limited information via an RDS-equipped FM set
Best for: most radio listeners who are comfortable with the current sound quality of their current station, who don't want or need extra choice.

Digital Satellite Radio via TV

In the days of analogue satellite television, all television channels came with around nine mono audio channels, primarily meant for transmitting different language versions of the same television programme. While some channels used this, notably Eurosport, many channels sublet their audio carriers to radio stations. Tuning in to radio in this way was difficult, and confusing - since the picture of the hosting channel remained on the screen - and consequently few people knew of the facility, let alone tried it.

With the advent of digital satellite transmissions in late 1999, radio stations were given their own channel numbers and displays, so publicly-accessible digital satellite radio became a reality. Sky and Freesat are the only satellite systems aimed specifically at the UK, and there are currently a large number of channels on the system. Media UK lists radio stations on Sky and the slightly smaller list of radio stations on Freesat (though most of the rest can be tuned-in manually). Radio stations are accompanied by a 'soft' dark-blue display, including information related to the current programme. On Freesat, some BBC stations are accompanied by a logo and now-playing information similar to DAB.

Digital Satellite Radio via TV in this way is also available in some parts of the world, notably South Africa and Australia.

Note: in the US, there is a satellite radio service called Sirius XM, which uses a number of terrestrial transmitters and satellites to broadcast a US-wide service of a few hundred stations. There is no such service in Europe, and due to the economics, no such service is likely.

Benefits: Satellite can deliver high quality audio over a large coverage area. Large choice.
Drawbacks: Few local radio stations use the system. No dynamic 'now playing' information on Sky. Limited information on Freesat. Fixed antennas only, so no mobile coverage.
Potential listeners: Around 2m FreeSat households; 7m Sky households (Ofcom, 2012)
Current listeners: Unrecorded (separately) by RAJAR. 4.7% of all radio listening is via the television (RAJAR Q2 2012).
Sound quality: Digital, between 96 and 192k, MP2 encoded.
Transmission cost: Around £20,000 for transponder space, and £20,000 for appearance in Sky's Electronic Programme Guide (and therefore allocation of a channel). Freesat also charges for appearance in their EPG.
Reception cost: A digibox is free with commencement of a SkyDigital subscription (the cheapest is around £258 a year), or you can buy without any subscription for a one-off £175. No subscription is required to receive the majority of radio stations. A FreeSat box is normally a one-off charge from £50.
Coverage: UK-wide (in practice, most of mainland Europe), to fixed antenna
Pay-radio capable: Yes (Sky only)
Now playing capability: None on Sky; limited on Freesat
Schedule capability: Now/Next only on Sky; a full schedule on Freesat
Display: limited to text in a standard format
Best for: people who don't want local information, but a wide choice of music, including the ethnic and the unusual. High amount of speech-based programming.

Digital Terrestrial Television

Freeview, launched on 30th October 2002, is a bouquet of entirely free channels through your aerial, including radio channels. Just like Sky, viewers can select a channel - starting at channel 700 - to listen to a limited choice of radio services. Media UK lists radio on Freeview.

Radio in this way is also available in other countries, notably Australia.

Benefit: High quality audio.
Drawbacks: Limited station choice. A significant lack of choice for commercial radio stations. No ethnic programming except that provided by the BBC. Listeners in Scotland lose radio in the evenings due to multiplex capacity.
Potential listeners: 12 million homes claim that Freeview is their primary television supplier (Ofcom, 2012). It should be noted that Freeview, due to its low cost, is a good second-set option (for bedrooms and kitchens) so total potential audience is rather higher.
Current listeners: Unrecorded (separately) by RAJAR. 4.7% of all radio listening is via the television (RAJAR Q2 2012).
Sound quality: Digital, between 96 and 192k, MP2 encoded.
Transmission cost: Rumoured to be upwards of £120,000 per year for a channel. Space is very limited.
Reception cost: DTV set-top boxes available from one-off £19.
Coverage: Dependent on transmitter multiplex, but total coverage approaching 99% of the population. Fixed antenna only.
Pay-radio capable: No
Now playing capability: Limited
Schedule capability: Yes - for up to seven days in advance
Display: text and a station graphic; some stations have interactive applications
Best for: people who want a few extra stations for free with their television - including the BBC's digital bouquet.

DAB Digital Radio

The oldest of all digital transmission facilities, DAB Digital Radio actually started in 1990, with a London-wide transmitter network by 1994. DAB Digital Radio uses a network of local transmitters covering particular areas of the country - transmitters which are designed to work together, unlike current AM, FM and TV transmissions. Media UK lists radio stations on DAB.

Benefit: Existing stations mostly available on DAB Digital Radio, and extra choice. Great audio difference between AM and DAB; higher signal-to-noise ratio than FM. Portable and easy to tune.
Drawbacks: Variable audio quality and coverage
Potential listeners: 41.7% of all adults live in a household with a DAB receiver (RAJAR Q2 2012)
Current listeners: 20.1% of all radio listening is to DAB (RAJAR Q2 2012)
Sound quality: Digital. Music stations from 112k to 192k (most at 128k) MP2 stereo. Some lower and mono.
Transmission cost: Around £30,000 for carriage on a local multiplex. Higher in London. Dependent on bitrate.
Reception cost: Receivers available for less than £15.
Coverage: Around 92% of population, and growing. Some blackspots. Designed for mobile reception.
Pay-radio capable: No
Now playing capability: Yes
Schedule capability: On supporting sets from supporting broadcasters. In practice, few sets support the EPG.
Display: constantly-updating 128 character display. Some use of DAB slideshow (full-colour images) on suitably-equipped radios.
Best for: people who want to listen to their current stations, as well as gain more choice and higher quality than AM transmissions. Best choice for high quality mobile reception subject to local reception conditions.

DAB+ Digital Radio

An update to the original MP2 digital radio that the UK uses, DAB+ is in use in many countries across the world: from Germany to Norway, Australia to Belgium. Any DAB+ radio will also pick up DAB transmissions, and it's likely that most radios bought in the future will support DAB+ out of the box.

Benefits to DAB+ over original DAB are mostly related to a better audio compression codec, AAC plus, which allows broadcasters to squeeze more stations into the same space. Usage elsewhere would seem to suggest that DAB+ does not appear to result in higher quality audio. Additionally, there is a better error correction on DAB+, which subtly improves reception in marginal areas.

There are currently no DAB+ tranmissions in the UK, and currently no plans to start using DAB+.

Cable

Digital radio on digital cable carries a smaller choice than digital satellite, and is normally sourced direct from digital satellite (therefore audio quality is at least similar, if not inferior, to digital satellite). Normally available through the TV, not (as in some countries) on FM rebroadcast. Media UK lists radio stations on Virgin Media, the UK's largest cable service.

Benefit: Some extra choice.
Drawbacks: Fixed reception, and limited choice.
Potential listeners: Around 4,000,000 subscribers to Virgin Media.
Current listeners: Unrecorded (separately) by RAJAR. 4.7% of all radio listening is via the television (RAJAR Q2 2012).
Sound quality: Digital. Similar to digital satellite, but may be subject to extra encoding/decoding generation.
Transmission cost: Rumoured to be around £15,000 for carriage per cable network.
Reception cost: Cable boxes normally free with subscription.
Coverage: Cable unavailable in many areas.
Pay-radio capable: Yes
Now playing capability: Limited (offered by BBC only)
Schedule capability: Some providers do have full EPG, but sources for schedules may not be direct from the radio station so can be out of date.
Display: Limited to programme information for most. For BBC, similar to Freeview.
Best for: people who appreciate some extra choice, but who already have cable.

Internet Radio

With the uptake of broadband, the internet can be used for reception of radio stations from around the world: either on a laptop computer or a standalone internet radio.

Benefit: Huge extra choice, including everything from international stations to one-man-band stations. Much larger amounts of interactivity, like webcams and direct chat to the presenters.
Drawbacks: Many stations have poor audio quality, sometimes as bad as AM. Unreliability ('buffering' and unavailability)
Potential listeners: Hundreds of millions worldwide.
Current listeners: 4.6% of all radio listening in the UK is via the internet.
Sound quality: Variable. Some stations sound better than DAB or FM, at up to 192kbps AAC. Some stations sound poor.
Transmission cost: Dependent on listener base, but from £20 a month to hundreds of thousands.
Reception cost: Compatible computers can start from £180. Standalone internet radios start from £99. Broadband internet is £10 a month. Reception while mobile is still difficult.
Coverage: Worldwide, subject to rights
Pay-radio capable: Yes
Now playing capability: Yes
Schedule capability: Full EPGs from websites, though a lack of co-ordinated information.
Display: Video and text is virtually unlimited
Best for: people who listen to a wide variety of eclectic styles, or want to hear speech programming worldwide.