FM transmitter (personal device)

"This article is concerned with low powered transmitters used in some countries for interfacing personal audio devices "FM transmitter" can also refer to high powered broadcast equipment used by pirate radio and licensed broadcast stations"

An FM transmitter is a portable device that plugs into the headphone jack or proprietary output port of a portable audio or video device, such as a media player, CD player, or satellite radio system. The sound is then broadcast through the transmitter, and plays through an FM frequency. Purposes for an FM transmitter include playing music from a device through a car stereo, or any radio without an audio input jack.

The FM-transmitter plugs into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios. Most devices on the market typically have a short range of up to 30 feet (9 meters) with any average radio (up to about 75 feet (23 meters) with a very good radio under perfect conditions) and can broadcast on any FM frequency from 76.0 to 108.0 MHz (or 87.9 to 107.9 in the US). Some lower-cost transmitters are hard-wired to the 87.7-91.9 MHz band allocated to educational broadcasts in the United States, or a certain other smaller range of frequencies.

FM transmitters are usually battery driven, but some use the cigarette lighter socket in cars, or draw their power from the device itself. They are typically used with portable audio devices such as MP3 or CD players, but are also used to broadcast other outputs (such as that from a computer sound card) throughout a home or other building.

Limitations

* The relatively low power output of FM transmitters sometimes makes it unsuitable for use in some large urban areas because of the number of other radio signals. This is compounded by the fact that strong FM signals can bleed over into neighboring frequencies making the frequencies unusable with the transmitter. Removing a car's radio antenna has been found to significantly improve transmitter reception. [ [http://barnacle.godsong.org/ask_the_box/0000192.html q the z: not a blog] ]

* Some models which connect via ports other than the headphone jack have no means of controlling the volume, which can force the sound to transmit out from the device harshly (causing over modulation, audio distortion and possible radio interference), or too low. In theory a device could use an automatic level control or audio limiter circuit to overcome this problem although there are few (if any) devices with such a facility available on the market yet.

European legality of FM transmitters

The European Union's Radio Spectrum body the [http://www.ero.dk/ ERO] (European Radiocommunications Office) has recently introduced a [http://www.ero.dk/documentation/docs/doc98/official/pdf/REC7003E.PDF recommendation document] (Table/Annex 13) for Member States to include Transmitters in the FM Band for Music Devices. The underlying specification suggests that the radio transmitter will only emit a maximum of 50 nanowatts Effective radiated power. It is not known what the current "iTrip" device emits although it is known that some devices supposedly manufactured to the US "FCC Part 15" standard emit considerably more. It also has to be ratified and entered into law in each European State, meaning that consultation will normally take place with the users of the spectrum in each country, a protracted and sometimes lengthy process. Until the recommendation is put in place and the law in the country of residence changed, an FM transmitter remains illegal to operate in many EU countries. Due to the minuscule range of these devices the existing legislation is rarely enforced against end users, although retailers in some jurisdictions have been threatened with prosecution.

UK legality developments

Regulations to legalise the use of certain types of FM transmitter came into force on 8 December 2006.From the end of 2006 the iTrip and other FM transmitters can be used without licence in the United Kingdom. To be legal, it must carry a CE mark which indicates their approval for sale in the European Union. Some FM transmitters have been manufactured for sale and use specifically in the US. These devices do not carry a CE mark and will remain illegal to use in the UK.

The new Regulations set out the technical specifications for legal devices. This is to minimise the risk of interference to other radio devices. In particular, the Regulations set a 50 nanoWatts power limit for legal devices, which limits the distance at which they can broadcast to up to 8 meters.

The changes to the Wireless Telegraphy Act 1949 were announced in a [http://www.ofcom.org.uk/consult/condocs/exemption/statement/ statement] from Ofcom.

Use of FM transmitters is now governed by [http://www.ofcom.org.uk/radiocomms/isu/licence_exempt/regulations/ Wireless Telegraphy Exemption Regulations]

Models

There are several universal models, as well as those specifically designed for and by certain leading brands; for instance, Griffin Technology manufactures the iTrip which can exclusively be used on iPods. Other examples of FM transmitters are [http://www.wholehousefmtransmitter.com Whole House FM Transmitter] , Arkon's SoundFeeder, or Belkin's FM TuneCast. Belkin manufactures a premium FM transmitter for iPods called the TuneBase FM with ClearScan. This device mounts an iPod on a sturdy flexible arm (which encases the fm transmitter and power connectors) and allows the iPod to be charged from a car cigarette lighter or power outlet.

Most electronic retail stores sell several different models of FM transmitter, including generic brands, which may vary greatly in price, even though quality of audio is indistinguishable. However, the signal strength varies.

ee also

*Small-scale use of the FM broadcast band
*Wireless microphone

References

External links

* [http://badacetechshow.com/psp_itrip.htm iTrip in a PSP - This site also has a wire diagram of the iTrip]
* [http://www.griffintechnology.com/products/itrip/ iTrip product info page by Griffin Technology]
* [http://www.theregister.co.uk/2003/07/31/uk_bans_ipod_radio_addon/ 2003 Register article about the legality of the iTrip in the UK]
* [http://www.fuzzmagazine.com/Features%20%26%20Reviews/869C8FD5-2211-4771-84EF-60CCCF52ECD1.html FUZZ Music Magazine: Gear Review/iTrip Auto]
* [http://www.itripsireland.com iTrips Ireland: iTrip News and Info]
* [http://www.zen22142.zen.co.uk/Circuits/rf/txcct.htm A simple FM transmitter circuit]
* [http://www.fmtransmitterreview.com Quick list of several FM transmitters currently on the market]
* [http://www.ebu.ch/CMSimages/en/tec_text_r120-2007_tcm6-49264.pdf Technical article about potential problems due to legalisation of personal transmitters in Europe]
* [http://www.nprlabs.org/reports/FMModulatorUsage.pdf Technical article about potential problems due to overpowered personal transmitters in the United States]