FM-transmitter
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 licenced broadcast stations
The FM-transmitter plugs into an audio output (usually the headphone socket) 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 with any average radio (up to about 75 feet with a very good radio under perfect conditions) and can broadcast on any FM frequency from 87.9 to 107.9 MHz (or 76.0 to 108.0 outside the US). 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 portablee audio devices such as Mp3 or CD players but can also be used with satellite recievers or computer soundcards to distribute their audio to adjacent rooms. These devices are commonly refered to as "I-trips" although strictly speaking "I-trip" is just one of many brands of such device on the market.
Limitations
- The relatively low power output of FM-transimitters sometimes akes 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.
- Some models which connect via ports other than the headphone socket have no means of controlling the audio level (volume) which can cause problems with tracks being too loud (causing overmodulation, audio distortion and possible radio interference) or too low. In theory a device could use an automatic level control or audio limiter circut 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 ERO (European Radiocommunications Office) has recently introduced a 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 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. Which means 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, in its current form due to the miniscule range of such devices to such legislation is rarely enforced against end users although retailers in some juristictions have being threatened with prosecution.
UK legality developments
The United Kingdom Radio Communications Agency OFCOM has launched a consultation, with a view to legalise the iTrip within the UK. Using the 2003 Wireless Telegraphy Exemption Regulations Act, it will be able to adjust the Wireless Telegraphy Act 1949, to provide such devices. The agency says that it hopes it could have a Europe-wide legislative framework in place by the end of the year.
This could mean that by 2007 the iTrip can be used without licence in the United Kingdom, however the device may require technical modification so that it conforms and can obtain its CE mark to be used in Europe/UK.
OFCOM's press release details the proposed changes to the Wireless Telegraphy Act 1949 and also links to the Consultation Document.
Regulations will legalise the use of certain types of FM transmitter when they come into force on 8 December 2006.
legal FM transmitters 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.
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.
Most electronic retail stores sell several different models of FM transmitter, which may vary greatly in price.