FM transmitter (personal device)
A personal FM transmitter is a low-power FM radio transmitter which purpose is to allow music or other audio content from a portable media player, CD player or other portable audio system to be played through a nearby FM radio. It can either be a capability built into a portable media player, or a portable appliance that plugs into the headphone jack or proprietary output port of a portable media player or video device. The sound is then broadcast through the transmitter on an FM broadcast band frequency and picked up by the receiver. The purpose is generally to allow portable audio devices to make use of the better sound quality of a home audio system or car stereo without requiring a wired connection between them. Often called simply an FM transmitter, it is a low power device that should not be confused with high powered commercial FM broadcasting transmitters.
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 100 feet (30 metres) with any average radio (up to about 300 feet (100 metres) with a very good radio under perfect conditions) this range can also be enhanced if operated in fixed locations of good high elevation, such as a multi-story apartment or tall building and can broadcast on any FM frequency from 87.5 to 108.0 MHz in most of the world, (or 88.1 to 107.9 in the US and Canada). 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 (and sometimes outside of vehicular use), or draw their power from a mains powered wall socket or the device itself. They are typically used with portable audio devices such as MP3 players, as well as hi-fi systems, message systems, etc.. They are also used to broadcast other outputs (such as that from a computer sound card) throughout a home or other building.
- 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. Some frequencies below 88.1 have even been supported as reception frequencies on some car stereos, and some indegeneous FM transmitters even take advantage of those unused frequencies which are generally more reliable as no frequency below 88.1 is used for mainstream broadcasters in the US.
- 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 out 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 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.
Within the European Radiocommunications Office in the case the of the Members States that also belong to the European Union the situation is as follows. In 2006, the legislative powers for harmonisation of the technical conditions for use of spectrum for a wide variety of short-range devices, including applications such as alarms, local communications equipment, door openers and medical implants were transferred from the EU Member States to the European Union by Decision of the European Commission 2006/771/EC. Therefore European States no longer have legislative powers in this field, but the powers to police and impose sanctions for non-respect of this EU acts remain in the hands of the Member States. Following the recommendation of the European Radiocommunications Office, by Decision of the European Commission 2009/381/CE amending Decision 2006/771/EC on harmonisation of the radio spectrum for use by short-range devices , in the frequencies of 87,5-108 MHz, it is allowed to use micro FM transmitters of less than 50 nanowatts Effective radiated power. Since then, these FM transmitters are automatically allowed to enter and being marketed in the territory of the European Union for the use of consumers, as long as they have the CE mark. For countries that belong to the European Radiocommunications Office but are not Member States of the European Union, national law applies and reference has to be made to legal procedures of each country.
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, such as the popular Belkin Tunecast 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 around 30 metres in stereo, using a modern hi gain FM radio, however by receiving the signal in mono this range can be increased due to the lack of audible noise when receiving in mono, as opposed to the inherent noise generated when listening to FM broadcasts using the Zenith GE stereo subcarrier system. Mounting these FM transmitters in a clear line of sight and in the highest part of a building, will sometimes improve its effective range to some extent as well.
Use of FM transmitters is now governed by Wireless Telegraphy Exemption Regulations
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. Even though several models of MP3 player have built-in FM receivers; other models have had built-in FM transmitters to eliminate the need for a separate device. Some cheaper brand units can exhibit very poor audio quality with sibilance (distorted T and S sounds) and poor stereo imaging. A single search on FM Transmitters gives a list of most popular models.