FM transmitter (personal device)
A personal FM transmitter is a low-power FM radio transmitter that broadcasts music (often from an MP3 player) to a standard FM radio. It can be utilized in cars. Most of these transmitters plug into the music player's headphone jack and then broadcast the signal over an FM broadcast band frequency, so that it can be picked up by any nearby radio. This allows 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 which only works over distances of a few metres, compared to high powered commercial FM broadcasting transmitters which can be received many miles away.
Most transmitters on the market typically have a short range of 100–300 feet (30–100 metres), depending on the quality of the receiver and the propagating conditions. This range can also be enhanced by operating in fixed locations at high elevation, such as a multi-story apartment or tall building. These transmitters can broadcast on any FM frequency from 87.5 to 108.0 MHz in most of the world (or 88.1 to 107.9 MHz 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 to 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 MP3 players. They are also used to broadcast other outputs (such as that from a computer sound card) throughout a home or other building.
Personal FM transmitters serve as a workaround dongle to allow an electronic device equipped with an AM/FM receiver to support features and functions not native to the stereo itself.
Examples of applications
- AM radio AM R
- Bluetooth audio receiver R
- cordless headphones (headphones with integrated FM tuner) AM C CF F R T W
- handsfree cellular M N S
- HD radio receivers F* O R
- internet radio F* I N
- Mac/PC desktop/laptop computers CF USB
- portable CD player C CF
- portable media players (e.g. iPod) CF
- protection of playback equipment from watery environments, such as if a shower radio were to be used when hearing one's MP3 player, or non-waterproof satellite or HD radio system CF R W WP
- tape player (cassette/8-track) T
- turntables (45/LP vinyl records) A
- satellite radio (Sirius XM Holdings) O R
- smartphones CF
- tablet computers (e.g. iPad) CF
- USB/SD card MP3 players CF USB
- voice advertising for soapboxes M
- cordless transmission of AUX in W
- other FM radios F** R
Note: some parameters are to be observed when using FM transmitters:
A NOT RECOMMENDED for use in automobiles, due to tactile force
AM a workaround if an FM radio without AM support is being used
C workaround if a stereo does not support CD insertion
CF grants support for audio formats of computer files [e.g. MP3, WAV, FLAC, ATRAC (proprietary Sony format), etc.], as in using portable CD players on car radios with a CD player of their own that does not support MP3s, and also solid-state devices that support these file formats
F this method makes it possible to hear FM radio stations on other frequencies, and a * indicates that this standard is largely a "mirror" of most analog FM frequencies. A ** indicates the possibility of "feedback" if the receiving frequency is the same as the transmitting frequency for this type of setup.
I supplies streamable content from the Internet
M a microphone is also required for this application
O this device offers a future opportunity to decommission analog FM frequencies
R addition of another radio standard
S ensures more safety to the driver
T recommended for AM/FM radios without a tape deck (8-track/cassette)
USB externalizes USB support to radios that do not natively support it
W an optional measure for radios, even if they already have AUX in, so as to ensure a "wireless" connection instead. Bluetooth can be an alternative to this in some cases.
WP an application where non-waterproof devices on the transmitting end are transmitting to a water-resistant AM/FM receiver (such as a shower radio)
Procedures for use of FM transmitters
Note that FM transmitters are recommended largely for AM/FM (sometimes FM-only) systems that do not have TRS connector or RCA jack AUX in, since AUX inputs are to be used if they are available, because FM transmitter use can be troublesome depending on parameters. However, first-time use of FM transmitters using AM/FM systems as a testbed even with AUX input availability is recommended, to ensure reliability in future applications where AUX inputs are unavailable.
Use of FM on devices that do not natively support FM
The opposite tactic of using an FM transmitter to use AM/FM radios without AUX inputs takes advantage of AUX inputs on stereos that do not internally support FM reception. For instance, devices like inexpensive stereos made for iPods (iHome for instance) and MP3 players in general can simply take advantage of the built-in FM tuner on MP3 players. Computer speakers generally do not internally support FM either, so they can be used in conjunction with computer-based devices such as PCs, laptops, iPods, smartphones, and other devices that supply radio reception to utilize the speakers. Sometimes FM transmitters can be tested for reliability using an iPod on FM reception mode, utilizing stereos that do not internally support FM.
Special applications where both an external FM receiver and an FM transmitter would be used are special scenarios where USB/SD flash drive MP3 players with a built-in FM transmitter (but no headphone out port) are used with stereos that do not have a built-in FM tuner.
- The relatively low power output of FM transmitters sometimes makes them 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 indigenous FM transmitters even take advantage of these 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, but there are few (if any) devices on the market yet with such a facility available.
- When trying out FM transmitters for the first time, it is recommended to test them on any FM radio (outside of a car), and to nominate them for radios without AUX inputs when testing them on the first available FM receiver (regardless of AUX input presence), since being lenient about parameters is the best approach to ensuring reliability in the beginning, because of these limitations.
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 emit a maximum of only 50 nanowatts of 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. This standard 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 jurisdiction of the European Radiocommunications Office, the situation in the case of Member States that also belong to the European Union is as follows. In 2006, 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 power to police and impose sanctions for non-respect of this EU act remains 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 , at frequencies of 87.5-108 MHz, it is allowed to use micro FM transmitters of less than 50 nanowatts of effective radiated power. Since this decision these FM transmitters are automatically allowed to enter and be 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. Since 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, they 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 with 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 a maximum of around 30 metres in stereo, using a modern high 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 their 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. While several models of MP3 player have built-in FM receivers, other models have had FM transmitters built in 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.
- Bluetooth (can be used as an alternative in some cases)
- Car audio
- Small-scale use of the FM broadcast band
- Wireless microphone
- Cassette adapter