FM transmitter (personal device)
Encyclopedia
- 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 radioPirate radioPirate radio is illegal or unregulated radio transmission. The term is most commonly used to describe illegal broadcasting for entertainment or political purposes, but is also sometimes used for illegal two-way radio operation...
and licensed broadcast stations
An FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
is a portable device that plugs into the headphone jack or proprietary output port of a portable audio or video device, such as a portable media player
Portable media player
A portable media player or digital audio player, is a consumer electronics device that is capable of storing and playing digital media such as audio, images, video, documents, etc. the data is typically stored on a hard drive, microdrive, or flash memory. In contrast, analog portable audio...
, CD player, or satellite radio
Satellite radio
Satellite radio is an analogue or digital radio signal that is relayed through one or more satellites and thus can be received in a much wider geographical area than terrestrial FM radio stations...
system. The sound is then broadcast through the transmitter, and plays through an FM broadcast band
FM broadcast band
The FM broadcast band, used for FM broadcast radio by radio stations, differs between different parts of the world. In Europe and Africa , it spans from 87.5 to 108.0 megahertz , while in America it goes only from 88.0 to 108.0 MHz. The FM broadcast band in Japan uses 76.0 to 90 MHz...
frequency. Purposes for an FM transmitter include playing music from a device through a car stereo, or any radio.
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 300feet (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 appartment or tall building and can broadcast on any FM frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the 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
Cigar lighter receptacle
The cigarette lighter receptacle in an automobile, initially designed to power an electrically heated lighter, became a de-facto standard DC connector to supply electrical power for portable accessories used in or near an automobile...
in cars, 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 hifi systems, message systems, etc.. They are also used to broadcast other outputs (such as that from a computer sound card
Sound card
A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...
) 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.
- Some models which connect via ports other than the headphone jack have no means of controlling the volumeLoudnessLoudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...
, 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 controlAutomatic gain controlAutomatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
or audio limiterAudio signal processingAudio signal processing, sometimes referred to as audio processing, is the intentional alteration of auditory signals, or sound. As audio signals may be electronically represented in either digital or analog format, signal processing may occur in either domain...
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 powerEffective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
. It is not known what the current "iTrip
ITrip
The iTrip is an FM-transmitter made by Griffin Technology as a third party accessory for Apple's popular iPod MP3 player and iPhone.The iTrip plugs into the headphone socket of the iPod and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car...
" 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
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
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 http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:119:0032:0039:EN:PDF, in the frequencies of 87,5-108 MHz, it is allowed to use micro FM transmitters of less than 50 nanowatts Effective radiated power
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
. 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
CE mark
CE marking is a mandatory conformity mark for products placed on the market in the European Economic Area . With the CE marking on a product the manufacturer ensures that the product conforms with the essential requirements of the applicable EC directives...
. 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. To be legal, it must carry a CE mark
CE mark
CE marking is a mandatory conformity mark for products placed on the market in the European Economic Area . With the CE marking on a product the manufacturer ensures that the product conforms with the essential requirements of the applicable EC directives...
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 it's effective range to some extent as well.
The changes to the Wireless Telegraphy Act 1949 were announced in a statement from Ofcom
Ofcom
Ofcom is the government-approved regulatory authority for the broadcasting and telecommunications industries in the United Kingdom. Ofcom was initially established by the Office of Communications Act 2002. It received its full authority from the Communications Act 2003...
.
Use of FM transmitters is now governed by Wireless Telegraphy Exemption Regulations
Models
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.
See also
- Car audioCar audioCar audio/video , auto radio, mobile audio, 12-volt and other terms are used to describe the sound or video system fitted in an automobile. While 12-volt audio and video systems are also used, marketed, or manufactured for marine, aviation, and buses, this article focuses on cars as the most common...
- Small-scale use of the FM broadcast band
- Wireless microphoneWireless microphoneA wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated...