Audio induction loop systems, also called audio-frequency induction loops ( AFILs ) or hearing loops , are an assistive listening technology for individuals with reduced ranges of hearing.
70-493: A hearing loop consists of one or more physical loops of cable which are placed around a designated area, usually a room or a building. The cable generates an electromagnetic field throughout the looped space which can be picked up by a telecoil -equipped hearing aid , a cochlear implant (CI) processor, or a specialized hand-held hearing loop receiver for individuals without telecoil-compatible hearing aids. The loops carry baseband audio-frequency currents; no carrier signal
140-586: A power amplifier as a loudspeaker would be driven. The coupling of magnetic fields is described mathematically by Faraday's law of induction . A summary of the theory necessary for AFILs is included in British Standard BS ;7594 , which is a guide to the design and installation of induction loops. A basic AFIL using a general purpose amplifier, as described above, has some disadvantages. The loop driver amplifier requires some additional circuits to overcome these. Using anything other than
210-682: A printed circuit board with a connector mounted to the housing). Cable assemblies can also take the form of a cable tree or cable harness , used to connect many terminals together. Electrical cables are used to connect two or more devices, enabling the transfer of electrical signals or power from one device to the other. Long-distance communication takes place over undersea communication cables . Power cables are used for bulk transmission of alternating and direct current power, especially using high-voltage cable . Electrical cables are extensively used in building wiring for lighting, power and control circuits permanently installed in buildings. Since all
280-406: A cable, radiates an electromagnetic field . Likewise, any conductor or cable will pick up energy from any existing electromagnetic field around it. These effects are often undesirable, in the first case amounting to unwanted transmission of energy which may adversely affect nearby equipment or other parts of the same piece of equipment; and in the second case, unwanted pickup of noise which may mask
350-430: A compressor can make the sound appear to have a more sustained tail. Guitar sounds are often compressed to produce a fuller, more sustained sound. Most devices capable of compressing audio dynamics can also be used to reduce the volume of one audio source when another audio source reaches a certain level; this is called side-chaining . In electronic dance music , side-chaining is often used on basslines , controlled by
420-561: A correctly designed loop driver amplifier is not only unsatisfactory, but may result in a loop installation that can generate harmonics when driven into distortion, and these will cause radio interference . This must be prevented, both for sound quality and for legal reasons as it is illegal to cause such interference in these circumstances. In Europe, the EMC Directive applies, and it is also illegal to supply or install unsuitable electronic and electrical equipment. A second factor
490-618: A hand drill and turning while maintaining moderate tension on the line. Where the interfering signal has a wavelength that is long compared to the pitch of the twisted pair, alternate lengths of wires develop opposing voltages, tending to cancel the effect of the interference. Electrical cable jacket material is usually constructed of flexible plastic which will burn. The fire hazard of grouped cables can be significant. Cables jacketing materials can be formulated to prevent fire spread (see Mineral-insulated copper-clad cable ) . Alternately, fire spread amongst combustible cables can be prevented by
560-413: A lower threshold (e.g. −60 dB) means a larger portion of the signal is treated. When the signal level is below the threshold, no processing is performed and the input signal is passed, unmodified, to the output. Thus a higher threshold of, e.g., −5 dB, results in less processing, less compression. Threshold timing behavior is subject to attack and release settings (see below ). When
630-428: A more relaxed compression that more closely relates to human perception of loudness. A compressor in stereo linking mode applies the same amount of gain reduction to both the left and right channels. This is done to prevent image shifting that can occur if each channel is compressed individually. Because a downward compressor only reduces the level of the signal, the ability to add a fixed amount of make-up gain at
700-408: A part of the method of enabling a two-way conversation over a single pair of wires. The telecoil enabled the hearing aid user to hear the phone conversation clearly without picking up background noise. From this, the natural development was to generate electromagnetic fields representing the audio, which the telecoil could receive. The simplest form of AFIL is a single wire around a room, driven from
770-492: A problem that TV viewers often notice: when a station switches from minimally compressed program material to a heavily compressed commercial, the volume sometimes seems to increase dramatically. Peak loudness might be the same—meeting the letter of the law—but high compression puts much more of the audio in the commercial at close to the maximum allowable, making the commercial seem much louder. Record companies, mixing engineers and mastering engineers have been gradually increasing
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#1732764667603840-495: A shooting range to converse normally, while sharply attenuating the much louder sounds of the gunshots, and similarly for musicians to hear quiet music but be protected from loud noises such as drums or cymbal crashes. In applications of machine learning where an algorithm is training on audio samples, dynamic range compression is a way to augment samples for a larger data set. Compression and limiting are identical in process but different in degree and perceived effect. A limiter
910-451: A short attack time . Compression is used to improve performance and clarity in public address systems , as an effect and to improve consistency in mixing and mastering . It is used on voice to reduce sibilance and in broadcasting and advertising to make an audio program stand out. It is an integral technology in some noise reduction systems. There are two types of compression: downward and upward. Both types of compression reduce
980-539: A similar standard (DIN VDE 0292). Audio level compression Dynamic range compression ( DRC ) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or compressing an audio signal 's dynamic range . Compression is commonly used in sound recording and reproduction , broadcasting , live sound reinforcement and some instrument amplifiers . A dedicated electronic hardware unit or audio software that applies compression
1050-465: Is earth loops , where different pieces of equipment are connected together by signal wires, but powered from different power sockets in different parts of the room or building. The combination of the mains earth and signal earth creates a receiving loop that produces an interference signal proportional to the area within the earth loop. Various steps are used to prevent interference on audio and video equipment. Powering signal sources and output devices from
1120-486: Is a compressor with a high ratio and, generally, a fast attack time. Compression with ratio of 10:1 or more is generally considered limiting. Brick wall limiting has a very high ratio and a very fast attack time. Ideally, this ensures that an audio signal never exceeds the amplitude of the threshold. Ratios of 20:1 all the way up to ∞:1 are considered brick wall . The sonic results of more than momentary and infrequent brick-wall limiting are harsh and unpleasant, thus it
1190-442: Is also used to provide lubrication between strands. Tinning was used to help removal of rubber insulation. Tight lays during stranding makes the cable extensible (CBA – as in telephone handset cords). In the 19th century and early 20th century, electrical cable was often insulated using cloth, rubber or paper. Plastic materials are generally used today, except for high-reliability power cables. The first thermoplastic used
1260-403: Is called a compressor . In the 2000s, compressors became available as software plugins that run in digital audio workstation software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect. A limiter is a compressor with a high ratio and, generally,
1330-421: Is more common as a safety device in live sound and broadcast applications. Some bass amps and PA system amplifiers include limiters to prevent sudden volume peaks from causing distortion or damaging the speakers. A compressor with a side-chain input controls gain from main input to output based on the level of the signal at the side-chain input. An early innovator of side-chain compression in an effects unit
1400-426: Is not greatly effective against low-frequency magnetic fields, however - such as magnetic "hum" from a nearby power transformer . A grounded shield on cables operating at 2.5 kV or more gathers leakage current and capacitive current, protecting people from electric shock and equalizing stress on the cable insulation. Coaxial design helps to further reduce low-frequency magnetic transmission and pickup. In this design
1470-467: Is reduced to 1 dB over the threshold. The gain and output level has been reduced by 3 dB. Another way of stating this is that any input signal level over the threshold will, in this case, be output at a level which is only 25% (i.e. 1 over 4) as much over the threshold as its input level was. The highest ratio of ∞ {\displaystyle \infty } :1 is often known as limiting , and effectively denotes that any signal above
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#17327646676031540-461: Is that many forms of hearing impairment mean that sound levels must be kept fairly constant. An effective loop driver will have an automatic level control to compress the signal, providing a constant loop amplitude for a wide range of source levels. Meeting this requirement is likely to meet the interference requirement at the same time. To do this, the loop driver should give constant output for at least 30 dB input range. A third problem
1610-421: Is the inductance of the loop cable, and its effect upon the higher frequencies of sound. To overcome this, many loop drivers operate as current mode amplifiers instead of voltage mode. By setting the amplifier characteristic between voltage and current mode, the overall performance is optimised for good bandwidth with minimum distortion. There are other options for reducing the effect of cable inductance, including
1680-482: Is used in voice communications in amateur radio that employ single-sideband (SSB) modulation to make a particular station's signal more readable to a distant station, or to make one's station's transmitted signal stand out against others. This is applicable especially in DXing . An SSB signal's strength depends on the level of modulation . A compressor increases the average level of the modulation signal thus increasing
1750-442: Is used to drive the compression of the delayed signal, which then appears at the output. This way a smooth-sounding slower attack rate can be used to catch transients. The cost of this solution is added audio latency through the processor. Compression is often applied in audio systems for restaurants, retail, and similar public environments that play background music at a relatively low volume and need it compressed, not just to keep
1820-487: Is used. The benefit is that it allows the sound source of interest – whether a musical performance or a ticket taker's side of the conversation – to be transmitted to the hearing-impaired listener clearly and free of other distracting noise in the environment. Typical installation sites include concert halls, ticket kiosks, high-traffic public buildings (for PA announcements), auditoriums, places of worship, courtrooms, meeting rooms, and homes. In
1890-401: The dynamic range of an audio signal. Downward compression reduces the volume of loud sounds above a certain threshold. The quiet sounds below the threshold remain unaffected. This is the most common type of compressor. A limiter can be thought of as an extreme form of downward compression as it compresses the sounds over the threshold especially hard. Upward compression increases
1960-570: The loudness war . Noise reduction systems use a compressor to reduce the dynamic range of a signal for transmission or recording, expanding it afterward, a process called companding . This reduces the effects of a channel or recording medium with limited dynamic range. Instrument amplifiers often include compression circuitry to prevent sudden high-wattage peaks that could damage the speakers. Electric bass players often use compression effects, either effects units available in pedal, rackmount units, or built-in devices in bass amps, to even out
2030-539: The EBU PLOUD group, which consists of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published EBU R 128 which introduces a new way of metering and normalizing audio . The Recommendation uses ITU-R BS.1770 loudness metering. As of 2016 , several European TV stations have announced their support for the new norm and over 20 manufacturers have announced products supporting
2100-649: The United Kingdom, as an aid for disability, their provision, where reasonably possible, is required by the Equality Act 2010 and previously by the Disability Discrimination Act 1995 , and they are available in "the back seats of all London taxis, which have a little microphone embedded in the dashboard in front of the driver; at 18,000 post offices in the U.K.; at most churches and cathedrals", according to Prof. David G. Myers. In
2170-637: The United States, an alternative technology using FM transmission to "neck loop" receivers was more widely adopted due to economic advantages. In comparison, hearing loop systems require a greater initial investment by the facility operator, but offer greater convenience and avoid the social stigma and hygienic concerns entailed by the FM system's paraphernalia for those who have hearing aids. Another alternative system, used primarily in theatres, uses invisible infrared radiation; compatible headsets can pick up
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2240-434: The algorithms are used to emulate the above analog technologies. A number of user-adjustable control parameters and features are used to adjust dynamic range compression signal processing algorithms and components. A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold . Threshold is commonly set in decibels ( dBFS for digital compressors and dBu for hardware compressors), where
2310-417: The application of fire retardant coatings directly on the cable exterior, or the fire threat can be isolated by the installation of boxes constructed of noncombustible materials around the bulk cable installation. CENELEC HD 361 is a ratified standard published by CENELEC, which relates to wire and cable marking type, whose goal is to harmonize cables. Deutsches Institut für Normung (DIN, VDE) has released
2380-424: The attack and release times are adjustable by the user. Some compressors, however, have the attack and release times determined by the circuit design and cannot be adjusted. Sometimes the attack and release times are automatic or program dependent , meaning that the behavior may change depending on the input signal. Another control a compressor might offer is hard knee or soft knee selection. This controls whether
2450-474: The audio volume into the listener's hearing range. To help the patient perceive the direction sound comes from, some hearing aids use binaural compression. Compressors are also used for hearing protection in some electronic active hearing protection earmuffs and earplugs , to let sounds at ordinary volumes be heard normally while attenuating louder sounds, possibly also amplifying softer sounds. This allows, for example, shooters wearing hearing protection at
2520-413: The bend in the response curve between below threshold and above threshold is abrupt (hard) or gradual (soft). A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user. A soft knee reduces the potentially audible transition from uncompressed to compressed, and is especially applicable for higher ratio settings where the changeover at
2590-402: The circuit conductors required can be installed in a cable at one time, installation labor is saved compared to certain other wiring methods. Physically, an electrical cable is an assembly consisting of one or more conductors with their own insulations and optional screens, individual coverings, assembly protection and protective coverings. Electrical cables may be made more flexible by stranding
2660-413: The compressor in a parallel signal path is known as parallel compression . It is a form of upward compression that facilitates dynamic control without significant audible side effects so long as the ratio is relatively low and the compressor's sound is relatively neutral. On the other hand, a high compression ratio with significant audible artifacts can be chosen in one of the two parallel signal paths. This
2730-518: The desired signal being carried by the cable, or, if the cable is carrying power supply or control voltages, pollute them to such an extent as to cause equipment malfunction. The first solution to these problems is to keep cable lengths in buildings short since pick up and transmission are essentially proportional to the length of the cable. The second solution is to route cables away from trouble. Beyond this, there are particular cable designs that minimize electromagnetic pickup and transmission. Three of
2800-399: The foil or mesh shield has a circular cross section and the inner conductor is exactly at its center. This causes the voltages induced by a magnetic field between the shield and the core conductor to consist of two nearly equal magnitudes which cancel each other. A twisted pair has two wires of a cable twisted around each other. This can be demonstrated by putting one end of a pair of wires in
2870-505: The grid-to-cathode voltage changes to alter the gain. Optical compressors use a photoresistor stimulated by a small lamp ( incandescent , LED , or electroluminescent panel ) to create changes in signal gain. Other technologies used include field effect transistors and a diode bridge . When working with digital audio, digital signal processing (DSP) techniques are commonly used to implement compression as audio plug-ins , in mixing consoles , and in digital audio workstations . Often
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2940-408: The input to reach the output gain determined by the ratio, or, to unity, once the input level has fallen below the threshold. Because the loudness pattern of the source material is modified by the time-varying operation of compressor, it may change the character of the signal in subtle to quite noticeable ways depending on the attack and release settings used. The length of each period is determined by
3010-431: The kick drum or a similar percussive trigger, to prevent the two from conflicting, and provide a pulsating, rhythmic dynamic to the sound. A compressor can be used to reduce sibilance ('ess' sounds) in vocals ( de-essing ) by feeding the compressor's side-chain an equalized version of the input signal, so that specific, sibilance-related frequencies (typically 4000 to 8000 hz) activate the compressor more. Compression
3080-423: The loop area. The calibration of field strength measuring devices is also included. Electrical cable An electrical cable is an assembly of one or more wires running side by side or bundled, which is used as an electrical conductor to carry electric current . Electrical cables are used to connect two or more devices, enabling the transfer of electrical signals , power , or both from one device to
3150-413: The mix with the other instruments (neither disappear during short periods of time, nor overpower the other instruments during short periods). Vocal performances in rock music or pop music are compressed for the same reason. Compression can also be used on instrument sounds to create effects not primarily focused on stabilizing the volume. For instance, drum and cymbal sounds tend to decay quickly, but
3220-466: The modulated infrared energy to reproduce sound. The first patented magnetic induction loop communication system was invented by Joseph Poliakoff (grandfather of Sir Martyn Poliakoff ) in Great Britain in 1937. The pickup coil in a hearing aid is known as a telecoil (or T-coil ) because its early form was to pick up a magnetic field from coils within a telephone . These were included as
3290-507: The new EBU Mode loudness meters. To help audio engineers understand what loudness range their material consists of (e.g. to check if some compression may be needed to fit it into the channel of a specific delivery platform), the EBU also introduced the Loudness Range (LRA) descriptor. Most television commercials are heavily compressed to achieve near-maximum perceived loudness while staying within permissible limits. This causes
3360-415: The noise gate make the quiet sounds (for instance: noise) quieter or even silent, depending on the floor setting. Upward expansion makes the louder sounds above the threshold even louder. The signal entering a compressor is split; one copy is sent to a variable-gain amplifier and the other to a side-chain where the signal level is measured and a circuit controlled by the measured signal level applies
3430-545: The on-air chain. Broadcasters use compressors in order that their station sounds louder than comparable stations. The effect is to make the more heavily compressed station jump out at the listener at a given volume setting. This is not limited to inter-channel differences; they also exist between programme material within the same channel. Loudness differences are a frequent source of audience complaints, especially TV commercials and promos that seem too loud. The European Broadcasting Union (EBU) has been addressing this issue in
3500-433: The other. Physically, an electrical cable is an assembly consisting of one or more conductors with their own insulations and optional screens, individual coverings, assembly protection and protective covering. One or more electrical cables and their corresponding connectors may be formed into a cable assembly, which is not necessarily suitable for connecting two devices but can be a partial product (e.g. to be soldered onto
3570-458: The output is usually provided so that an optimum output level is produced. The look-ahead function is designed to overcome the problem of being forced to compromise between slow attack rates that produce smooth-sounding gain changes, and fast attack rates capable of catching transients. Look-ahead is implemented by splitting the input signal and delaying one side (the audio signal) by the look-ahead time. The non-delayed side (the gain control signal)
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#17327646676033640-415: The overall loudness of commercial albums. This is achieved by using higher degrees of compression and limiting during mixing and mastering ; compression algorithms have been engineered specifically to accomplish the task of maximizing audio level in the digital stream. Hard limiting or clipping can result, affecting the tone and timbre of the music. The effort to increase loudness has been referred to as
3710-535: The principal design techniques are shielding , coaxial geometry, and twisted-pair geometry. Shielding makes use of the electrical principle of the Faraday cage . The cable is encased for its entire length in foil or wire mesh. All wires running inside this shielding layer will be to a large extent decoupled from external electrical fields, particularly if the shield is connected to a point of constant voltage, such as earth or ground . Simple shielding of this type
3780-528: The proper operation of the hearing loop. BS 7594 (published by the BSI and widely used in Britain) is a non-mandatory guideline for the design and installation of induction loops. It has a comprehensive guide to theory, as well as guidance for those considering the installation of AFILs in buildings for which they may be responsible. It also contains some valuable guidance relating to other equipment within
3850-401: The rate of change and the required change in gain. For more intuitive operation, a compressor's attack and release controls are labeled as a unit of time (often milliseconds). This is the amount of time it takes for the gain to change a set amount of dB or a set percentage towards the target gain. There is no industry standard for the exact meaning of these time parameters. In many compressors,
3920-420: The required gain to the amplifier. This design, known as a feed-forward type, is used today in most compressors. Earlier designs were based on a feedback layout where the signal level was measured after the amplifier. There are a number of technologies used for variable-gain amplification, each having different advantages and disadvantages. Vacuum tubes are used in a configuration called variable-mu where
3990-405: The same mains circuit to prevent formation of an earth loop; shielded cables or signal isolators may be used. An objective of the field strength requirements of standards for AFILs is to make the perceived loudness of sound from the loop the same as from the microphone in the hearing aid. This is the basis of the average field strength of 100mA/m used to generate today's performance standards around
4060-418: The signal level goes above threshold, compressor operation is delayed by the attack setting. For an amount of time determined by the release after the input signal has fallen below the threshold, the compressor continues to apply dynamic range compression. The amount of gain reduction is determined by ratio : a ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level
4130-414: The sound levels of their basslines . Gain pumping , where a regular amplitude peak (such as a kick drum) causes the rest of the mix to change in volume due to the compressor, is generally avoided in music production. However, many dance and hip-hop musicians purposefully use this phenomenon, causing the mix to alter in volume rhythmically in time with the beat. Hearing aids use a compressor to bring
4200-417: The threshold is brought down to the threshold level once the attack time has expired. A compressor may provide a degree of control over how quickly it acts. The attack is the period when the compressor is decreasing gain in response to the increased level at the input to reach the gain determined by the ratio. The release is the period when the compressor is increasing gain in response to reduced level at
4270-408: The threshold would be more noticeable. A peak-sensing compressor responds to the peak level of the input signal. While providing tighter peak level control, peak level sensing does not necessarily relate to human perception of loudness. Some compressors apply a power measurement function (commonly root mean square or RMS) on the input signal before comparing its level to the threshold. This produces
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#17327646676034340-608: The transmitted signal strength. Most modern amateur radio SSB transceivers have speech compressors built-in. Compression is also used in land mobile radio , especially in transmitted audio of professional walkie-talkies and remote control dispatch consoles . Compression is used extensively in broadcasting to boost the perceived volume of sound while reducing the dynamic range of source audio. To avoid overmodulation , broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast. Normally these limits are met by permanently inserted compression hardware in
4410-598: The use of a multi-core cable where the conductors are connected in parallel. Structural steel and other metalwork in buildings can cause problems by reducing the field strength unevenly across the loop area, causing frequency distortions. In most cases, a solution can be found using combinations of loops with phase shift between them, combined with frequency correction and increased signal strength. Audio induction loops create relatively high magnetic field levels. Other equipment must be designed and installed to work properly within this field. The most common cause of problems
4480-475: The volume fairly constant, but also to make quiet parts of the music audible over ambient noise. Compression can increase average output gain of a power amplifier by 50 to 100% with a reduced dynamic range. For paging and evacuation systems, this adds clarity under noisy circumstances and saves on the number of amplifiers required. Compression is often used in music production to make instruments more consistent in dynamic range, so that they "sit" more nicely in
4550-502: The volume of quiet sounds below a certain threshold. The louder sounds above the threshold remain unaffected. Some compressors also have the ability to do the opposite of compression, namely expansion . Expansion increases the dynamic range of the audio signal. Like compression, expansion comes in two types, downward and upward. Downward expansion makes the quiet sounds below the threshold even quieter. A noise gate can be thought of as an extreme form of downward expansion as
4620-465: The volume of the music. A sidechain with equalization controls can be used to reduce the volume of signals that have a strong spectral content within a certain frequency range: it can act as a de-esser , reducing the level of vocal sibilance in the range of 6–9 kHz. Another use of the side-chain in music production serves to maintain a loud bass track without the bass drum causing undue peaks that result in loss of overall headroom . Inserting
4690-548: The wires. In this process, smaller individual wires are twisted or braided together to produce larger wires that are more flexible than solid wires of similar size. Bunching small wires before concentric stranding adds the most flexibility. Copper wires in a cable may be bare, or they may be plated with a thin layer of another metal, most often tin but sometimes gold , silver or some other material. Tin, gold, and silver are much less prone to oxidation than copper, which may lengthen wire life, and makes soldering easier. Tinning
4760-491: The world. IEC 60118-4 (formerly Britain's BS 6083 part 4, also known as EN 60118-4) is now the main specification for international use. This standard specifies long term average field strength with allowance for program peaks, background noise level ,and frequency response. IEC 62489-1 specifies how to measure the performance of the system components in a hearing loop system. The standard recommends fixed and portable monitoring devices to allow verification of
4830-793: Was gutta-percha (a natural latex ) which was found useful for underwater cables in the 19th century. The first, and still very common, man-made plastic used for cable insulation was polyethylene . This was invented in 1930, but not available outside military use until after World War 2 during which a telegraph cable using it was laid across the English Channel to support troops following D-Day . Cables can be securely fastened and organized, such as by using trunking, cable trays , cable ties or cable lacing . Continuous-flex or flexible cables used in moving applications within cable carriers can be secured using strain relief devices or cable ties. Any current -carrying conductor, including
4900-536: Was the Eventide Omnipressor from 1974. With side-chaining, the compressor behaves in the conventional manner when both main and side-chain inputs are supplied with the same signal. The side-chain input is used by disc jockeys for ducking – lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the side-chain input so that whenever the DJ speaks the compressor reduces
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