High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts (audiophiles) to refer to high-quality reproduction of sound  or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment. Ideally, high-fidelity equipment has minimal amounts of noise and distortion and an accurate frequency response.
One effort to standardize the term was the 1973 German Deutsches Institut für Normung (DIN) standard DIN 45500. DIN 45 500 approval was intended to provide audio equipment buyers with reassurance that their equipment was capable of good quality reproduction. In theory, only stereo equipment that met the standard could bear the words 'hi-fi'. This standard was well intentioned but only mildly successful; in practice, the term was widely misapplied to audio products that did not remotely approach the DIN basis specifications.
After World War II, several innovations created the conditions for a major improvement of home-audio quality:
In the 1950s, the term high fidelity began to be used by audio manufacturers as a marketing term to describe records and equipment which were intended to provide faithful sound reproduction. While some consumers simply interpreted high fidelity as fancy and expensive equipment, many found the difference in quality between "hi-fi" and the then standard AM radios and 78 RPM records readily apparent and bought 33⅓ LPs, such as RCA's New Orthophonics and London's ffrrs, and high-fidelity phonographs. Audiophiles paid attention to technical characteristics and bought individual components, such as separate turntables, radio tuners, preamplifiers, power amplifiers and loudspeakers. Some enthusiasts assembled their own loudspeaker systems. In the 1950s, hi-fi became a generic term, to some extent displacing phonograph and record player.
In the late 1950s and early 1960s, the development of the Westrex single-groove stereophonic record cutterhead led to the next wave of home-audio improvement, and in common parlance, stereo displaced hi-fi. Records were now played on a stereo. In the world of the audiophile, however, high fidelity continued and continues to refer to the goal of highly-accurate sound reproduction and to the technological resources available for approaching that goal. This period is most widely regarded as "The Golden Age of Hi-Fi", when tube equipment manufacturers of the time produced many models considered endearing by modern audiophiles, and just before solid state equipment was introduced to the market, subsequently replacing tube equipment as mainstream.
A very popular type of system for reproducing music from the 1970s onwards was the integrated music centre which combined phonograph, radio tuner, tape player, preamp and power amplifier in one package, often sold with its own separate, detachable or integrated speakers. These systems advertised their simplicity; the consumer did not have to select and assemble the individual components. Purists generally avoid referring to these systems as high fidelity, though some are capable of very good quality sound reproduction.
Blind tests refer to experiments where researchers have sighted knowledge on the tested (audio) components, but not individuals undergoing the experiments. In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group and the experimental group. Only after all the data has been recorded (and in some cases, analyzed) do the researchers learn which individuals are which. A commonly-used variant of this test is the ABX test. This involves comparing two known audio sources (A and B) with either one of these when it has been randomly selected (X). There is no way to prove that a certain lossy methodology is transparent. To scientifically prove that a lossy method is not transparent, double-blind tests may be useful.
Scientific double-blind tests are often used to disprove certain audio components (such as expensive, exotic cables) have any real bearings on audible sound quality. These tests are not accepted by some "audiophile" magazines in their evaluation of audio equipment, such as Stereophile and The Absolute Sound. John Atkinson, current editor of Stereophile, stated (in a 2005 July editorial named Blind Tests & Bus Stops) that he once purchased a solid-state amplifier, the Quad 405, in 1978 after blind tests, but came to realize months later that "the magic was gone" until he replaced it with a tube amp. Robert Harley of The Absolute Sound wrote, in a 2008 editorial (on Issue 183), that "blind listening tests fundamentally distort the listening process and are worthless in determining the audibility of a certain phenomenon."http://www.avguide.com/forums/blind-listening-tests-are-flawed-editorial?page=2
Doug Schneider, editor of the online Soundstage network, refuted this position with two editorials in 2009. http://www.goodsound.com/editorial/200905.htmhttp://www.goodsound.com/editorial/200906.htm He stated: "Blind tests are at the core of the decades’ worth of research into loudspeaker design done at Canada’s National Research Council (NRC). The NRC researchers knew that for their results to be credible within the scientific community and to have the most meaningful results, they had to eliminate bias, and blind testing was the only way to do so." Many Canadian companies such as Axiom, Energy, Mirage, Paradigm, PSB, Revel use blind testing extensively in designing their loudspeakers. Many audio insiders like Sean Olive of Harman International share this view.http://seanolive.blogspot.com/2009/04/dishonesty-of-sighted-audio-product.html
As can be seen in many audio forums, ABX tests on audio components continue to be a controversial subject.
Stereophonic sound provided a partial solution to the problem of creating some semblance of the illusion of live orchestral performers by creating a phantom middle channel when the listener sits exactly in the middle of the two front loudspeakers. When the listener moves slightly to the side, however, this phantom channel disappears or is greatly reduced. An attempt to provide for the reproduction of the reverberation was tried in the 1970s through quadraphonic sound but, again, the technology at that time was insufficient for the task. Consumers did not want to pay the additional costs and space required for the marginal improvements in realism. With the rise in popularity of home theatre, however, multi-channel playback systems became affordable, and many consumers were willing to tolerate the six to eight channels required in a home theater. The advances made in signal processors to synthesize an approximation of a good concert hall can now provide a somewhat more realistic illusion of listening in a concert hall.
In addition to spatial realism, the playback of music must be subjectively free from noise to achieve realism. The compact disc (CD) provides about 90 decibels of dynamic range, which is about as much as most people can tolerate in an average living room.
Audio equipment must be able to reproduce frequencies high enough and low enough to be realistic. The human hearing range, for healthy young persons, is 20 Hz to 20,000 Hz.  Most adults can't hear higher than 15 kHz. CDs are capable of reproducing frequencies as low as 10 Hz and as high as 22.05 kHz, making them adequate, in range, to reproduce all the frequencies that people can hear.
The equipment must also provide no noticeable distortion of the signal or emphasis or de-emphasis of any frequency in this frequency range.
Integrated, mini, or lifestyle systems, also known as music centres or minisystems, contain one or more sources such as a CD player, a tuner, or a cassette deck together with a preamplifier and a power amplifier in one box. Such products are generally disparaged by audiophiles, although some high-end manufacturers do produce integrated systems. The traditional hi-fi enthusiast, however, will build a system from separates, often with each item from a different manufacturer specialising in a particular component. This provides the most flexibility for piece-by-piece upgrades.
For slightly less flexibility in upgrades, a preamplifier and a power amplifier in one box is called an integrated amplifier; with a tuner, it is a receiver. A monophonic power amplifier, which is called a monoblock, is often used for powering a subwoofer. Other modules in the system may include components like cartridges, tonearms, turntables, Digital Media Players, digital audio players, DVD players that play a wide variety of discs including CDs, CD recorders, MiniDisc recorders, hi-fi videocassette recorders (VCRs) and reel-to-reel tape recorders. Signal modification equipment can include equalizers and signal processors.
This modularity allows the enthusiast to spend as little or as much as he wants on a component that suits his specific needs. In a system built from separates, sometimes a failure on one component still allows partial use of the rest of the system. A repair of an integrated system, though, means complete lack of use of the system.
Another advantage of modularity is the ability to spend one's money on only a few core components at first and then later add additional components to one's system. Some of the disadvantages of this approach are increased cost, complexity, and space required for the components.
Modern hi-fi equipment can include signal sources such as digital audio tape (DAT), digital audio broadcasting (DAB) or HD Radio tuners. Some modern hi-fi equipment can be digitally connected using fibre optic TOSLINK cables, universal serial bus (USB) ports (including one to play digital audio files), or Wi-Fi support.
Another modern component is the music server consisting of one or more computer hard drives that hold music in the form of computer files. When the music is stored in an audio file format that is lossless such as FLAC, Monkey's Audio or WMA Lossless, the computer playback of recorded audio can serve as an audiophile-quality source for a hi-fi system.