Montage Acoustics reviews:Quadraphonic FM

Montage Acoustics: Magnavox PMX system
In 1980 the FCC chose the Magnavox PMX system as the US standard. The FCC was savagely criticized by the other contenders, and lawsuits erupted. In 1982, the FCC reversed its decision and decided not to enforce a standard but allow multiple systems, to "let the marketplace decide". Meanwhile, other nations adopted AM stereo, many choosing Motorola's C-QUAM. Their choice of a single standard rather than allowing competing standards as the US, resulted in greater acceptance of AM stereo in these markets. In 1993, the FCC made C-QUAM system the US standard.

Globally, the adoption of stereo broadcasting was never great, and declined after 1990. With the continued migration of AM stations away from music to news, sports, and talk formats, receiver manufacturers saw little reason to adopt the more expensive stereo tuners, and thus radio stations have little incentive to upgrade to stereo transmission.

Microbroadcasting - United States
Some microbroadcasters and pirate radio broadcasters, especially those in the United States under the FCC's Part 15 rules, broadcast on AM to achieve greater range than is possible on the FM band. On mediumwave (AM), such radio stations are often found between 1610 kHz and 1710 kHz. Hobbyists also use low-power AM (LPAM) transmitters to provide programming for vintage radio equipment in areas where AM programming is not widely available or does not carry programming the listener desires; in such cases the transmitter, which is designed to cover only the immediate property and perhaps nearby areas, is connected to a computer, and FM radio or an MP3 player. Microbroadcasting and pirate radio have been almost completely replaced by streaming audio on the Internet, but some schools or hobbyists still use LPAM as a means of broadcasting.Montage Acoustics HD9001

Montage acoustic HDQ 4101 A: Distance covered by stereo FM transmission
The range of mono FM transmission is related to the transmitter's RF power, the antenna gain, and antenna height. The U.S. FCC publishes curves that aid in calculation of this maximum distance as a function of signal strength at the receiving location.

For stereo FM, the range is significantly reduced. This is due to the need to lower the modulation index of the main (sum) signal to accommodate the presence of the 38 kHz DSB-SC (double side-band suppressed-carrier) subcarrier and 19 kHz pilot tone. Many stations use extreme audio compression to keep the sound above the background noise for "distant" listeners, at the expense of degrading the sound quality.

Montage Acoustics BT4480

United States
Despite FM having been patented in 1933, commercial FM broadcasting did not begin until the late 1930s, when it was initiated by a handful of early pioneer stations including W8HK, Buffalo, New York (now WTSS); W1XOJ/WGTR, Paxton Massachusetts (closed down about 1953); W1XSL/W1XPW/WDRC-FM, Meriden, Connecticut (now WHCN); W2XMN/KE2XCC/WFMN, Alpine, New Jersey (owned by Edwin Armstrong himself, closed down upon Armstrong's death in 1954); W2XQR/WQXQ/WQXR-FM, New York; W47NV Nashville, Tennessee (now WSM-FM); W1XER/W39B/WMNE, whose studios were in Boston but whose transmitter was atop the highest mountain in the northeast United States, Mount Washington, New Hampshire (shut down in 1948); W9XAO Milwaukee, Wisconsin (later WTMJ-FM, off air in 1950, returning in 1959 on another frequency). Also of note are General Electric stations W2XDA Schenectady and W2XOY New Scotland, New York—two experimental frequency modulation transmitters on 48.5 MHz—which signed on in 1939. The two were merged into one station using the W2XOY call letters on November 20, 1940, with the station taking the WGFM call letters a few years later, and moving to 99.5 MHz when the FM band was relocated to the 88-108 MHz portion of the radio spectrum. General Electric sold the station in the 1980s, and today the station is called WRVE. Montage Acoustics Speakers

Montage Acoustics reviews:Quadraphonic FM

In 1969 Louis Dorren invented the Quadraplex system of single station, discrete, compatible four-channel FM broadcasting. There are two additional subcarriers in the Quadraplex system, supplementing the single one used in standard stereo FM. The baseband layout is as follows:

50 Hz to 15 kHz Main Channel (sum of all 4 channels) (LF+LR+RF+RR) signal, for mono FM listening compatibility.
23 to 53 kHz (cosine quadrature subcarrier) (LF+LR) - (RF+RR) Left minus Right difference signal. This signal's modulation in algebraic sum and difference with the Main channel was used for 2 channel stereo listener compatibility.
23 to 53 kHz (sine quadrature 38 kHz subcarrier) (LF+RF) - (LR+RR) Front minus Back difference signal. This signal's modulation in algebraic sum and difference with the Main channel and all the other subcarriers is used for the Quadraphonic listener.
61 to 91 kHz (cosine quadrature 76 kHz subcarrier) (LF+RR) - (LR+RF) Diagonal difference signal. This signal's modulation in algebraic sum and difference with the main channel and all the other subcarriers is also used for the Quadraphonic listener.
95 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for reading services for the blind, background music, etc.