FM broadcasting - FM broadcasting (English spelling)

Japanese: FM放送 - えふえむほうそう（英語表記）FM broadcasting

Broadcasting using frequency modulation (FM). Frequency modulation is a modulation method that changes the frequency of a carrier wave according to the amplitude of the audio signal. FM broadcasting is also called VHF broadcasting because it uses very short waves as a carrier wave.

The FM system was invented by the American E. H. Armstrong in 1928, but it wasn't put to practical use for broadcasting until 1941. After World War II, due to a shortage of frequencies for medium wave radio, it became popular not only in the United States but also in Europe. In Japan, the NHK Tokyo Ultrashort Wave Experimental Station was opened in 1957 (Showa 32) for the purpose of collecting data on technical standards for FM broadcasting, and in 1963 experimental FM stereo broadcasting began. Since then, NHK has expanded its nationwide broadcasting network and promoted the spread of receivers. Frequency allocations for FM broadcasting were decided in 1968, regular broadcasting began in 1969, and frequency allocations were gradually made to private FM stations, leading to the present day.

[Satoshi Kimura, Toshiyuki Kaneki, Shokichiro Yoshikawa February 17, 2020]

Modulation Method

This method modulates the carrier frequency according to the amplitude of the audio signal, changing it so that the carrier frequency _is f ₀ + Δ f for the positive waveform of the audio signal and f ₀ - Δ f for the negative waveform, centered around the carrier frequency f 0 when the amplitude of the audio signal is zero. This modulation method is used not only for FM broadcasting, but also for the audio carrier wave in television broadcasting and the video carrier wave in satellite broadcasting. In the FM method, the deviation from the center frequency is called frequency deviation, and the maximum value for FM broadcasting is set at 75 kHz above and below, while the maximum value for the audio carrier wave in television broadcasting is set at 25 kHz above and below.

When a carrier wave is modulated by an audio signal, part of the energy of the carrier wave is distributed on both sides of the center frequency. This is called a sideband. The sideband in FM broadcasting is much wider than that in AM broadcasting, and is determined by the frequency and amplitude of the audio signal. The bandwidth including these sidebands is the occupied bandwidth, which is 200 kHz above and below in FM broadcasting. As such, FM broadcasting requires a wide bandwidth for one broadcast wave, so it cannot be implemented when using a low carrier wave such as AM broadcasting (allocated frequencies in Japan are 526.5 to 1606.5 kHz). When FM broadcasting began in Japan, 76.1 to 89.9 MHz was allocated, but in 2014, 90.0 to 94.9 MHz was added, making 76.1 to 94.9 MHz available, and it is now used for FM complementary broadcasting (wide FM broadcasting) and other purposes. In the United States, the band was 40 MHz when broadcasting began, but it was later changed to 88 to 108 MHz.

[Satoshi Kimura, Toshiyuki Kaneki, Shokichiro Yoshikawa February 17, 2020]

FM radio station

The VHF radio waves used for FM broadcasting have a shorter range of 10 to 100 kilometers compared to the medium wave radio waves used for AM broadcasting, so the reception range is narrower and the service area is localized. Also, if there are mountains or tall buildings in the way of propagation, the radio waves will be blocked, so FM broadcasting transmitters are installed in high places such as on tall steel towers or on top of mountains.

[Shokichiro Yoshikawa February 17, 2020]

FM Receiver

A standard FM broadcast receiver uses the superheterodyne method, with an intermediate frequency of typically 10.7 MHz and a bandwidth of 200 kHz. Unlike amplitude modulation (AM) receivers used for medium wave broadcasting, it always has a built-in amplitude limiter, and the demodulation circuit uses a frequency discriminator that converts changes in the carrier frequency into changes in output voltage. In FM, the effect of noise in the high frequency range of the audio signal is large, so the high frequency components of the audio signal are emphasized (pre-emphasis) before being sent on the transmitting side, and after demodulating the audio signal on the receiving side, it is passed through a de-emphasis process, which has the exact opposite characteristics to the pre-emphasis process on the transmitting side. This results in a flat frequency response overall, while reducing high-frequency noise.

[Satoshi Kimura, Toshiyuki Kaneki, Shokichiro Yoshikawa February 17, 2020]

Features

FM broadcasting has various advantages over AM radio broadcasting, including the following:

(1) Good sound quality. Medium wave broadcasting channels are assigned at 9 kHz intervals, and it is not possible to transmit signals with high frequency components. However, FM broadcasting channels are assigned at 100 kHz carrier frequency intervals, and the signal band ranges from 30 to 15,000 Hz, making it possible to broadcast sound that is fairly faithful to the original sound.

(2) Less noise. Medium wave broadcasting is greatly affected when noise radio waves emitted from automobile spark plugs and various electrical devices get mixed into the receiver. However, FM broadcasting is less affected by noise because the amplitude components of the noise mixed into the broadcast waves can be removed by a limiter built into the receiver.

(3) Less interference. The ultra-short wave frequency used in FM broadcasting has a short reach, so the reception range of a single station is narrow. Even if there are two FM broadcast waves, the stronger wave has the effect of suppressing the weaker wave, so there is little interference.

(4) Stable reception conditions: VHF signals are less affected by the ionosphere both day and night, and the propagation conditions are stable. Therefore, the volume and sound quality are stable.

(5) Stereo broadcasting is possible. FM broadcasting has a sufficient frequency band, making it possible to broadcast stereo with a three-dimensional feel using just one signal.

[Satoshi Kimura, Toshiyuki Kaneki, Shokichiro Yoshikawa February 17, 2020]

FM and AM modulation methods
©Shogakukan ">

FM and AM modulation methods

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

周波数変調（FM：frequency modulation）方式を使った放送。周波数変調は、音声信号の振幅に応じて搬送波の周波数を変化させる変調方式であり、FM放送は搬送波に超短波を用いることから、超短波放送ともよばれる。

　FM方式は1928年にアメリカのE・H・アームストロングによって発明されているが、放送として実用化されたのは1941年からで、第二次世界大戦後は中波ラジオ用周波数の不足から、アメリカはもちろんヨーロッパでも盛んに行われるようになった。日本では1957年（昭和32）にNHK東京超短波実験局がFM放送の技術基準に関する資料収集を目的に開設され、1963年にはFMステレオ実験放送も開始、以後NHKによって全国放送網が拡充されて受信機の普及が図られてきた。1968年FM放送用の周波数割当てが決定、1969年に本放送開始、民間FM局にも周波数割当てが順次行われ、今日に至っている。

［木村　敏・金木利之・吉川昭吉郎　2020年2月17日］

変調方式

音声信号の振幅がゼロの場合の搬送波周波数f₀を中心に、音声信号の正側の波形に対してはf₀＋Δfと、負側の波形に対してはf₀－Δfとなるように、音声信号の振幅によって搬送波の周波数を変化させて変調する方式をとっている。この変調方式は、FM放送のほかに、テレビ放送の音声搬送波、衛星放送の映像搬送波などにも使われている。FM方式においては、中心周波数からのずれを周波数偏移とよび、FM放送ではその最大値を上下75キロヘルツに、またテレビ放送の音声搬送波では、最大値を上下25キロヘルツに規定している。

　搬送波を音声信号で変調すると、搬送波のエネルギーの一部が中心周波数の両側に分布する。これを側波帯とよぶ。FM放送における側波帯は、中波放送の側波帯よりはるかに広く、音声信号の周波数と振幅によって決まる。これら両側波帯を含む帯域幅が占有帯域幅で、FM放送では上下200キロヘルツである。このように、FM放送は一つの放送波に広い帯域幅を必要とするので、中波放送（日本の割当て周波数は526.5～1606.5キロヘルツ）のように低い搬送波を使用する場合には実施できない。日本ではFM放送開始時に76.1～89.9メガヘルツが割り当てられたが、2014年（平成26）に90.0～94.9メガヘルツが加えられたため76.1～94.9メガヘルツが使用可能となり、現在ではFM補完放送（ワイドFM放送）などでも使用されるようになった。なお、アメリカでは放送開始当時40メガヘルツ帯であったが、その後88～108メガヘルツに変更されている。

［木村　敏・金木利之・吉川昭吉郎　2020年2月17日］

FM放送局

FM放送に使用される超短波の電波は、中波放送に使用される中波の電波に比べて到達距離が10～100キロメートルと短いため受信範囲が狭く、サービスエリアは局地的である。また伝搬途中に山岳や高いビルなどがあると、電波がこれに遮られてしまうため、FM放送の送信所は高い鉄塔や山の上など高所に設置される。

［吉川昭吉郎　2020年2月17日］

FM受信機

標準的なFM放送用受信機はスーパーヘテロダイン方式を採用しており、中間周波数は通常10.7メガヘルツ、帯域幅は200キロヘルツである。中波放送に使われる振幅変調（AM：amplitude modulation）用受信機と異なり、振幅制限器（リミッター）をかならず内蔵しており、復調回路には搬送波の周波数変化を出力電圧の変化として取り出す周波数弁別器（ディスクリミネーター）を使用している。FM方式では、音声信号の高い周波数領域における雑音の影響が大きいため、送信側でまえもって音声信号の高域成分を強調（プリエンファシス）して送り出し、受信側で音声信号を復調したのち、送信側のプリエンファシスとちょうど逆特性をもつディエンファシスを通している。これによって、総合的にみて平坦(へいたん)な周波数特性を得るとともに、高域の雑音を低減させている。

［木村　敏・金木利之・吉川昭吉郎　2020年2月17日］