FM broadcasting
FM broadcasting is a method of radio broadcasting that uses frequency modulation to modulate the carrier wave. In November 1919, Hans Idzerda began broadcasts, using narrow-band FM, over station PCGG, located at The Hague in the Netherlands. However, most early broadcasting stations instead employed Amplitude Modulation transmisons. In 1933, American engineer Edwin Armstrong invented wide-band FM, which is used worldwide to transmit high-fidelity sound over broadcast radio. FM broadcasting offers higher fidelity—more accurate reproduction of the original program sound—than other analog broadcasting techniques, such as AM broadcasting. It is also less susceptible to common forms of interference, having less static and popping sounds than are often heard on AM, but with a more limited broadcast distance. Therefore, FM is used for most broadcasts of music and general audio. FM radio stations use the very high frequency range of radio frequencies.
Broadcast bands
Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum. Usually 87.0 to 108.0 MHz is used, or some portion of it, with few exceptions:- 87.0 to 87.5 is not available in ITU region 1.
- In the former Soviet republics, and some former Eastern Bloc countries, the older 65.8–74 MHz band is also used. Assigned frequencies are at intervals of 30 kHz. This band, sometimes referred to as the OIRT band, is slowly being phased out. Where the OIRT band is used, the 87.0–108.0 MHz band is referred to as the CCIR band.
- In Japan, the band 76–95 MHz is used.
- In Brazil, until the late 2010s, FM broadcast stations only used the 88–108 MHz band, but with the phasing out of analog television, the 76–88 MHz band are allocated for old local MW stations which have moved to FM in agreement with ANATEL.
To minimize inter-channel interference, stations operating from the same or nearby transmitter sites tend to keep to at least a 500 kHz frequency separation even when closer frequency spacing is technically permitted. The ITU publishes Protection Ratio graphs, which give the minimum spacing between frequencies based on their relative strengths. Only broadcast stations with large enough geographic separations between their coverage areas can operate on the same or close frequencies.
In most countries the maximum permitted frequency error of the unmodulated carrier is specified, which typically should be within 2 kHz of the assigned frequency.
Technology
Modulation
FM broadcasting shows better resistance to amplitude noise because the information is carried in the instantaneous frequency rather than in the signal amplitude. In wideband FM systems such as FM broadcasting, most amplitude variations are rejected by the detector, and further reduction is obtained through limiting. In his 1936 paper, Edwin Armstrong explained that wideband FM with a limiter has much greater immunity to amplitude noise than amplitude-modulated systems, and he also noted that this advantage does not hold for narrowband FM.Frequency modulation or FM is a form of modulation which conveys information by varying the frequency of a carrier wave; the older amplitude modulation or AM varies the amplitude of the carrier, with its frequency remaining constant. With FM, frequency deviation from the assigned carrier frequency at any instant is directly proportional to the amplitude of the input signal, determining the instantaneous frequency of the transmitted signal. Because wideband FM signals require greater bandwidth than AM signals, FM broadcasting is allocated to higher-frequency bands where wider channels are available.
The maximum frequency deviation of the carrier is specified and regulated by the licensing authorities in each country. For a stereo broadcast, the maximum permitted carrier deviation is typically ±75 kHz, although higher are allowed in the United States when SCA systems are used. For a monophonic broadcast, the most common permitted maximum deviation is ±75 kHz, although some countries specify a lower value for monophonic broadcasts, such as ±50 kHz.
Bandwidth
The bandwidth of an FM transmission can be approximated using Carson's rule, which states that the necessary bandwidth is twice the maximum deviation plus twice the highest modulating frequency. For a transmission that includes RDS this give =. This is also known as the necessary bandwidth.Noise
FM broadcasting offers improved resistance to amplitude noise compared to AM broadcasting due to its transmission of a constant-envelope. Limiting restores the constant-envelope.Pre-emphasis and de-emphasis
Random noise has a triangular spectral distribution in an FM system, with the effect that noise occurs predominantly at higher audio frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. These processes are known as pre-emphasis and de-emphasis, respectively. Employing pre-emphasis and de-emphasis improves the signal-to-noise ratio at higher audio frequencies, compensating for the frequency-dependent noise characteristics of FM transmission, and was originally described by Armstrong. The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world, a time constant is used. In the Americas and South Korea, is used. This applies to both mono and stereo transmissions. For stereo, pre-emphasis is applied to the left and right channels separately before multiplexing.More generally, pre-emphasis is effective in systems where the desired signal contains most of its energy at low frequencies. Higher frequencies are deliberately boosted before transmission. Noise introduced along the transmission path is then added to the signal. At the receiver, the corresponding de-emphasis removes the boost, with the desired effect of reducing the level of high-frequency noise relative to the recovered signal.
The use of pre-emphasis can become problematic because many forms of contemporary music contain more high-frequency energy than the musical styles prevalent at the birth of FM broadcasting. Pre-emphasizing these high-frequency components can cause excessive deviation of the FM carrier. Modulation control devices are used to prevent this. Systems more modern than FM broadcasting tend to use either programme-dependent variable pre-emphasis; e.g., dbx in the BTSC TV sound system, or none at all.
Pre-emphasis and de-emphasis were used in the earliest days of FM broadcasting. According to a BBC report from 1946, was originally considered in the US, but was subsequently adopted.
Stereo FM
Long before FM stereo transmission was considered, FM multiplexing of other types of audio-level information was experimented with. Edwin Armstrong, who invented FM, was the first to experiment with multiplexing, at his experimental 41 MHz station W2XDG located on the 85th floor of the Empire State Building in New York City.These FM multiplex transmissions started in November 1934 and consisted of the main channel audio program and three subcarriers: a fax program, a synchronizing signal for the fax program and a telegraph order channel. These original FM multiplex subcarriers were amplitude modulated.
Two musical programs, consisting of both the Red and Blue Network program feeds of the NBC Radio Network, were simultaneously transmitted using the same system of subcarrier modulation as part of a studio-to-transmitter link system. In April 1935, the AM subcarriers were replaced by FM subcarriers, with much improved results.
The first FM subcarrier transmissions emanating from Major Armstrong's experimental station KE2XCC at Alpine, New Jersey occurred in 1948. These transmissions consisted of two-channel audio programs, binaural audio programs and a fax program. The original subcarrier frequency used at KE2XCC was 27.5 kHz. The IF bandwidth was ±5 kHz, as the only goal at the time was to relay AM radio-quality audio. This transmission system used 75 μs audio pre-emphasis like the main monaural audio and subsequently the multiplexed stereo audio.
In the late 1950s, several systems to add stereo to FM radio were considered by the FCC. Included were systems from 14 proponents including Crosby, Halstead, Electrical and Musical Industries, Ltd, Zenith, and General Electric. The individual systems were evaluated for their strengths and weaknesses during field tests in Uniontown, Pennsylvania, using KDKA-FM in Pittsburgh as the originating station. The Crosby system was rejected by the FCC because it was incompatible with existing subsidiary communications authorization services which used various subcarrier frequencies including 41 and 67 kHz. Many revenue-starved FM stations used SCAs for "storecasting" and other non-broadcast purposes. The Halstead system was rejected due to lack of high-frequency stereo separation and reduction in the main channel signal-to-noise ratio. The GE and Zenith systems, so similar that they were considered theoretically identical, were formally approved by the FCC in April 1961 as the standard stereo FM broadcasting method in the United States and later adopted by most other countries. It is important that stereo broadcasts be compatible with mono receivers. For this reason, the left and right channels are algebraically encoded into sum and difference signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel.
The signal is limited to 30 Hz to 15 kHz to protect a 19 kHz pilot signal. The signal, which is also limited to 15 kHz, is amplitude modulated onto a 38 kHz double-sideband suppressed-carrier signal, thus occupying 23 kHz to 53 kHz. A 19 kHz ± 2 Hz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it, as defined by the formula below, is also generated. The pilot is transmitted at 8–10% of overall modulation level and used by the receiver to identify a stereo transmission and to regenerate the 38 kHz sub-carrier with the correct phase. The composite stereo multiplex signal contains the Main Channel, the pilot tone, and the difference signal. This composite signal, along with any other sub-carriers, modulates the FM transmitter. The terms composite, multiplex and even MPX are used interchangeably to describe this signal.
The instantaneous deviation of the transmitter carrier frequency due to the stereo audio and pilot tone is
where A and B are the pre-emphasized left and right audio signals and =19 kHz is the frequency of the pilot tone. Slight variations in the peak deviation may occur in the presence of other subcarriers or because of local regulations.
Another way to look at the resulting signal is that it alternates between left and right at 38 kHz, with the phase determined by the 19 kHz pilot signal. Most stereo encoders use this switching technique to generate the 38 kHz subcarrier, but practical encoder designs need to incorporate circuitry to deal with the switching harmonics. Converting the multiplex signal back into left and right audio signals is performed by a decoder, built into stereo receivers. Again, the decoder can use a switching technique to recover the left and right channels.
In addition, for a given RF level at the receiver, the signal-to-noise ratio and multipath distortion for the stereo signal will be worse than for the mono receiver. For this reason many stereo FM receivers include a stereo/mono switch to allow listening in mono when reception conditions are less than ideal, and most car radios are arranged to reduce the separation as the signal-to-noise ratio worsens, eventually going to mono while still indicating a stereo signal is received. As with monaural transmission, it is normal practice to apply pre-emphasis to the left and right channels before encoding and to apply de-emphasis at the receiver after decoding.
In the U.S. around 2010, using single-sideband modulation for the stereo subcarrier was proposed. It was theorized to be more spectrum-efficient and to produce a 4 dB s/n improvement at the receiver, and it was claimed that multipath distortion would be reduced as well. A handful of radio stations around the country broadcast stereo in this way, under FCC experimental authority. It may not be compatible with very old receivers, but it is claimed that no difference can be heard with most newer receivers. At present, the FCC rules do not allow this mode of stereo operation.