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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[general] Amplitude modulated transmission of electromagnetic waves used for wireless signal transmission. The mechanism of operation is distinctly different from FM radio. The radio signal is generated by an alternating dipole in oscillatory fashion over the length of an antenna generating a carrier wave with a fixed frequency, which is modulated in amplitude by the superimposed signal wave. The accompanying alternating electric and magnetic field is described for the dipole (seedipole, alternating). The superposition principle applies to electromagnetic waves. The broadcast frequency range is split in the following wavelength bands. The carrier wave is denoted by three denominations. The first transmission designation is long wave radio: 148.5–283.5 kHz. Long-wave (LW-) radio generally uses 9 kHz channel spacing. Next is medium-wave (MW-) radio: 520–1,710 kHz. Channel spacing varies between 9 and 10 kHz. Medium-wave radio is associated with the generally available information channels and music for commercial use. The last band is short wave broadcasting in the 1.711–30.0 MHz range. Short-wave (SW-) radio is subdivided into 15 bands; channels are generally separated by 5 kHz (see Figure A.62).
Modulation and demodulation
Published in Geoff Lewis, Communications Technology Handbook, 2013
Dynamic carrier control (DCC). This technique, which produces carrier level compression at the transmitter, relies on the normal automatic gain control (AGC) function of the receiver to provide the necessary expansion. The full carrier level is transmitted during low levels of modulation, to ensure a good signal to noise (S/N) ratio. As the audio level increases, the carrier level is automatically reduced. The corresponding worsening of the S/N ratio at the receiver is masked by the higher level of output signal. The transmitter power consumption thus decreases with increasing depth of modulation. A significant power saving can be made during the transmission of modern music programmes that consistently produce higher levels of modulation. From the reception point of view, the system works better in areas of high signal levels and with medium wave broadcasting. The carrier control signal has to have fast attack and slow release times in order to correctly control the level of compression.
Distribution of audio signals
Published in Michael Talbot-Smith, Audio Engineer's Reference Book, 2012
The propagation of radio waves in the broadcast bands at medium frequency (MF), high frequency (HF), and very high frequency (VHF) depends on the frequency being considered and the medium through (or over) which the waves travel. Ground conductivity and the properties of the ionosphere over the transmission path determine the resultant field strengths at MF. The properties of the ionosphere determine the resultant field strengths at HF. The nature of the terrain and the properties of the troposphere over the transmission path determine received signal levels at VHF. Assessment of received field strength for known transmitter powers and antenna conditions requires the application of free-space theory modified by the inclusion of data obtained from numerous measurements of the properties of the transmission medium. Prediction involves the statistical extension of this data, based on historical observations and resulting trends. The free-space conditions which will be summarized and modified with idealized theory, are complemented by practical modifications using methods and data noted by the CCIR (the International Radio Consultative Commmittee which serves the International Telecommunications Union (ITU)). The Broadcast range of frequencies is as follows. Medium wave (MF): 526.5–1606.5 kHz. Very high frequency (VHF Band 2): nominally 88–108 MHz. Short wave (SW) or high frequency (HF): 3900–4000 kHz (75 m band) 5950–6200 kHz (49 m band) 7100–7300 kHz (41 m band) 9500–9775 kHz (31 m band) 11 700–11 975 kHz (25 m band) 15 100–15 450 kHz (19 m band) 17 700–17 900 kHz (16 m band) 21 450–21 750 kHz (13 m band) 25 600–26 100 kHz (11 m band)
The Changing Face of Public Broadcasting in India
Published in IETE Journal of Education, 2023
AM broadcasting remained as the mainstay for a long time and medium wave (MW) stations in the 550–1650 KHz frequency range formed the back bone of the regional services. Short wave stations in 3–30 MHz frequency bands, allocated by Wireless Allocation Radio Frequency Committee (WARC), were used for long distance broadcasting targeted at far flung corners of the world. The frequencies in the MW band had a restricted range during the daytime and SW stations had to change frequencies to take care of variations in the ionospheric conditions.