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Basic principles
Published in Michael Talbot-Smith, Audio Engineer's Reference Book, 2012
The signal/noise ratio can be improved further by using high-frequency pre-emphasis. The choice of the pre-emphasis characteristic must be made with care. If too much high-frequency boost is applied this will cause the sound balancer to reduce the general modulation level, which would be counterproductive. The curves for pre-emphasis are designed making assumptions about the programme content's spectrum. For example, the 50 s pre-emphasis used for FM radio in Europe was chosen at a time when single mic balancing of music was the norm. Modern mic techniques and electronic instruments like synthesizers have considerably increased the high-frequency content of programme material causing transmitter limiters to operate on much modern music. Modern design choices tend to roll off the rate of increase of boost at high frequencies. A typical pre-emphasis curve is the J17 equalization used by NICAM (near instantaneously companded audio multiplex) stereo broadcasts (Figure 1.84). Compact disc and DAT, among other systems, allow the user a choice of whether to use pre-emphasis or not. A typical decision might be to use pre-emphasis for classical orchestra recording but not to use it for pop music. For the first, a wide dynamic range places a premium on low noise at low modulation levels. For pop recordings there is a perceived requirement to make them sound as loud as possible requiring the highest possible modulation level. The commonplace use of electrical and synthesizer instruments or very close mic balances of acoustic instruments means that the high-frequency content is usually very
Video recording
Published in John Watkinson, Television Fundamentals, 1996
Fortunately it was discovered that if the gain of the lower side band is increased whilst the gain of the upper sideband is reduced by the same amount, no distortion occurs. Thus if the replay signal, after equalization, is subject to a filter with a frequency response falling at 6 dB per octave, the demodulated signal is undistorted and a useful improvement in signal-to-noise ratio is obtained because the higher noise level in the upper sideband has less effect on the demodulated signal. A further improvement may be obtained if the video signal is subject to pre-emphasis on recording and a matching de-emphasis on replay. Pre-emphasis causes high frequencies to be boosted with respect to low frequencies.
Radiocommunications systems
Published in L. Tetley, D. Calcutt, Understanding GMDSS, 2012
The outline technical principles of VHF transceivers are similar to those of other transceivers. Major differences do exist between VHF and MF/HF transceivers and these are briefly listed below.Frequency modulation is used because the increased spectrum bandwidth required for this method of modulation is available on the VHF transmission band.The use of FM leads to the use of a pre-emphasis circuit in the TX which modifies the audio spectrum to assist in overcoming noise problems and improve link quality. A reciprocal de-emphasis circuit is provided in the receiver.The frequency modulator consists of a voltage controlled oscillator (VCO) and PLL circuit. Signal demodulation in the RX takes place in a discriminator circuit.Low transmitter output power requirement leads to the design of compact transceiver units.By definition, the use of very high transmission frequencies enables physically smaller antennae to be used.There is no requirement for manual tuning of the antenna output stage because the frequency spectrum bandwidth containing the whole range of communication channels is relatively small as compared with the very high frequency band used.
Vibration monitoring for the West-East Gas Pipeline Project of China by phase optical time domain reflectometry (phase-OTDR)
Published in Instrumentation Science & Technology, 2021
Gang Li, Kehong Zeng, Bin Zhou, Wenming Yang, Xiaohui Lin, Fei Wang, Yun Chen, Xutao Ji, Dahai Zheng, Barerem-Melgueba Mao
The pre-emphasis process is important in order to improve the signal-to-noise ratio (SNR) by reducing interferences from individual characteristics. It has been reported that the pre-emphasis makes the recognition remains valid even when the human voice is altered by illness.[17] In the distributed vibration sensing system, the mission of the pre-emphasis is more complicated for three reasons.