The basic introduction

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Abstract:-

The concept of pre-emphasis & De-emphasis is broad subject ,howrver I will try to give you the basic introduction,circuit diagram,explanation of preeemphasis and Demphasis and there advantage & necessities of using it in FM.

INTODUCTION

The term 'FM band' is effectively shorthand for 'frequency band in which FM is used for broadcasting'. It can upset purists, because it conflates a modulation scheme with a range of frequencies .

Throughout the world, the broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 - 108.0 MHz is used

Pre-emphasis

" Improving the signal to noise ratio by increasing the magnitude of higher frequency signals with respect to lower frequency signals"

De-emphasis

" Improving the signal to noise ratio by decreasing the magnitude of higher frequency signals with respect to lower frequency signals"

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Noise at the higher-modulating-signal frequencies is inherently greater in amplitude than noise at the lower frequencies. This includes both signal frequency interference and thermal noise.Therefore,for information signals with a uniform signal level,a nonuniform signal-to-noise ratio is produced,and the higher modulating signal frequencies have a lower signal to noise ratio than the lower frequencies.It can be seen that the S / N ratio is lower at the high frequencies ends of the triangle.To compensate for this, the high frequency modulating signals are emphasiszed or boosted in amplitude in the transmitter prior to performing modulation.To compensate for this boost, the high frequency singals are attenuated or deemphasized in the receiver after demodulation has been performed.Deemphasis is the reciprocal of preemphasis and,therefore,a deemphasis network restores the original amplitude versus frequency characteristics to the information signals.In essence,the preemphasis network allows the high frequency modulating signals to modulate the carrier at a higher level and thus,cause more frequency deviation than their original amplitudes would have poroduced.The high frequency signals are propagated through the system at an elevated ,demodulated,and then restored to their original amplitude properties.

A preemphasis network is a high pass filter and a deemphasis network is a low pass filter.A preeemphasis network provides a constant increase in the amplitude of the modulating signal with an increase in frequency.With FM,approximately 12dB of improvement in noise performance is achieved using pre and deemphasis.The break frequency is determined by the RC or L /R time constant of the network.The break frequency occurs at the frequency where Xc or XL equals R.Mathematically,the break frequency is

fb =1/2pieRC --------(1)

fb=1/2pieL/R ---------(2)

An active rather than a passive preemphasis network is used because a passive preemphasis network provides loss to all frequencies with more loss introduced at the lower modulating signal frequencies.The result of using apassive network would be a decrease in the signal to noise ratio at the lower modulating signal frequencies rather than an increase in the signal to noise ratio at the higher modulating signal frequencies.

From equation

m=K1Vm /fm --------(3)

where

m= modulation index (unitless)

K1= deviation sensitivity(cycles per seconds per volt or hertz per volt)

Vm = peak modulating singal amplitude (volts)

fm= cyclic frequency (hertz per second)

From equation (3) it can be seen that if changes in the frequency of the modulating signal (fm)produce corresponding changes in its amplitude (Vm), the modulation index(m) remains constant with frequency.This ofcourse , is a characteristic of phase modulation.Consequently with commercial broad cast band modulators, frequencies below 2120 Hz produce frequency modulation, and frequencies above 2120 Hz produce phase modulation . converting FM to PM is not the function of a preemphasis network, however, but rather a consequence .

The noise generated internally in FM demodulators inherently increases with frequency,which produces a non uniform signal to noise ratio at the output of the demodulator.The signal to noise ratios are lower for the higher modulating signal frequencies than for the lower modulating singal frequencies.Using a preemphasis network in front of the FM modulator and a deemphasis network at the output of the FM demodulator improves the signal to noise ratio for the higher modulating signal frequencies, thus producing a more uniform signal to noise ratio at the output of the demodulator.

CIRCUIT DIAGRAM

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Pre-emphasis is a strong high frequency boost before the transmitter, and an equally strong high frequency attenuation in every radio.

FM radio is inherently a noisy medium. The noise has triangular distribution, meaning the noise is louder at higher frequencies.

We see the noise very clearly in both the oscilloscope and spectrum analyzer above, as well as on the meters. This is "Silence" during noisy reception! In the spectrum analyzer you clearly see the 19000 Hz pilot tone, and all the noise around it. Notice also that there is more noise at higher frequencies, and less noise at lower frequencies. In the image above it also looks like the noise rises again at the very lowest frequencies (all the way to the left) but this is not so -- it's simply an artifact of how linear FFT spectrum analyzers work. More frequencies (several octaves, actually) get bunched into just a pixel or two, and the noise energy in each all contribute to the noise.

Anyway. We can all agree that FM is inherently very noisy. To overcome this, FM broadcast uses pre-emphasis. Pre-emphasis is a progressive treble boost, designed to boost the level of high frequencies over the noise floor. In every radio receiver, a corresponding de-emphasis network attenuates the treble by exactly the same amount, yielding flat requency response, albeit with lower headroom for high frequencies.

In the image above, MpxTool was set to 75us de-emphasis. Notice the significantly lower noise level on the De-emph meters compared to De-mod meters.

North America uses 75us pre-emphasis. Europe and Australia uses 50us pre-emphasis. 50us is lighter boost, yielding slightly worse noise reduction than 75us, but more high frequency headroom.

Stereo reception is more noisy than mono reception because of the Stereo Subcarrier, located between 22 and 54 kHz. Due to the triangular noise distribution, background noise is very high under this subcarrier, and decoding the stereo subcarrier into Left and Right means that this noise gets shifted down into the audible range. This makes pre-emphasis even more important for FM Stereo reception -- despite the fact that pre-emphasis "goes the wrong way" for half of the subcarrier. At these high frequencies, the direction doesn't matter as much -- they're all noisy.

Notice how pre-emphasis lifts the high frequencies well above the noise floor. Without pre-emphasis, the high frequencies get buried in noise.

In these audio examples we can hear how pre-emphasis affects the audio. Audio clip courtesy of WHIO.

All (.pls) All (.m3u) 75us, Stereo 50us, Stereo No pre-emph, Stereo 75us, Mono 50us, Mono No pre-emph, Mono

We will probably agree that it's a good thing we have pre-emphasis.

The image below shows the concept. Below the 2.1 KHz, both the transmitter and receiver are flat. Above that, the pre-emphasis at the transmitter is exactly offset by the de-emphasis at the receiver, so the net is a flat frequency response, measured from transmitter input to receiver output.

CONCLUSION:-

All radios have de-emphasis, so unless we broadcast with pre-emphasis, our audio will sound extremely muffled.

Nevertheless, it's difficult to control a pre-emphasized signal. The treble boost causes lots of little spikes in the waveforms, and during S-sounds, crash cymbals, saxophones and other treble-heavy sounds, it causes extremely tall spikes up to 17dB.17dB may not sound like a lot, but it is. 6dB is doubling the level. 6dB more is doubling it again.. That's 400%, and we're still only at 12dB.Add 5dB more just for good measure, and we're at over 700%.

FM channels are fixed bandwidth. Maximum deviation (+/- 75 kHz) is dictated by laws and physical limitations alike. If we go beyond +/- 75 kHz, we're in our neighbours channel already. So, the signal must stay within the bounds. Allowing for these occasional 700% spikes would mean our average modulation must be only 14% (or 10 kHz). This would be a mighty quiet, weak and noisy signal.

ACKNOWLEDGEMENT:-

This is to certify that I Manish Uzzwal .Roll.no.-50,Section-c6703,is the student of Lovely Professional University has made this term paper with the guidence of Miss Gurpreet Kaur the faculty of Analog Communication .He supported me alot to complete my paper.Without his support I would not be able to complete my paper.

REFERENCES

  1. www.repeater-builder.com/rbtip/predeemp.
  2. www.allinterview.com/showansen.
  3. wikipedia.org/wiki/FM_broadcasting wers/10635.html html
  4. wiki.answers.com/Q/What_is_pre_emphasis_de_emphasis
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