Pre-emphasis and De-emphasis.pptx

Gives information about Pre-emphasis and De-emphasis

1. ANALOG COMMUNICATION SYSTEMS By Ms. Swati Shrrpal Halunde Assistant Professor DEPT. of E.C.E S.I.T.C.O.E. Yadrav-Ichalkaranji Pre-emphasis and De-emphasis

2. Pre-emphasis and De-emphasis Pre and de-emphasis circuits are used only in frequency modulation. Pre-emphasis is used at transmitter and de-emphasis at receiver . Pre-emphasis In FM, the noise has a greater effect on the higher modulating frequencies. This effect can be reduced by increasing the value of modulation index (m f ), for higher modulating frequencies. This can be done by increasing the deviation ‘  ’ and ‘  ’ can be increased by increasing the amplitude of modulating signal at higher frequencies. Definition: The artificial boosting of higher audio modulating frequencies in accordance with prearranged response curve is called pre-emphasis. Pre-emphasis circuit is a high pass filter as shown in Fig.

3. As shown in Fig. 1, AF is passed through a high-pass filter, before applying to FM modulator. A s m o du l a t i n g f r e q u e n c y ( f m ) i n c r e a se s , c a p a c i t iv e r e a c t a n c e d e c r e a s e s a n d m o d ul a t i n g v ol t a g e g oe s o n i n c r e a s i n g . f m  Voltage of modulating signal applied to FM modulat Boosting is done according to pre-arranged curve as shown in Fig. 2 . Fi g. 2 : P r e - emp h asi s C u r v e

4. • The time constant of pre-emphasis is at 50  s in all CCIR standards. In systems employing American FM and TV standards, networks having time constant of 75  sec are used. The pre-emphasis is used at FM transmitter as shown in Fig. Fig. FM Transmitter with Pre-emphasis

5. De - e m p h as i s De-emphasis circuit is used at FM receiver . Definition: T h e a r t i f i c i a l b o o s t i n g o f h ig h e r m o d u l a t i n g fr e q u e n c i e s i n t h e process of pre-emphasis is nullified at receiver by process called de-emphasis. De-emphasis circuit is a low pass filter shown in Fig. Fig. De-emphasis Circuit

6. Fig. De-emphasis Curve As shown in Fig.5, de-modulated FM is applied to the de-emphasis circuit (low pass filter) where with increase in f m , capacitive reactance X c d ec r e a se s . So t h a t o u t pu t o f d e- e m ph a si s ci r c u i t a l s o r e du ce s • Fig. 5 shows the de-emphasis curve corresponding to a time constant 50  s. A 50  s de-emphasis corresponds to a frequency response curve that is 3 dB down at frequency given by, f = 1/ 2πRC = 1/ 2π x 50x 1000 = 3180 Hz

7. Comparison between Pre-emphasis and De-emphasis Parameter Pre-emphasis De-emphasis 1. Circuit used High pass filter. Low pass filter. 2. C i rc u i t d i a g r a m F ig. 2. 36 Fig . 2.37 3. Response curve Fig. 2.38 Fig. 2.39 4 . Tim e c o ns t a nt T = RC = 50  s T = RC = 50  s 5. D e f i n i t i o n Boosting of higher frequencies Removal of higher frequencies 6. U s e d at FM transmitter FM receiver.

8. Comparison between AM and FM Parameter AM FM 1 . De f i n i t i o n Amplitude of carrier is varied in accordance with amplitude of modulating signal keeping frequency and phase constant. Frequency of carrier is varied in accordance with the amplitude of modulating signal keeping amplitude and phase constant. 2. C o ns t a nt parameters Frequency and phase. Amplitude and phase. 3. M o d u l a t e d si g n a l 4 . M o d ul at i on I n d e x m=Em/Ec m =  / fm 5 . N um b e r o f sidebands Only two Infinite and depends on m f . 6. B a n d wi dt h BW = 2f m BW = 2 (  + f m (max) ) 7. A p p lic a t i o n MW, SW band broadcasting, video transmission in TV. Broadcasting FM, audio transmission in TV.