Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
College of Engineering and Architecture
Department of Electronics Engineering
ECE 078
Principles of Communication Systems
Semester II, SY 24-25
Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
Presented by
03-2324-032313
Benitez, Asheir Clerk P.
03-2324-034478
Curimao, Axel Ross T.
03-2324-035659
Gagute, Paul Benedict B.
03-2324-033899
Ramos, Darold Lawrence P.
BSECE2-01
Presented to
Engr. Stephanie M. Macanlalay
Instructor
March 9, 2025
Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
I. Objectives
1. Design a pre-emphasis and de-emphasis circuits.
2. Study pre-emphasis and de-emphasis used for FM.
3. Analyze the frequency response of the pre-emphasis and de-emphasis circuits.
II. Conceptual Framework
[1] Pre-emphasis is an artificially enhanced frequency change that is used at the
transmitter. It enhances the higher components of a signal before transmission or recording.
De-emphasis is used at the receiver. Its purpose is to transform the message signal back into its
original signal. The de-emphasis process attenuates the higher frequencies by the same amount
they were boosted during pre-emphasis.
III. Materials and Procedures
Materials:
1. 0-20 MHz Cathode Ray Oscilloscope (1 piece)
2. 0-2 MHz Audio Frequency Oscillator (1 piece)
3. 0-30 V, 1 A Regulated Power Supply (1 piece)
4. 1 kΩ Resistor (1 piece)
5. 2 kΩ Resistor (1 piece)
6. 10 kΩ Resistors (2 pieces)
7. 68 kΩ Resistor (1 piece)
8. 100 kΩ Resistor (1 piece)
9. 0.1 uF Capacitors (2 pieces)
10. 0.001 uF Capacitors (3 pieces)
11. Q2N2222 Transistor (1 piece)
12. 0.3 H Decade Inductance Box (1 piece)
Procedures:
[2]
1. The circuit connection is made in the simulator (PROTO).
2. A power supply of 10 V is given to the pre-emphasis circuit.
3. Set the input voltage at 2 V, 1 kHz for pre-emphasis and 1 V, 1 kHz for de-emphasis using
AFO.
4. For this constant value of input voltage, the values of the frequency is varied and
the output voltage is noted on the CRO.
5. A graph is plotted between gain and frequency in a semilog graph sheet for both
pre-emphasis and de- emphasis outputs.
Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
Figure 3.1: Pre-Emphasis Circuit
Figure 3.2: De-Emphasis Circuit
Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
[2] Frequency Response of Pre-Emphasis and De-Emphasis Circuits:
DESIGN
Pre-Emphasis:
The cut off frequency is given by the formula: f = R/2πL Let fc = 1 kHz
Assume R = 2 kΩ Therefore L = R/2πf L = 2000/(2π*1000) L = 0.3 H
De-Emphasis:
The cut off frequency is given by the formula: f = 1/2πRC Let fc = 1.6 kHz
Assume R = 10 kΩ Therefore C1 = 1/2πfR C1 = 1/(2π*1600*10000) C1 = 0.01 µF
IV. Data and Results
A. Values from the experiment from the Simulation App (PROTO)
Table 5.1: Pre-Emphasis
Frequency (Hz) Vo Gain=Vo/Vi Gain in dB=20log(Vo/Vi)
1 kHz 3.3 V 1.65 4.35
Table 5.2: De-Emphasis
Frequency (Hz) Vo Gain=Vo/Vi Gain in dB=20log(Vo/Vi)
1 kHz 4 uV 4 micro -107.96
V. Analysis and Discussion
Laboratory Exercise #7
Pre-Emphasis and De-Emphasis
[3] Threshold effect is the reduction in output signal to noise ratio. It is generally an
undesirable effect in the FM system and it occurs due to the presence of large noise. [1] By using
Pre-emphasis and De-emphasis. Amplifying high frequency signals during transmission and
attenuating it during reception greatly reduces the effect of noise during transmission. [4]
Generally; Pre-emphasis T = RC = 50uS (micro Siemens) De-emphasis T = RC = 75uS It is
calculated using T = RC typically 25uS, 50uS and 75uS. It varies depending on which country. Pre
emphasis is the boosting of higher frequencies is used at FM transmitter and compensated for
expected attenuation during transmission. While De Emphasis is the removal of higher
frequencies used at FM receiver and it restores original frequency balance of the received
signal.
VII. Conclusion
We have learned that demodulating is the process of recovering the original information
but our focus here is on amplitude demodulation. The amplitude demodulator circuit is easy to
build and understand and can be simply built by series and parallel of capacitors and resistors
and adding a diode, to rectify the signal during the negative cycle it becomes reverse biased.
VIII. References
[1] Pre-emphasis and De-emphasis - GeeksforGeeks
[2] ECE 078: Principles of Communication Systems LABORATORY Exercise #7, page 3,4,5
[3] Threshold effect at the receivers
[4] Pre-emphasis and De-emphasis - GeeksforGeeks and Introduction to Emphasis in FM
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