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Understanding Sound and Electrical Signals in Telecommunications

Learn the basics of sound waves and electrical signals, their properties, speed, and communication systems. Explore frequency, amplitude, audibility, and components of communication. Discover impedance, inductive and capacitive reactance in electrical signals.

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Understanding Sound and Electrical Signals in Telecommunications

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  1. The Basic – Sound, Electrical Signal, Dr. Farid Farahmand CET 533

  2. Sound • What is Sound? • Periodic variation in air pressure • Travels through media with different speed – air, metal, wood, etc. • Sound waves • Pressure waves that vibrate molecules in a medium • Similar to throwing a stone in water • Human Voice • Vibrations created in vocal cords traveling out of the mouth and changing (disturbing) the air around us http://www.grc.nasa.gov/WWW/K-12/airplane/sndwave.html

  3. Telecommunications Basics • Telecommunications is about sending data and voice • Telephony and data communication • The sound/data must be converted into electrical signals • Sound waves are characteristics by their physical properties • Frequency and wavelength • Power or strength • Amplitude or volume (loudness) • Communication circuits convert sound into electrical waves • Electrical waves are characterized by • Voltage • Current • Power • Impedance • Frequency and wavelength

  4. Sound Properties • Frequency • Number of cycles (events) over a time unit • Cycles per second (Hz) - FIGURE • Wavelength • Describes the length of the waveform • Amplitude • Determines the loudness (height of the signal) • Describes the loudness or softness • Higher volume  traveling longer (more power) • Hence: Distance µ Volume µ Power

  5. Sound Properties • Audibility • Frequency range that is sensible to human ear 20-20K Hz • Human speech: 300-4KHz • That is why the sampling rate for human speech is 8KHz! • Size of the eardrum determines the frequency sensibility • Dogs are more sensible to hear higher frequencies; they have small ear drums • Telephones tend to hear sounds with low frequencies • Teenagers are more sensitive to high pitch noises! One way to disperse them! • Telephone systems are designed for human speech • Filtering any frequencies beyond 300-4KHz • You can not listen to a Mozart’s concert too well! Assuming you are in space, would a very loud explosion hurt your ears drums? ANSWER http://www.school-for-champions.com/science/sound.htm

  6. The Speed of Sound speed of sound (m/s) = 331.5 + 0.60 T(°C) The motion relationship "distance = velocity x time" is the key to the basic wave relationship. With the wavelength as distance, this relationship becomes l=vT. Then using f=1/T gives the standard wave relationship http://library.thinkquest.org/11924/swchar.html http://library.thinkquest.org/19537/Physics4.html

  7. Communication Systems • Basic components • Converter (sound to electrical & electrical to sound) • TX and RX • Medium (air, copper, water, etc.) – typically a conductor - FIGURE • Speed of the electrical signals through a conductor is faster than air • Faster due to shock wave effect of electrons (similar to domino effect)

  8. Basics of Electrical Signals • Four basic properties • Current, Voltage, Power, and Impedance • Voltage: The force causing flow of electrons down the conductor (or medium) – (V) • Current is the flow of electrons (I) • More flow when there is less resistance in the medium • Can be direct or alternating (DC or AC) • Power defines as P=V.I AC Voltage: http://micro.magnet.fsu.edu/electromag/java/generator/ac.html

  9. Basics of Electrical Signals • A little more about AC Voltage • Generators typically generate AC voltage • A moving wire through magnetic field – FIGURE • It converts mechanical energy to electrical energy • AC voltage has amplitude and frequency • In US the amplitude from an outlet is 120 RMS with f=60 Hz FIGURE • RMS = 0.707 x ½ (Vpeak-to-peak) • The Vpeak-to-peak in US is +170 to -170  340 V • Hence: 0.707 x ½ (340) = 120 V RMS!! http://www.walter-fendt.de/ph11e/mfwire.htm http://micro.magnet.fsu.edu/electromag/java/compass/index.html

  10. Basics of Electrical Signals • Impedance • Measured in Ohm or W • Composed of Resistance, Inductive Reactance, Capacitive Reactance • Resistance is the force impeding the flow of electrons • All conductors have some resistance • Effects both AC and DC signals • V = I.R

  11. Basics of Electrical Signals • Inductive Reactance (XL) • Only impacts AC signals (alternating flow through Tip and Ring in telephone cables) • Defined as XL = 2fp.L • L is called the inductance and measured in Henry • Inductance depends on the wire gauge and the material • Example: f=100, L=0.01H, XL=0.63 Ohm • Note: Higher frequency results in higher Inductive Reactance

  12. Basics of Electrical Signals • Capacitive Reactance (XC) • Only impacts AC signals (capacitance effect between the Tip and Ring wires) • Defined as XC = 1/(2fp.C) • C is called the capacitance and measured in Farad • Capacitance is proportional to the wire length • Example: f=1000, C=0.083F, XC=1917 Ohm • Note: Higher frequency results lower XC

  13. Basics of Electrical Signals • Remember: f  Very High, then XL0 and XC¥ • We can couple the capacitance and inductance effects to cancel them! • If XL = XC; What is the frequency? • This is called the resonant frequency • In this case we call the circuit Tuned • Example: f=10KHz, C=253.3 nF, L=1mH • XL=62.83 Ohm • XC=62.83 Ohm • The resonant frequency = 10KHz

  14. Power in Telecommunication Systems • Often we are interested in comparing power levels • Input power vs. output power in an amplifier • Ratio between good signal and bad signal (noise) • Loss of signal on a wire • Power in telecom systems is defines as 1 mW of electricity flowing through one ohm of resistance • P = VxI = RxI2  Double the current what happens? • We use dBm to express signal power in telecommunications • Converting Power in mW to dBm: • Example: If Power is 10mW -> Power is 10 dBm

  15. Power in Telecommunication Systems • We are also interested to see what happened to the power level • The unit to compare is decibel (dB) • Gain = P2/P1 = Power_out / Power_in If P1=2W and the gain is 3dB, find P1!

  16. Power in Telecommunication Systems • Remember: • Example 1: if P2=2mW and P1 = 1mW  3dB • Example 2: if P2=1KW and P1=10W 20dB • What if dB is given and you must find P2/P1? • P2/P1 = Antilog(dB/10) = 10 dB/10 . • Example 3: if dB is +10 what is P2/P1? • P2/P1 = Antilog(+10/10) = 10 +10/10 = 10

  17. Power Analysis (FIGURE)

  18. Organizations • Who deals with telecommunication issues? • Who can use what spectrum • Signal formats and technology • Connectivity, addressing, etc. • FCC (Federal Communication Commission) – Controls all communication at federal levels. • Standard Organizations • International Telecommunication Union (ITU) – UN-based • Has several branches: ITU-T (telecommunication), ITU-R (radio) • CCITT world organized standards organization for telecommunications • American National Standards Institute (ANSI) • ANSI X.3T9.5 Focusing on US telecommunications – similar to ITU • International Standard Organization (ISO) – Liaison between ANSI and ITU

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