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

2. Sound What is Sound? Sound waves Human Voice
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

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

4. Sound Properties Frequency Wavelength Amplitude
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

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

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:

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 = x ½ (Vpeak-to-peak)
The Vpeak-to-peak in US is +170 to -170  340 V
Hence: x ½ (340) = 120 V RMS!!

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

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