Introduction Modulation Signals at Different perspectives – Frequency and Time AM, FM, PM

1. Introduction • Modulation • Signals at Different perspectives – Frequency and Time • AM, FM, PM • Signal conversion – Analog to Digital and Digital to Digital.

2. Signal Modulation • The process of varying the electrical energy is modulation. • The high amplitude, high frequency, and fixed frequency level of energy that flows through the channel is the signal carrier. • An electronic device varies this carrier to reflect the information contained in a weaker voice or data signal. • Original computer and telephone signals often lack the strength to go far enough to be useful. • A modulation device can generate a combined signal strong enough to make it to its destination • Another device called a demodulator has to separate the signal that arrives at its destination

3. The Time Perspective of analog signal

4. Time perspective of digital signal.

5. The Frequency Perspective • A signal has either a constant frequency over a period or a frequency that varies over time. Whether the frequency remains constant or changes, it has a maximum and a minimum. • If the frequency is constant, the maximum and minimum frequency are the same. • This frequency range is the frequency perspective of a signal.

6. Frequency perspective of analog signals

7. Analog Signal Modulation • The three primary analog techniques are • amplitude modulation, frequency modulation, and phase modulation. • The fourth technique applies to digital communications channel only and is called digital modulation. • A matching demodulation technique requires on the receiving end of the link

8. Amplitude Modulation • First used this technique for radio broadcast. • Easy to implement on the transmitting end, and the demodulation was easy and Inexpensive on the receiving end. • Constant frequency but varies in amplitude over time to convey information. • Lower-frequency information signal is imposed on the carrier • The amplitude of the resulting compound signal varies within an envelope that matches the form of the signal.

9. Disadvantages: • Any noise can adversely affect the demodulation of the AM signal. • Electrical noise can result in misinterpretation of the signal • Amplifiers amplify both the signal and the noise that has contaminated the signal.

10. Amplitude modulation signal generation

11. Frequency Modulation • Has a constant amplitude but varies in frequency over time to convey information. • Imposing the lower-frequency information signal on the carrier • The frequency of the resulting compound signal varies to match the form of the information signal. • The disadvantage of FM is the bandwidth it takes to transmit a signal has a wider spectrum Being wider in bandwidth the number of FM signals transmitted through a medium is smaller than the number of AM signals. • Low-speed modems use FM technique to convert digital signals into FM signals. • This process of frequency shift keying (FSK)

12. Frequency modulation signal generation and FM signal bandwidth

13. Phase Modulation • This technique does not apply widely to radio communications, • Vendors use the technique to convey color information in color television broadcasts. • PM a better alternative to AM and FM techniques. • More difficult to understand than AM and FM • Allows computers to communicate at high data rates through the telephone system. • Requires at least two analog signals. • The first signal is the carrier; all other signals modify the carrier signal to convey information.

14. Imposes information on the carrier by changing the shape of the carrier’s signal curve at a given points in time. • Signal cross the amplitude reference line at different times and are therefore different in phase. • The different in phase between two sine waves is phase angle, from 0 to 360 • The break point is the phase change that conveys the information for the compound signal. • A reference pattern allows designers to design and build PM circuits. • Can build a circuit that locks onto a carrier’s frequency to eliminates it to leave the information signal.

15. Carrier and information signals 180 degrees different in phase, and a phase modulated signal.

16. Very accurate and efficiency • Multiple phase shift angles can be introduced. • Quadrature phase modulation incorporates four different phase angles, • Each represents two bits: • 45 degree phase shift = 11; • 135 degree phase shift = 10; • 225 degree phase shift = 01; • 315 degree phase shift = 00. • Quadrature phase modulation has double the efficiency of simple phase shift modulation.

17. Four phase angles of 45, 135, 225, and 315 degrees.

18. Pulse code modulation (PCM) and pulse amplitude modulation (PAM).

19. Analog-to Digital Conversion (CODEC) • Codec (short for coder/decoder) translates analog voice signals into digital signals. • The codec samples the signal • Process is called pulse amplitude modulation (PAM). • Final modulation is pulse code modulation (PCM). • PCM converts the stream of continuous varying PAM signals into a stream of binary signals.

20. Digital to Digital Conversion Three common coding techniques for Ethernet data: 1. Non Return to Zero (NRZ) 2. Manchester Coding, Differential Manchester Coding, and 3. 4B/5B.

21. Non-Return to Zero Digital Encoding Schemes • Non-Return to Zero-Level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages. • It is also simple and inexpensive to implement. • Non-Return to Zero-Inverted (NRZ-I has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0. • Problem: long sequences of 0s in the data produce a signal that never changes. • The receiver requires signal changes to produce a synchronization.

22. Manchester Digital Encoding Schemes • Ensures that each bit has some type of signal change • Solves the synchronization problem. • There is always a transition in the middle of the interval. • Manchester code signal: • Changes from high to low in the middle of the interval to transmit a 0, and • Changes from low to high in the middle of the interval.

23. Differential Manchester • Used in most local area networks • No transmission in the beginning interval = 1 • Transmission in the beginning interval = 0 • Receiver can synchronize itself with the incoming bit stream. • Disadvantage: Nearly half of the time there will be two transitions during each bit. Hardware has to work twice as hard of that of NRZ encode signal. • Therefore it can be inefficient.

24. Examples of four digital encoding schemes

25. 4B/5B Digital Encoding Scheme • Solve the synchronization problem • Relatively inefficient. • Satisfy the synchronization problem and avoid the inefficiency problem. • The 4B/5B takes 4 bits of data, converts the four bits into a unique five bit sequence, and encodes the five bits using NRZ-I. • Looking at next four bits in a data stream to be transmitted and replaced the four bits with a new 5-bit code. • Notice that there are no more than two consecutive zeros in all the new 5-bit code.

26. 4B/5B Conversion Table

27. 0000 11110 Original becomes 5-bit encoded 1 1 1 1 0data data NRZ-I Encoded signal