UNIT 2- UNDERSTANDING DIGITAL SIGNALS PART 2
- 1. UNIT 2: UNDERSTANDING DIGITAL SIGNALS
- 2. Unit Outline 1. Analog signals 2. Digital signals 2.1 Bit rate 2.2 Bit interval 2.3 Digital signals as a composite Analog signal 2.4 Digital signal through a wide bandwidth medium 2.5 Digital signal through a band limited medium 2.6 Digital versus Analog bandwidth 3. Modulation of digital data 3.1 Aspects of Analog to digital conversion
- 3. Digital signals as a composite Analog signal • A digital signal, in the time domain, comprises connected vertical and horizontal line segments. • A vertical line in the time domain means a frequency of infinity (sudden change in time); • A horizontal line in the time domain means a frequency of zero (no change in time). • Going from a frequency of zero to a frequency of infinity (and vice versa) implies all frequencies in between are part of the domain. • Fourier analysis can be used to decompose a digital signal. • If the digital signal is periodic, which is rare in data communications, the decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies. • If the digital signal is non-periodic, the decomposed signal still has an infinite bandwidth, but the frequencies are continuous.
- 4. Digital signals as a composite Analog signal… • Figure: shows a periodic and a non-periodic digital signal and their bandwidths.
- 5. Digital versus Analog bandwidth • Bandwidth can be defined as the portion of the electromagnetic spectrum occupied by the signal • It may also be defined as the frequency range over which a signal is transmitted. • Different types of signals have different bandwidth. • Ex. Voice signal, music signal, etc • Bandwidth of analog and digital signals are calculated in separate ways; • analog signal bandwidth is measured in terms of its frequency (hz) • digital signal bandwidth is measured in terms of bit rate (bits per second, bps) • Bandwidth of signal is different from bandwidth of the medium/channel
- 6. Bandwidth of an analog signal • Bandwidth of an analog signal is expressed in terms of its frequencies. • It is defined as the range of frequencies that the composite analog signal carries. • It is calculated by the difference between the maximum frequency and the minimum frequency. Consider the signal shown in the diagram below:
- 7. Bandwidth of an analog signal… • Fig: Bandwidth of a signal in time domain and frequency domain • The signal shown in the diagram is an composite analog signal with many component signals. • It has a minimum frequency of F1 = 30Hz and maximum frequency of F2 = 90Hz. • Hence the bandwidth is given by F2 – F1 = 90 – 30 = 60 Hz
- 8. Bandwidth of a digital signal • Bandwidth of a digital signal is defined as the maximum bit rate of the signal to be transmitted. It is measured in bits per second.
- 9. Bandwidth of a Channel • A channel is the medium through which the signal carrying information will be passed. • In terms of analog signal, bandwidth of the channel is the range of frequencies that the channel can carry. • In terms of digital signal, bandwidth of the channel is the maximum bit rate supported by the channel. i.e. the maximum amount of data that the channel can carry per second. • The bandwidth of the medium should always be greater than the bandwidth of the signal to be transmitted else the transmitted signal will be either attenuated or distorted or both leading in loss of information. • The channel bandwidth determines the type of signal to be transmitted i.e. analog or digital.
- 10. Digital signal through a wide bandwidth medium • If a medium has a wide bandwidth, we can send a digital signal through it . • Some frequencies will be weakened or blocked; still, enough frequencies will be passed to preserve a decent signal shape • This makes it possible for Baseband Transmission
- 11. Digital signal through a band limited medium • Data is sent through a band-limited medium using a band-limited telephone line to the Internet in Hz. • When digital data are transmitted through a band limited channel, dispersion in the channel causes an overlap in time between successive symbols. • The channels carrying these pulses never have enough bandwidth required to ensure the preservation of the shape of the pulse • This form of dispersion, known as intersymbol interference can pose a serious problem to the quality of reception if it is uncontrolled. • Pulse shaping techniques enable perfect reception in the absence of noise.
- 12. Pulse shaping
- 13. Transmission of digital signals • Two approaches can be used to transmit a digital signal 1. Baseband transmission 2. Broadband Transmission
- 14. Baseband Transmission • Baseband Transmission: - Baseband transmission means sending a digital signal over a channel without changing the digital signal to an analog signal. • Baseband transmission requires that we have a low-pass channel, a channel with a bandwidth that starts from zero • A digital signal is a composite analog signal with an infinite bandwidth. • Ex - the entire bandwidth of a cable connecting two computers is one single channel not allowing more than two stations to communicate at the same time.
- 15. Broadband Transmission • Broadband Transmission (Using Modulation):- Broadband transmission or modulation means changing the digital signal to an analogy signal for transmission. • Modulation allows us to use a bandpass channel-a channel with a bandwidth that does not start from zero. This type of channel is more available than a low-pass channel. • Modulation allows us to use a bandpass channel - a channel with a bandwidth that does not start from zero. • At the receiver, the received analog signal is converted to digital, and the result is a replica of what has been sent.
- 16. Modulation of digital data • Modulation of a digital signal for transmission on a bandpass channel
- 17. Encoding and Modulating • Data stored in a computer are in the form of Os and 1s. To be carried from one place to another, data are usually converted to digital signals. This is called digital-to-digital conversion or encoding digital data into a digital signal • Sometimes, we need to convert an analog signal into a digital signal. This is called analog-to-digital conversion or digitizing an analog signal
- 18. EncodingandModulating… • At other times, we want to send a digital signal coming out of a computer through a medium designed for an analog signal. The signal produced by the computer should be converted to an analog signal. This is called digital- to-analog conversion or modulating a digital signal • Often an analog signal such as voice or music is sent over long distances using analog media. However, the frequency of the voice or music is not appropriate for this kind of transmission; the signal should be carried by a higher-frequency signal; This is called analog-to analog conversion or modulating an analog signal
- 19. Analog-to-Digital Conversion • In ADC, we are representing the information contained in continuous wave form as a series of digital pulses • The problem is how to translate information from an infinite number of values to a discrete number of values without sacrificing sense or quality • A well-known of ADC technique is called pulse code modulation (PCM)
- 20. Pulse Amplitude Modulation (PAM) • PAM is the first step in ADC and it is the foundation of the PCM • PAM takes an analog signal, samples it, and generates a series of pulses based on the results of the sampling • The term sampling means measuring the amplitude of the signal at equal intervals
- 21. Pulse Code Modulation (PCM) • PCM modifies the pulses created by PAM to create a completely digital signal • To do so, PCM first quantizes the PAM pulses. Quantization is a method of assigning integral values in a specific range to sampled instances
- 22. PCM • Then, each quantized value is translated into its 7- bit binary equivalent. The 8th bit indicates the sign • The binary digits are then transformed into a digital signal using one of the line coding techniques
- 23. From analog signal to PCM digital code
- 24. Digital-to-Analog Conversion • Digital-to-analog conversion or digital-to-analog modulation is the process of changing one of the characteristics of an analog signal based on the information in a digital signal (Os and 1s)
- 25. Most UsefulModulation TechniquesforDataCommunications • A sine wave is defined by 3 characteristics: amplitude, frequency, and phase; thus by changing one aspect of the sine wave, we can use it for represent digital data • There are 3 mechanisms for modulating digital data into an analog signal: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK) • In addition, there is a fourth mechanism that combines changes in both amplitude and phase called quadrature amplitude modulation (QAM)
- 26. Amplitude Shift Keying (ASK) • In ASK, the strength of the carrier signal is varied to represent binary 1 or 0 • Both frequency and phase remain constant while the amplitude changes • ASK transmission is highly susceptible to noise interference • Binary ASK (BASK) • ASK is normally implemented using only two levels and is hence called binary amplitude shift keying. • Bit 1 is transmitted by a carrier of one particular amplitude. • To transmit Bit 0 we change the amplitude keeping the frequency constant
- 27. Amplitude Shift Keying (ASK)…
- 28. Frequency ShiftKeying (FSK) • In FSK, the frequency of the carrier signal is varied to represent bit 1 or 0 • The frequency of the signal during each bit duration is constant and its value depends on the bit: both peak amplitude and phase remain constant • FSK avoids most of the noise problems of ASK since the receiving device is looking for specific frequency changes over a given number of perioas
- 29. Phase Shift Keying (PSK) • In PSK, the phase of the carrier is varied to represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase changes
- 30. Quadrature Amplitude Modulation (QAM) • Quadrature amplitude modulation is a combination ofASKand PSK • In QAM, it ispossible totransfer more bitsper position as there aremultiple points of transfer;thus, themodulation efficiencyis increased