Digital Transmission PDF

Digital Transmission  Digital Transmission ⮚ Digital Transmission is the transmittal of digital signals between two or more points. ⮚ The signals can be binary or any other form of discrete- level digital pulses in a communications system. ⮚ The original source of information m...

Digital Transmission  Digital Transmission ⮚ Digital Transmission is the transmittal of digital signals between two or more points. ⮚ The signals can be binary or any other form of discrete- level digital pulses in a communications system. ⮚ The original source of information may be in digital form, or it could be analog signals that have been converted to digital pulses prior to transmission and converted back to analog signals in the receiver 2  Digital Transmission ❑ The Information needs to be converted to either a digital signal or an analog signal for transmission Conversion methods Digital/Digital Analog/Digital Digital/Analog Analog/Analog encoding encoding encoding encoding Unipo Pola Bipo PCM DM ASK FSK PSK lar r lar AM FM PM Multileve Multitransiti l on QAM  Digital Transmission ⮚ With digital transmission systems, a physical facility, such as a pair of wires, coaxial cable, or an optical fiber cable, is required to interconnect the various points within the system. ⮚ Digital pulses cannot be propagated through a wireless transmission system, such as Earth’s atmosphere or free space (vacuum). 4  Advantages of Digital Transmission over Analog ⮚ Noise immunity ⮚ Digital pulses are better suited for processing and multiplexing ⮚ Uses signal regeneration rather than signal amplification ⮚ Simpler to measure and evaluate. 5  Disadvantages ⮚ Requires more bandwidth ⮚ Analog signals must be converted to digital codes before transmission and converted back at the receiver ⮚ Requires precise time synchronization between the transmitter and the receiver ⮚ Incompatible with existing analog systems 6  Analog to Digital Conversion ⮚ An analog signal varies continuously with time ⮚ Analog signals must be converted to digital pulses prior to transmission and converted back to their original analog form at the receiver. ⮚ If we want to transmit such a signal digitally, that is, as a series of numbers, we must first sample the signal ⮚ This involves finding its amplitude at discrete time intervals. Only in this way can we arrive at a finite series of numbers to transmit. 7 Analog to Digital Conversion A typical system describing the functions in the analog-to-digital and digital-to- analog chain. 8  Pulse Modulation ⮚ Pulse modulation consists essentially of sampling analog information signals and then converting those samples into discrete pulses and transporting the pulses from a source to a destination over a physical transmission medium. ⮚ The four predominant methods of pulse modulation include: 1. Pulse Width Modulation (PWM), 2. Pulse Position Modulation (PPM), 3. Pulse Amplitude Modulation (PAM), 4. Pulse Code Modulation (PCM). 9  Pulse Modulation 1. PWM is sometimes called pulse duration modulation (PDM) or pulse length modulation (PLM), as the width of a constant amplitude pulse is varied proportional to the amplitude of the analog signal at the time the signal is sampled. 2. With PPM, the position of a constant-width pulse within a prescribed time slot is varied according to the amplitude of the sample of the analog signal. 3. With PAM, the amplitude of a constant width, constant- position pulse is varied according to the amplitude of the sample of the analog signal 10  Pulse Modulation 4. With PCM, the analog signal is sampled and then converted to a serial n-bit binary code for transmission. Each code has the same number of bits and requires the same length of time for transmission. ⮚PAM is used as an intermediate form of modulation with PSK, QAM, and PCM, although it is seldom used by itself. ⮚PWM and PPM are used in special-purpose communications systems mainly for the military but are seldom used for commercial digital transmission systems. ⮚ PCM is by far the most prevalent form of pulse modulation 11  Pulse Modulation Pulse modulation: (a) analog signal; (b) sample pulse; (c) PWM; (d) PPM; (e) PAM; (f) PCM 12  Pulse Code Modulation (PCM) ⮚ PCM is the only digitally encoded modulation technique that is commonly used for digital transmission. ⮚ The term pulse code modulation is somewhat of a misnomer, as it is not really a type of modulation but rather a form of digitally coding analog signals. ⮚ In PCM the available range of signal voltages is divided into levels, and each is assigned a binary number. ⮚ Each sample is then represented by the binary number representing the level closest to its amplitude, and this number is transmitted in serial form. 13 PCM System 14  Pulse Code Modulation (PCM) ❖ In the Transmitter, ⮚ The bandpass filter limits the frequency of the analog input signal to the standard voice-band frequency range of 300 Hz to 3000 Hz. ⮚ The sample-and-hold circuit periodically samples the analog input signal and converts those samples to a multilevel PAM signal. ⮚ The analog-to-digital converter (ADC) converts the PAM samples to parallel PCM codes, which are converted to serial binary data in the parallel-to-serial converter and then outputted onto the transmission line as serial digital pulses. 15  Pulse Code Modulation (PCM) ❖ In the Receiver, ⮚ The serial-to-parallel converter converts serial pulses to parallel PCM codes. ⮚ The digital-to-analog converter(DAC) converts the parallel PCM codes to multilevel PAM signals. ⮚ The hold circuit is basically a low pass filter that converts the PAM signals back to its original analog form. ❖ Codec (coder/decoder) - is an integrated circuit that performs the PCM encoding and decoding functions is called a 16  Sampling ❖ An analog signal varies continuously with time. To transmit an analog signal digitally first, we must sample the signal. This involves finding its amplitude at discrete time intervals. In that way, we can arrive at a finite series of numbers to transmit.  Sampling Pulse amplitude modulation has some applications, but it is not used by itself in data communication. However, it is the first step in another very popular conversion method called pulse code modulation. Term sampling means measuring the amplitude of the signal at equal intervals.  Sampling Technique Three different sampling methods for PCM 19  Sampling Technique 1. Natural sampling ❖ It yields a sample pulse whose shape follows that of the original signal Natural Sampling. 20  Sampling Technique Flat-topped sampling - samples by using a sample-and-hold circuit, which maintains the signal level at the start of the sample pulse - The most common method used for sampling voice signals in PCM systems - the purpose of a sample-and-hold circuit is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant-amplitude PAM voltage levels. - the input voltage is sampled with narrow pulse and then held relatively constant until the next sample is taken. 21  Sampling Technique Flat-topped sampling The most common method used for sampling voice signals in PCM systems. Sampling is done using a sample-and-hold circuit in which the input voltage is sampled with narrow pulse and then held relatively constant until the next sample is taken. 22  Sampling Technique ❖ Flat-topped sampling 23 Sample and Hold Circuit 24  Sample and Hold Circuit ⮚ The FET acts as a simple analog switch. ⮚ When turned on, Q1 provides a low-impedance path to deposit the analog sample voltage across capacitor C1. ⮚ The time that Q1 is on is called the aperture or acquisition time. ⮚ Essentially, C1 is the hold circuit. When Q1 is off, C1 does not have a complete path to discharge through and, therefore, stores the sampled voltage. ⮚ The storage time of the capacitor is called the A/D conversion time ⮚ If the input to the ADC is changing while it is performing the conversion, aperture distortion results. ⮚ Droop – is the gradual discharge across the capacitor during conversion time 25  Sample and Hold Circuit Input and output waveforms 26  Sampling Theorem ⮚ The Nyquist sampling theorem establishes the minimum sampling rate (fs) that can be used for a given PCM system. ⮚ For a sample to be reproduced accurately in a PCM receiver, each cycle of the analog input signal (fa) must be sampled at least twice. ⮚ Consequently, the minimum sampling rate is equal to twice the highest audio input frequency. ⮚ If the sampling rate is too low, a form of distortion called aliasing or foldover distortion is produced. ⮚ Once aliasing is present, it cannot be removed. 27  Nyquist Sampling Theorem. ❖ It establishes the minimum sampling rate (fs) that can be used for a given PCM system. ❖ For a sample to be reproduced accurately in a PCM receiver, each cycle of the analog input signal (fa) must be sampled at least twice. ❖ Mathematically, the minimum Nyquist sampling rate is expressed below where: 𝒇𝒔 ≥ 𝟐𝒇𝒂 𝒇𝒔 = 𝒕𝒉𝒆 minimum Nyquist sampling rate 𝒇𝒂 = the highest frequency to be sampled in Hz Aliasing or Foldover Distortion ⮚ The distortion created by using a low sampling rate when coding an analog signal. 28  Example A PCM has a 30 kHz sampling frequency. What is the maximum input frequency? 29  Example A PCM has a 30 kHz sampling frequency. What is the maximum input frequency? Solution: Given in the problem is sampling rate which is 30 kHz, substituting it in the formula for the minimum Nyquist sampling rate we get, 𝒇𝒔 ≥ 𝟐𝒇𝒂 𝟑𝟎 𝒌𝑯𝒛 ≥ 𝟐𝒇𝒂 𝒇𝒔=30 kHz/2=15 kHz Therefore, for a 30 kHz sampling frequency the maximum input frequency is 15 kHz. 30  Sampling Theorem Satisfactory sampling rate 31  Sampling Theorem Sampling rate too low 32  Output spectrum for a sample-and-hold circuit 33  Quantization Quantization ❖It is the process of converting an infinite number of possibilities to a finite number of conditions. ⦿ Quantizing ⮚It refers to the assigning of PCM codes to absolute magnitudes 34  Quantization ⦿ Resolution ⮚ It is equal to the voltage of the minimum step size. ⮚ It is the minimum voltage other than 0V that can be decoded by the digital- to- analog converter in the receiver. Three-Bit PCM Code. 35 36  Dynamic Range (DR) ❖ It is the ratio of the largest possible magnitude to the smallest possible magnitude that can be decoded by the Digital-to Analog converter (DAC). ❖ Mathematically, 𝑽𝒎𝒂𝒙 𝑫𝑹 = 𝑽 𝒎𝒊𝒏 = 𝒓𝒆𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 𝑽𝒎𝒂𝒙 ❖ Dynamic Range in dB 𝑽𝒎𝒂𝒙 𝑫𝑹(𝒅𝑩) = 𝟐𝟎𝒍𝒐𝒈 𝑽 𝒎𝒊𝒏 where: 𝑽𝒎𝒂𝒙 = the maximum voltage magnitude that can be decoded by the DAC 𝑽𝒎𝒊𝒏 = the minimum voltage magnitude that can be decoded by the DAC 37  Number of Bits required for a PCM code 𝟐𝒏 − 𝟏 ≥ 𝑫𝑹 where: n = the number of PCM bits, excluding the sign bit 𝑫𝑹 = the absolute value of dynamic range 38  Example A PCM system specified a 56 dB of dynamic range. How many bits are required to satisfy the dynamic range specification? 39 Overload or peak limiting magnitude of the sample exceeds the highest quantization interval. 40 resolution magnitude of the minimum step size. minimum voltage other than 0V that can be decoded by the DAC at the receiver. 41 Note The smaller the magnitude of the minimum step size, the better the resolution and the more accurate the quantization interval will resemble the actual analog sample 42 Quantization error or results of rounding off Dynamic Range (DR) ratio of the largest possible magnitude to the smallest magnitude that can be decoded by the DAC. 43 Dynamic Range (DR) To determine the no. of bits required for a PCM code 2n - 1 > DR where n = no. of PCM bits DR = absolute value 44  Quantization Amplitude quantizing: Mapping samples of a continuous amplitude waveform to a finite set of amplitudes. Out In ▪ Average quantization noise power ▪ Signal peak power Quantized values ▪ Signal power to average quantization noise power 45  Encoding (PCM) Pulse code modulation (PCM): Encoding the quantized signals into a digital word (PCM word or codeword). Each quantized sample is digitally encoded into an l bits codeword where L in the number of quantization levels and 46  Pulse Code Modulation (continued) Binary values are later converted to an analog signal Waveform similar to original results 47  Pulse Code Modulation (continued) The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution 48  Pulse Code Modulation (continued) Because the human voice has a fairly narrow bandwidth Telephone systems digitize voice into either 128 levels or 256 levels Called quantization levels If 128 levels, then each sample is 7 bits (2 ^ 7 = 128) If 256 levels, then each sample is 8 bits (2 ^ 8 = 256) 49  Pulse Code Modulation (continued) How fast do you have to sample an input source to get a fairly accurate representation? Nyquist says 2 times the bandwidth Thus, if you want to digitize voice (4000 Hz), you need to sample at 8000 samples per second 50  Delta Modulation An analog waveform is tracked using a binary 1 to represent a rise in voltage and a 0 to represent a drop 51  Delta Modulation uses a single bit PCM code to achieve digital transmission of analog signals 2 problems associated with delta modulation 1. slope overload 2. granular noise 52  slope overload the slope of the analog signal is greater than the delta modulation can maintain 53  granular noise when the origin analog input signal has a relatively constant amplitude, the reconstructed signal has variation that were not present in the original signal. 54  Adaptive delta modulation delta modulation system where the step size of the DAC is automatically varied depending on the amplitude characteristic of the analog input signal. 55 Adaptive delta modulation 56  Source Coding To eliminate redundancy Huffman Coding Shannon-Fano Coding To maximize information rate in a transmission What is Information Rate ? Information per bit 🡪 Entropy 57  Channel Coding Error Control Coding To reduce the impact of channel errors by controlled introduction of redundancy Decrease in effective data rate Increased coding gain Forward Error Correcting Codes Linear Block Codes Convolutional Codes ARQ methods 58  Line Coding Formats (Converting Data into Signals) Numerous techniques – NRZ-L NRZ-I Manchester Differential Manchester Bipolar AMI 59 Converting Data into Signals (continued) 60 Where do you need modulation ? Orthogonal Frequency Division Multiplexing ADSL 61 62 63 64 65 Digital Codes ❖ It is used to represent data such as text, numerical or symbols processed and stored by computers. 66 1. Morse Code The first digital code created by the inventor of the telegraph, Samuel Morse. It consists of a series of “dots” and “dashes” that represent letters of the alphabet, numbers, and punctuation marks. 67  1. Morse Code The Morse codes. A dot (.) is a short click; a dash (—) is a long click. 68  2. Baudot Code -Developed by Thomas Murray, a French postal engineer named in 1875 and named the code after Emile Baudot, an early pioneer in telegraph printing. -It was the first fixed-length character code developed where 5 bits were used to represent letters, numbers and symbols. -It was used primarily for low-speed teletype equipment. -It is recommended by the CCITT as the International Alphabet No. 2. 69 2. Baudot Code The latest version of the Baudot Code. 70  3. American Standard Code for Information Interchange (ASCII Code) -It is a 7-bit binary code, which can represent 128 numbers, letters, punctuation marks, and other symbols. -It is the standard character set for source coding the alphanumeric character set that humans understand but computers do not (computers only understand 1s and 0s). -It is often used with 8th bit (bit b7) which is not part of the ASCII code but is generally reserved for an error detection bit called the parity bit. -It is recommended by the ITU as the International Alphabet No.5. 71 3. American Standard Code for Information Interchange (ASCII Code) Direction of propagation of ASCII Code. 72  3. American Standard Code for Information Interchange (ASCII Code) ASCII-77: Odd Parity. 73 4. Extended Binary Coded Decimal Interchange Code (EBCDIC) -It was developed in 1962 by the International Business Machines Corporations (IBM), an eight-bit fixed length character set allowing a maximum of 256 characters to be represented. -Its primary use is in IBM and IBM- compatible computing systems and equipment. 74 4. Extended Binary Coded Decimal Interchange Code (EBCDIC) 75  4. Extended Binary Coded Decimal Interchange Code (EBCDIC) EBCDIC Code. 76 77 78 Binary Transmission Conventions Data Transmission 79 1. Parallel Transmission -In parallel transmission, bits are transmitted character at a time or n bits at a time, and each bit has its own wire or line. -Parallel data transmission is not practical for long-distance communication. Data transfers in long-distance communication systems are made serially; each bit of a word is transmitted one after another 80 1. Parallel Transmission Parallel Transmission 81 2. Serial Transmission -In serial transmission, bits are transferred over a single line one at a time over two communicating devices. -Serial transmission is generally used where the cost of the communication medium is high. 82 2. Serial Transmission Serial Transmission 83 ⦿ Asynchronous Transmission -The transitions of the signals do not necessarily occur at the same nominal rate. -Information is received and translated by agreed upon patterns such as sending 1 start bit (0) at the beginning and 1 or more stop bits (Is) at the end of each byte and there may be a gap between each byte. 84 ⦿ Asynchronous Transmission Asynchronous Transmission 85 ⦿ Synchronous Transmission -The digital transitions in the signals occur at exactly the same rate. -Data are transmitted as an unbroken string of 1s and 0s, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information. 86 ⦿ Synchronous Transmission 87 88 ⦿Isochronous Transmission -It provides synchronization for the entire stream of bits which guarantees that the data arrive at a fixed rate. 89 Thank you 90