ENCODING AND DECODING ANALOG AND DIGITAL SIGNALS
For communication to take place both transmitting and receiving must occur successfully. Transmitting involves the sender encoding the message and transmitting it over the medium. Receiving involves the receiver understanding the organisation of the encoded message – based on the protocols agreed upon during handshaking with the transmitter. The receiver can then decode the message based on the rules of the agreed protocols. In essence both encoding and decoding are organising information processes. Encoding organises the data into a form suitable for transmission along the communication medium. Decoding changes the organisation of the received data into a form suitable for subsequent information processes. Prior to transmission data is encoded into a signal according to the rules of the transmission protocols being used and suited to the transmission media along which the message will travel. When messages reach their destination the receiver reverses this process by decoding the signal and transforming it back into data. Data that originates or is stored on a computer is always in binary digital form. Digital data is all data that is represented (or could be represented) using whole distinct numbers – in the case of computers a binary representation is used. Continuous data that usually originates from the real world is analogy. Both analogy and digital data can be encoded and transmitted on electromagnetic waves. Note that in reality all waves are continuous hence they are analogy. For our purpose, it is how we choose to interpret the data carried on these analogy waves that we shall use to distinguish between digital signals and analogy signals. A digital signal is being used when digital data is encoded onto an analogy wave. An analogy signal is being used when analogy data is encoded onto an analogy wave. To encode analogy data into a digital signal requires that the data first be converted into digital using an analogy to digital converter (ADC). Similarly to encode digital data into an analogy signal the data must be converted to analogy data using a digital to analogy converter (DAC).
Analogy Data to Analogy Signal
When the data is analogy the waveform varies continuously in parallel with the changes in the original analogy data. For example microphones collect analogy sound waves and encode them as an infinitely variable electromagnetic wave
The voltage transmitted from the microphone varies continuously in parallel with the soundwaves entering the microphone. An analogy signal is produced as the entire analogy wave represents the original analogy data. All points on the analogy wave have significance – this is not true of digital signals.
Analog signals are transmitted alongtraditional PSTN telephone lines. For voice(audio) microphones are used as thecollection device and speakers as the displaydevices. The microphone encodes the analogdata and the speaker performs the decoding process. The electromagnet within thespeaker moves in and out in response to thereceived analog signal. This causes thespeaker’s diaphragm to move in and outwhich in turn creates compression wavesthrough the air that we finally hear as sound.Traditional analog radio and analog TV are further examples of analog datatransmitted as an analog signal – including broadcasts through air and also analogaudio and video cassettes (VHS). In both cases an analog signal is transmitted thatvaries continuously. This analog signal is decoded and displayed by the receivingradio/stereo or television set.
Digital Data to Digital Signal
Digital signals are produced when digital data is encoded onto analog waves. Todecode the wave and retrieve the encoded digital data requires the receiver to read thewave at the same precise time intervals. The receiver determines the characteristics of the wave at each time interval based on the details of the coding scheme. As aconsequence each particular waveform can be decoded back into its original bit pattern.There are two commonly used techniques for encoding digital data. The first alters thevoltage present in a circuit to represent different bit patterns. This technique is usedover short distances, including communication within a computer and between nodeson a baseband LAN. Note that altering voltage changes the power or amplitude of thewave. The second alters characteristics of a constant frequency electromagnetic wavecalled a carrier wave. The carrier wave is modified (modulated) to represent different bit patterns by altering a combination of amplitude, phase and/or frequency. Themodulation (and subsequent demodulation) process is used for most long distance broadband communication. Both the above encoding techniques create differentwaveforms (often called symbols) that represent different numbers (bit patterns). Thewaveforms are changed at regularly spaced time intervals to represent each new pattern of bits.The time between each interval is known as the “bit time”. For example on a100baseT Ethernet network the bit time is 10 nanoseconds. Therefore a transmittingnetwork interface card (NIC) on a 100baseT network ejects one bit every 10nanoseconds. Similarly all receiving nodes must examine the wave every 10nanoseconds. On 100baseT protocol networks a single bit is represented after each bit time using Manchester encoding. Thereceiver detects the transitions to not onlydecode the signal but also to remain insynchronisation with the sender.Each transition from high to low or low to high occurs over time. Therefore the actualwave has rounded edges.
Higher speed and/or longer distance protocols represent multiple bits within each distinct waveform. Consider DSL and cable modems which modulate the carrier wave’s amplitude and phase within a predetermined range of frequencies. QAM (Quadrature Amplitude Modulation) is currently the dominant protocol. A modem (DSL or cable) that uses 256QAM represents 8 bits after each bit time elapses. As there are 256 different combinations of 8 bits then 256QAM uses 256different waveforms known as symbols. Each distinct symbol having a unique combination of phase and amplitude. Current cable modems using256QAM typically transmit (and receive) more than 5Msym/s (5 million symbols per second). As each symbol represents 8 bits then speeds around 40Mbps are achievable.
In reality each different waveform is repeated continually during each bit time. Encoding schemes, like QAM, that modulate carrier waves are used within all long-distance and/or high-speed low-level protocols (OSI layer 1 and 2). This includes long-distance Gigabit and faster Ethernet standards, SONET, FDDI and ATM. These protocols operate on various types of transmission media including wire, fibre optic and wireless mediums. For digital signals the speed of transmission can be increased in two fundamental ways – by increasing the number of bits represented by each symbol or by decreasing the bit time (equivalent to increasing the symbol rate). The quality of the transmission media and limitations of the transmitting and receiving hardware determine the extent to which distinct symbols can be determined. As the number of symbols increases the difference between each symbol is more difficult to determine. Similarly as bit times decrease the accuracy of synchronisation between sender and receiver must increase.
Digital Data to Analogy Signal
Converting digital data to an analogy signal requires the data to first be converted to analogy prior to its transmission as an analogy signal. A digital to analogy converter (DAC) performs this process. Digital to analogy conversion is used between video cards and analogy monitors and is also used to connect dial-up modems to traditional analogy PSTN telephone lines. It is also used when playing audio CDs and DVDs. As digital data contains distinct rather than continuous data then during digital to analogy conversion it is necessary to estimate the intermediate waveforms between each known digital data point. In
Fig 3.66
The dotted lines represent each of the digital samples and the solid line represents the analogy signal. Note that the analogy signal is a smooth curve produced by estimating the shape of the curve between pairs of digital samples. Audio CDs use PCM (Pulse Code Modulation) to encode the original analogy music as a sequence of 16-bit digital sound samples – approximately 44100 per second. When a CD is playing the waveform between each digital sample is estimated based on the values of the adjoining digital samples (refer
Fig 3.66
). For audio CDs the digital samples are so close together that such estimations are imperceptible to listeners. Today dial-up modems are rarely used to connect to the Internet, however they are routinely used to transmit fax data over traditional PSTN lines. In the past the infrastructure at local telephone exchanges was built to deal exclusively with analog voice signals. A total bandwidth of 3200Hz, ranging from 200Hz to 3400Hz, was used as these frequencies encompass the normal frequencies present in natural speech. Frequencies above 3400Hz were filtered out of the signal completely. As a consequence the signal transmitted (and received) by dial-up modems had to simulate and operate within the same frequency range as analog voice signals. The devices connecting telephone exchanges did not differentiate between voice and other data transmissions. In terms of encoding and decoding processes occurring within the PSTN both voice and data were both transmitted and received identically as analog signals. Consider the following
This DAC makes no formal attempt to smooth its analog output; however some smoothing occurs as the output signal moves from one level to another during switching. In this case each sample contains just 4 bits. Each bit activates a switch that allows current to flow (or not flow) through a resistor. Each resistor allows a different proportion of the voltage through. In the diagram the digital sample 1010 is being processed. If the input voltage is 5 volts then the first 1 in the sample allows five volts through and the next 1 allows just one quarter of 5 volts through – the finally output being 6.25 volts.
Analog Data to Digital Signal
In this case we have continuous analog data that is to be represented digitally during its transmission. Today this routinely occurs when transmitting audio and video analog data within all types of communication networks including the PSTN, VoIP, cable TV network and digital TV network. For analog data to be transmitted digitally first requires the data to be converted to digital using an analog to digital converter (ADC).Telephone calls from normal home phones are transmitted as analog signals to the local exchange. The analog data is converted to digital data at the exchange where it travels using a digital signal to the receiver’s local exchange. At the receiver’s local exchange the digital signal is received, the data is converted back to analog and then transmitted as an analog signal to the receiver’s residence. Mobile phones convert the analog sound waves to digital within each phone; therefore digital signals are used exclusively to transmit data between mobile phones. Analog to digital converters (ADCs) repeatedly sample the analog data and convert each sample to a binary number. ADCs are present within many collection devices including sound cards, video capture cards, TV cards, optical mice, scanners and digital still and video cameras. The analog to digital conversion process produces sequences of binary numbers that represent the analog data at particular regular points. For images the sampling points are known as pixels, whilst for audio the sampling points are time based. Video includes both pixel and time based samples.
For communication to take place both transmitting and receiving must occur successfully. Transmitting involves the sender encoding the message and transmitting it over the medium. Receiving involves the receiver understanding the organisation of the encoded message – based on the protocols agreed upon during handshaking with the transmitter. The receiver can then decode the message based on the rules of the agreed protocols. In essence both encoding and decoding are organising information processes. Encoding organises the data into a form suitable for transmission along the communication medium. Decoding changes the organisation of the received data into a form suitable for subsequent information processes. Prior to transmission data is encoded into a signal according to the rules of the transmission protocols being used and suited to the transmission media along which the message will travel. When messages reach their destination the receiver reverses this process by decoding the signal and transforming it back into data. Data that originates or is stored on a computer is always in binary digital form. Digital data is all data that is represented (or could be represented) using whole distinct numbers – in the case of computers a binary representation is used. Continuous data that usually originates from the real world is analogy. Both analogy and digital data can be encoded and transmitted on electromagnetic waves. Note that in reality all waves are continuous hence they are analogy. For our purpose, it is how we choose to interpret the data carried on these analogy waves that we shall use to distinguish between digital signals and analogy signals. A digital signal is being used when digital data is encoded onto an analogy wave. An analogy signal is being used when analogy data is encoded onto an analogy wave. To encode analogy data into a digital signal requires that the data first be converted into digital using an analogy to digital converter (ADC). Similarly to encode digital data into an analogy signal the data must be converted to analogy data using a digital to analogy converter (DAC).
Analogy Data to Analogy Signal
When the data is analogy the waveform varies continuously in parallel with the changes in the original analogy data. For example microphones collect analogy sound waves and encode them as an infinitely variable electromagnetic wave
The voltage transmitted from the microphone varies continuously in parallel with the soundwaves entering the microphone. An analogy signal is produced as the entire analogy wave represents the original analogy data. All points on the analogy wave have significance – this is not true of digital signals.
Analog signals are transmitted alongtraditional PSTN telephone lines. For voice(audio) microphones are used as thecollection device and speakers as the displaydevices. The microphone encodes the analogdata and the speaker performs the decoding process. The electromagnet within thespeaker moves in and out in response to thereceived analog signal. This causes thespeaker’s diaphragm to move in and outwhich in turn creates compression wavesthrough the air that we finally hear as sound.Traditional analog radio and analog TV are further examples of analog datatransmitted as an analog signal – including broadcasts through air and also analogaudio and video cassettes (VHS). In both cases an analog signal is transmitted thatvaries continuously. This analog signal is decoded and displayed by the receivingradio/stereo or television set.
Digital Data to Digital Signal
Digital signals are produced when digital data is encoded onto analog waves. Todecode the wave and retrieve the encoded digital data requires the receiver to read thewave at the same precise time intervals. The receiver determines the characteristics of the wave at each time interval based on the details of the coding scheme. As aconsequence each particular waveform can be decoded back into its original bit pattern.There are two commonly used techniques for encoding digital data. The first alters thevoltage present in a circuit to represent different bit patterns. This technique is usedover short distances, including communication within a computer and between nodeson a baseband LAN. Note that altering voltage changes the power or amplitude of thewave. The second alters characteristics of a constant frequency electromagnetic wavecalled a carrier wave. The carrier wave is modified (modulated) to represent different bit patterns by altering a combination of amplitude, phase and/or frequency. Themodulation (and subsequent demodulation) process is used for most long distance broadband communication. Both the above encoding techniques create differentwaveforms (often called symbols) that represent different numbers (bit patterns). Thewaveforms are changed at regularly spaced time intervals to represent each new pattern of bits.The time between each interval is known as the “bit time”. For example on a100baseT Ethernet network the bit time is 10 nanoseconds. Therefore a transmittingnetwork interface card (NIC) on a 100baseT network ejects one bit every 10nanoseconds. Similarly all receiving nodes must examine the wave every 10nanoseconds. On 100baseT protocol networks a single bit is represented after each bit time using Manchester encoding. Thereceiver detects the transitions to not onlydecode the signal but also to remain insynchronisation with the sender.Each transition from high to low or low to high occurs over time. Therefore the actualwave has rounded edges.
Higher speed and/or longer distance protocols represent multiple bits within each distinct waveform. Consider DSL and cable modems which modulate the carrier wave’s amplitude and phase within a predetermined range of frequencies. QAM (Quadrature Amplitude Modulation) is currently the dominant protocol. A modem (DSL or cable) that uses 256QAM represents 8 bits after each bit time elapses. As there are 256 different combinations of 8 bits then 256QAM uses 256different waveforms known as symbols. Each distinct symbol having a unique combination of phase and amplitude. Current cable modems using256QAM typically transmit (and receive) more than 5Msym/s (5 million symbols per second). As each symbol represents 8 bits then speeds around 40Mbps are achievable.
In reality each different waveform is repeated continually during each bit time. Encoding schemes, like QAM, that modulate carrier waves are used within all long-distance and/or high-speed low-level protocols (OSI layer 1 and 2). This includes long-distance Gigabit and faster Ethernet standards, SONET, FDDI and ATM. These protocols operate on various types of transmission media including wire, fibre optic and wireless mediums. For digital signals the speed of transmission can be increased in two fundamental ways – by increasing the number of bits represented by each symbol or by decreasing the bit time (equivalent to increasing the symbol rate). The quality of the transmission media and limitations of the transmitting and receiving hardware determine the extent to which distinct symbols can be determined. As the number of symbols increases the difference between each symbol is more difficult to determine. Similarly as bit times decrease the accuracy of synchronisation between sender and receiver must increase.
Digital Data to Analogy Signal
Converting digital data to an analogy signal requires the data to first be converted to analogy prior to its transmission as an analogy signal. A digital to analogy converter (DAC) performs this process. Digital to analogy conversion is used between video cards and analogy monitors and is also used to connect dial-up modems to traditional analogy PSTN telephone lines. It is also used when playing audio CDs and DVDs. As digital data contains distinct rather than continuous data then during digital to analogy conversion it is necessary to estimate the intermediate waveforms between each known digital data point. In
Fig 3.66
The dotted lines represent each of the digital samples and the solid line represents the analogy signal. Note that the analogy signal is a smooth curve produced by estimating the shape of the curve between pairs of digital samples. Audio CDs use PCM (Pulse Code Modulation) to encode the original analogy music as a sequence of 16-bit digital sound samples – approximately 44100 per second. When a CD is playing the waveform between each digital sample is estimated based on the values of the adjoining digital samples (refer
Fig 3.66
). For audio CDs the digital samples are so close together that such estimations are imperceptible to listeners. Today dial-up modems are rarely used to connect to the Internet, however they are routinely used to transmit fax data over traditional PSTN lines. In the past the infrastructure at local telephone exchanges was built to deal exclusively with analog voice signals. A total bandwidth of 3200Hz, ranging from 200Hz to 3400Hz, was used as these frequencies encompass the normal frequencies present in natural speech. Frequencies above 3400Hz were filtered out of the signal completely. As a consequence the signal transmitted (and received) by dial-up modems had to simulate and operate within the same frequency range as analog voice signals. The devices connecting telephone exchanges did not differentiate between voice and other data transmissions. In terms of encoding and decoding processes occurring within the PSTN both voice and data were both transmitted and received identically as analog signals. Consider the following
This DAC makes no formal attempt to smooth its analog output; however some smoothing occurs as the output signal moves from one level to another during switching. In this case each sample contains just 4 bits. Each bit activates a switch that allows current to flow (or not flow) through a resistor. Each resistor allows a different proportion of the voltage through. In the diagram the digital sample 1010 is being processed. If the input voltage is 5 volts then the first 1 in the sample allows five volts through and the next 1 allows just one quarter of 5 volts through – the finally output being 6.25 volts.
Analog Data to Digital Signal
In this case we have continuous analog data that is to be represented digitally during its transmission. Today this routinely occurs when transmitting audio and video analog data within all types of communication networks including the PSTN, VoIP, cable TV network and digital TV network. For analog data to be transmitted digitally first requires the data to be converted to digital using an analog to digital converter (ADC).Telephone calls from normal home phones are transmitted as analog signals to the local exchange. The analog data is converted to digital data at the exchange where it travels using a digital signal to the receiver’s local exchange. At the receiver’s local exchange the digital signal is received, the data is converted back to analog and then transmitted as an analog signal to the receiver’s residence. Mobile phones convert the analog sound waves to digital within each phone; therefore digital signals are used exclusively to transmit data between mobile phones. Analog to digital converters (ADCs) repeatedly sample the analog data and convert each sample to a binary number. ADCs are present within many collection devices including sound cards, video capture cards, TV cards, optical mice, scanners and digital still and video cameras. The analog to digital conversion process produces sequences of binary numbers that represent the analog data at particular regular points. For images the sampling points are known as pixels, whilst for audio the sampling points are time based. Video includes both pixel and time based samples.