Analog Signal Processing

Analog signals (continuously varying signals) may be processed by filters, shaping circuits, combiners, and amplifiers to change their shape and modify their content.

Signal Filtering
Filters may remove (band-reject) or allow (band-pass) portions of analog (possibly audio signals) that contain a range of high and low frequencies that are not necessary to transmit. In some cases, additional signals (at different frequencies) may be combined with audio signals prior to transmission. These signals may be multiple channels or may be signals that are used for control purposes. If control signals are added to an analog signal that is transmitted, they are usually removed from the audio signal in the receiver by filtering.

Figure below shows typical audio signal processing for a communications transmitter. In this example, the audio signal is processed through a filter to remove very high and very low frequency parts (audio band-pass filter). These unwanted frequency parts are possibly noise and other out of audio frequency signals that could distort the desired signal. The high frequencies can be seen as rapid changes in the audio signal. After an audio signal is processed by the audio band-pass filter, the sharp edges of the audio signal (high frequency components) are removed.


Audio Signal Filtering

Signal Amplification
Signal amplification is a process of sensing an input (usually low level) signal and converting the signal into a larger version of itself. An amplifier device provides this conversion process. Amplifiers increase both the desired signal and unwanted noise signals. Noise signals are any random disturbance or unwanted signal in a communication system that tends to obscure the clarity of a signal in relation to its intended use.

Figure below shows how a signal may be amplified. This diagram shows that the input signal is increased in value by an amplifier that can vary its gain (amount of amplification).


Signal Amplification

Signal Shaping
Audio signals may be processed by shaping circuits to add or remove emphasis of frequency (tone) or intensity (volume). When the signal processing involves differences of amplification of specific frequency components of an input signal, it is called pre-emphasis and de-emphasis. Signal processing that involves relative changes in the amount of amplification dependent on the level of input signal, it is called companding and expanding.

Some analog transmission systems use pre-emphasis circuits to amplify the high frequency components of the audio input signal which allow the modulation system to be more effective. Certain modulation systems do not respond well to low amplitudes of high frequency input signals. By boosting the high frequency component of the input audio signal, the modulator better translates the input signal into a modulated carrier signal. When pre-emphasis is used for transmission, a matched de-emphasis system is used in the receiver to convert the boosted high frequency component back into its original low signal level.

The intensity of an audio signal can vary dramatically because some people talk loudly and others talk softly. A system that reduces the amount of amplification (gain) of an audio signal for larger input signals (e.g., louder talker) is called companding. The use of companding allows the level of audio signal that enters the modulator to have a smaller overall range (higher minimum and lower maximum). High signals and low signals input to a modulator may have a different conversion level (ratio of modulation compared to input signal level). This can create distortion so companding allows the modulator to convert the information signal (audio signal) with less distortion. Of course, the process of companding must be reversed at the receiving end, called expanding, to recreate the original audio signal.

Figure below shows the basic signal companding and expanding process. This diagram shows that the amount of amplifier gain is reduced as the level of input signal is increased. This keeps the input level to the modulator to a relatively small dynamic range. At the receiving end of the system, an expanding system is used to provide additional amplification to the upper end of the output signal. This recreates the shape of the original input audio signal.


Analog Signal Companding and Expanding

Communication Systems: Simplex, Half Duplex, Full Duplex (FDX), Time Division Duplex (TDD)

Communication systems transfer information between 2 or more users. Communications systems may transfer information in one direction at a time on the same channel (simplex), in two directions on different time at different times (half duplex), or simultaneously on two different channels (full duplex). There are various approaches including TDD that allow the appearance of full duplex operation although the actual transmission system uses simplex or half duplex operation.

Simplex
Simplex communication allows the transmission of information between users, but only one direction at a time on the same channel or frequency. The common use of Simplex systems is traditional television or audio broadcast radio systems that transmit a signal from a single transmitter to many receivers.

Half Duplex
Half duplex communication provides the ability to transfer voice or data information in either direction between communications devices but not at the same time. The information may be transmitted on the same frequency or divided into different channels. When divided into different channels, one channel of frequency is used for transmitting and the other channel or frequency is used for receiving.

The use of different frequencies is common in half duplex radio transmission because the transmitter and receiver are commonly connected to the same antenna. If the same transmitter and receiver frequency were used, the high transmitter power would probably destroy the receiver circuitry.

Full Duplex (FDX)
Full Duplex communication is the process of transferring of voice or data signals in both directions at the same time. Full duplex operation normally assigns the transmitter and receiver to different communication channels. When the communications system uses two different frequencies for simultaneous communication, it is called frequency division duplex (FDD). One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction.

The definition of full duplex becomes confusing when it is applied to the end result of simultaneous voice and data communication. This is because it is possible to provide information at the input and output of a communication system while not actually sending the information simultaneously in a communication system. When a communication system provides for simultaneous two-way communication by time sharing, it is called time division duplex (TDD).

Time Division Duplex (TDD)
Time division duplex (TDD) communication uses a single channel or frequency to provide simultaneous two-way communications between devices by time-sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously. Figure below shows the basic operation of FDD and TDD system.

Telecom Made Simple : Modulation

Signal modulation is the process of modifying the characteristics of a carrier wave signal using an information signal (such as voice or data). The characteristics that can be changed include amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM). A pure electrical, radio, or optical carrier signal carries no information aside from either being in on or off state. When the carrier signal is modified from a normalized state, it is called a modulated signal. This modulated signal is the carrier of the information that is used to modify the carrier signal. When the carrier signal is received, its signal is compared to an unmodulated signal to reverse the process (called demodulation). This allows the extraction of the original information signal. A carrier wave signal can be carried by wire, fiber, or electromagnetic waves transmitted through the air (radio).

When a carrier signal is modulated, the frequency changes above and below its reference frequency. The difference between the upper and lower maximum frequency changes is called the bandwidth. The relationship between the amount of frequency bandwidth of an information signal (the baseband) and the channel bandwidth of the modulated carrier determines if the system is a narrowband or wideband system. Narrowband systems have a relatively small communications channel bandwidth, typically below 100 kbps. When the bandwidth of the broadband carrier is much higher than the bandwidth of the information source, it is called a wideband system.

The device that modifies the carrier signal with the information source (baseband signal) is called a modulator. An assembly or device that combines the function of modulating and demodulating signals is called a Modulator/DEModulator (MODEM).

Combined types of Modulation
Today’s sophisticated modulation systems can use all three variable parameters: frequency, amplitude, or timing (phase) at the same time to transfer analog or digital information. One of the more popular forms of combined modulation technologies is Quadrature Amplitude Modulation (QAM). QAM is a modulation technique that enables two information signals to modulate a single carrier frequency. The two different signals’ amplitude modulates two samples of the carrier that are of the same frequency, but differ in phase by 90 degrees. The resultant two signals can be added together and both signals recovered at a decoder where they are then demodulated 90 degrees apart.

Basic Communication Technologies : Signal Types

Telecommunication technology involves the transfer of information signals through wires, fiber, or through the air by the by means of electrical or optical signals. Communication signals are usually characterized by their intensity (voltage and current) and frequency (cycles per second). To allow information to be transferred using communication signals, an information source (audio, data or video) is either represented by the signal itself (called the baseband signal) or the information slightly changes the wave shape of the communication signal (called the broadband signal). The information is imposed on the carrying signal (called the carrier) by varying the signal level or time changes (frequency shift).

Signal Types
There are two basic types of signals: analog and digital. Many communication systems receive analog signals (e.g., audio signals), convert them to a digital format, transport the digital signals through a network, and reconvert the digital signals back to their analog form when they reach their destination.

Analog
An analog signal can vary continuously between a maximum and minimum value and it can assume an infinite number of values between the two extremes.

Figure below shows a sample analog signal created by sound. In this example, as the sound pressure from a person’s voice is detected by a microphone, it is converted to its equivalent electrical signal. Also, the analog audio signal continuously varies in amplitude (height, loudness, or energy) as time progresses.



Digital
Digital signals have a limited number of discrete states, usually two, in contrast to analog signals that vary continuously and have an infinite number of states. Digital signals transfer discrete signal levels at predetermined time intervals. Digital signals typically have two levels: on (logic 1) and off (logic 0). The information contained in a single time period is called a bit. The number of bits that are transferred in one second is called the data transfer rate or bits per second (bps). Because many bits are typically transferred in 1 second, the data rate is typically preceded by a multiplier k (thousand) or M (million). For example, if the data transfer rate is 3 million bits per second, 3 Mbps would indicate this. Bits are typically combined into groups of 8 bits to form a byte. When the reference is made to bytes instead of bits, the b is capitalized. For example, 10 thousand bytes is represented by kB. Figure below shows a sample digital signal. In this example, the bits 01011010 are transferred in 1 second. This results in a bit rate of 8 bps.



The earliest form of digital radio communication was Morse Code. To send Morse Code, the radio transmitter was simply turned on and off to form dots and dashes. The receiver would sense (detect) the radio carrier to reproduce the dots and dashes. A code book of dots and dashes was used to decode the message into symbols or letters. The on and off pulses or bits that comprise a modern digital signal is sent in a similar way.

The trend in communication systems, just as in other types of electronics products such as compact discs, is to change from analog systems to digital systems. Digital systems have a number of important advantages including the fact that digital signals are more immune to noise. Unlike analog systems, even when noise has been introduced, any resulting errors in the digital bit stream can be detected and corrected. Also, digital signals can be easily manipulated or processed in useful ways using modern computer techniques.

Teleservices

Teleservices are telecommunication services that provide added processing or functionality to the transfer of information between users. Teleservices are categorized by their high level (application) characteristics, the low level attributes of the bearer service(s) that are used as part of the teleservice, and other general attributes. High level attributes include: application type (for example voice or messaging) and operation of the application. The low-level description includes a list of the bearer services required to allow the teleservice to operate with their data transfer rate(s) and types. Other general attributes might specify a minimum quality level for the teleservice or other special condition. The categories of teleservices available include voice (speech), short messaging, facsimile, and group voice.

Figure below shows a typical teleservice. In this diagram, a telephone user wishes to send a fax to a recipient who is traveling. The designated recipient has setup a fax forwarding service where the delivery of incoming faxes can be instructed. The sender is given the recipient’s fax number. When the sender dials the number, the call is routed through the telephone network to the fax forwarding service provider (step 1). When the incoming call is detected, the fax forwarding service receives the fax into a fax mailbox (step 2). Later that day, the recipient of the fax forwarding service calls in and enters a fax forwarding number (step 3). The fax forwarding service then checks the fax mailbox and automatically sends all the waiting faxes to the new number (possibly a hotel fax number) that has been updated by the recipient (step 4). Because this service involves both the transport and processing of information, it is categorized as a teleservice.


Teleservice

Custom Calling Features
Custom local area signaling services (CLASS) are telephone service features available in a local access and transport area (LATA) that are primarily based on information that can be processed inside the telephone network. CLASS features include call forwarding, caller identification, and three-way calling.

Voice Mail (VM)
Voice mail (VM) is a service that provides a telephone customer with an electronic storage mailbox that can answer and store incoming voice messages. Voice mail systems use interactive voice response (IVR) technology to prompt callers and customers through the options available from voice mailbox systems. Voice mail systems offer advanced features not available from standard answering machines including message forwarding to other mailboxes, time of day recording and routing, special announcements, and other features.

Central Exchange (Centrex)
Centrex is a service offered by a local telephone service provider that allows the customer to have features that are typically associated with a private branch exchange (PBX). These features include 3 or 4 digit dialing, intercom features, distinctive line ringing for inside and outside lines, voice mail waiting indication, and others. Centrex services are provided by the central-office switching facilities that are located in the local telephone network.

Call Center
A call center is a place where calls are answered and originated, typically between a company and a customer. Call centers assist customers with requests for new service activation and help with product features and services. A call center usually has many stations for call center agents that communicate with customers. When call agents assist customers, they are typically called customer service representatives (CSRs).

Call centers use telephone systems that usually include sophisticated automatic call distribution (ACD) systems and computer telephone integration (CTI) systems. ACD systems route the incoming calls to the correct (qualified) customer service representative (CSR). CTI systems link the telephone calls to the accounting databases to allow the CSR to see the account history (usually producing a “screen-pop” of information).

Operator Services
An operator service is a telecommunication service that uses an operator to assist in the handling of a call. These special handling services include collect calling (billing to a called number), third party charging (billing to another phone or calling card), identification of a person who has called (call trace services), call information services (assistance with directory number location), rate information services (call charge rates), or any other service that requires an operator for special call processing services.

Information Services
Information services involve the processing of information that is transferred through a communications system. Information services add value to information by generating, acquiring, storing, transforming, processing, retrieving, utilizing, or making available information via telecommunications. Examples of information services include fax store and forward, electronic publishing, text to voice conversion, and news services.

Bearer Services

Bearer services are telecommunication services that are used to transfer user data and control signals between two pieces of equipment. Bearer services can range from the transfer of low speed messages (300 bps) to very high-speed data signals (10+ Gigabits).

Bearer services are typically categorized by their information transfer characteristics, methods of accessing the service, interworking requirements (to other networks), and other general attributes. Information characteristics include data transfer rate, direction(s) of data flow, type of data transfer (circuit or packet) and other physical characteristics. The access methods determine what parts of the system control could be affected by the bearer service. Some bearer services must cross different types of networks (e.g., wireless and wired) and the data and control information may need to be adjusted depending on the type of network. Other general attributes might specify a minimum quality level for the service or special conditional procedures such as automatic re-establishment of a bearer service after the service has been disconnected due to interference. Some categories of bearer services available via the telephone system include synchronous and asynchronous data, packet data, and alternate speech and data.

Figure below shows a typical bearer service. In this diagram, a customer decides to send a data file to a computer that is connected to a public telephone network (at the office). In this example, the bearer service is circuit-switched data. The customer uses a modem to adapt their portable computer to the telephone network. The portable dials the office computer telephone number via the modem. The telephone system routes this call to a modem that connects the office computer to the telephone network. When the office computer modem accepts the call, the customer’s modem begins to send data directly on the telephone line at channel at 28 kbps. All the telephone system provides is a standard voice communications path between the portable computer modem and the office computer modem. The key bearer service attribute is the transfer of real-time information in the audio frequency range of 300 Hz to 3300 Hz. This is the bearer service.


bearer service

Constant Bit-rate (CBR)
Constant bit-rate (CBR) service is a class of telecommunications service that provides an end-user with constant bit data transfer rate. CBR service is often used when real-time data transfer rate is required such as for voice service.

Variable Bit-rate (VBR)
Variable bit-rate (VBR) is a category of telecommunications service that provides the users with a data transmission rate of service that can vary. Applications that use VBR services usually require some real-time interactivity with bursts of data transmission. An example of a VBR application is videoconferencing.

Available Bit-rate (ABR)
Available bit-rate (ABR) is a communications service category that provides the user with a data transmission rate that varies dependent on the availability of the network resources. ABR service may provide the user with feedback as to the changed data transfer rate and may have established minimum and maximum levels of data transmission rates.

Unspecified Bit-rate (UBR)
Unspecified bit-rate (UBR) is a category of telecommunications service that provide an unspecified data transmission rate of service to end-user applications. Applications that use UBR services do not require real-time interactivity nor do they require a minimum data transfer rate. UBR applications may not require the pre-establishment of connections. An example of a UBR application is Internet web browsing.

Committed Information Rate (CIR)
Committed information rate (CIR) is a guaranteed minimum data transmission rate of service that will be available to the user through a network. Applications that use CIR services include voice and real-time data applications. CIR can be measured in bits per second, burst size, and burst interval.

Some service providers allow users to transmit data above the CIR level. However, when data is transmitted above the CIR level, some of the data may be selectively discarded if the network becomes congested.

Distribution Services

Distribution is the transfer of information throughout a geographic area or through a network. Distribution services include broadcast, multicast, and point-to-point communication.

Broadcast
Broadcast transmission is the distribution of an information signal to a specified geographic area or network system. Broadcasting allows the same information to be received by all customers in that geographic area that can successfully receive (demodulate) and decode the information.

Figure below shows broadcast communication service. This diagram shows two broadcast examples: radio broadcast and network broadcast. Part (a) shows a radio broadcast tower that is sending an audio broadcast to all radios that are within its radio signal coverage area. Part (b) shows a network broadcast system that sends a data message that is coded to indicate the message is a broadcast message. This message contains an address that indicates it is a broadcast message. When routers or other data distribution devices receive this message, each distribution device forwards the data broadcast message to the other network parts for which it is connected to. All communication devices that are connected to the network can receive the broadcast message.


broadcast communication

Multicast
Multicast transmission is a communications service where a single message or information transmission contains an address (code) that is designated for several devices (nodes) in a network. Devices must contain the matching code to successfully forward or decode the message.

Figure below shows examples of how multicast services can be implemented. The first method uses encoded video broadcast transmission and encoded messaging to allow only a select group to view the received information. While all the television broadcast receivers all receive the same radio signal, only the receivers with the correct code will be able to descramble the television signal. The second method uses multicast routing in the Internet to store and forward data to an authorized group of recipients that are connected to its router. When a router in the Internet that is capable of multicast service receives a multicast message, it will store the message for forwarding. It then uses the multicast address to lookup a list of authorized recipients in its routing table. The stored message is then forwarded to the authorized receiving device or next router that is part of the multicast service.



multicast communication

Point-to-Point
Point-to-point communication is the transmission of signals from one specific point to another. Point-to-point communication uses addressing to deliver information to a specific receiver of the information. It is possible to implement point-to-point communication through a broadcast network by using device addressing or through a network using network routing.

Figure below shows examples of how point-to-point services can be implemented. The first method is a paging system that uses device addressing to uniquely identify a specific receiver of the information. While all the pager devices receive the same radio signal, only the receiver that has the correct paging code will be able to descramble the paging message. The second method uses network routing in the Internet to store and forward data to a specific recipient in the network. When a router in the Internet receives a point-to-point message, it will use the address to lookup the best forwarding path to transfer the information towards its destination. Using point-to-point communications by network routing, only the designated recipient will receive the data.


point-to-point communication