digital signal: Information from Answers.com

The term digital signal is used to refer to more than one concept. It can refer to discrete-time signals that are digitized, or to the waveform signals in a digital system.

Discrete-time signals

Digital signals are digital representations of discrete-time signals, which are often derived from analog signals.

An analog signal is a datum that changes over time—say, the temperature at a given location; the depth of a certain point in a pond; or the amplitude of the voltage at some node in a circuit—that can be represented as a mathematical function, with time as the free variable (abscissa) and the signal itself as the dependent variable (ordinate). A discrete-time signal is a sampled version of an analog signal: the value of the datum is noted at fixed intervals (for example, every microsecond) rather than continuously.

If individual time values of the discrete-time signal, instead of being measured precisely (which would require an infinite number of digits), are approximated to a certain precision—which, therefore, only requires a specific number of digits—then the resultant data stream is termed a digital signal. The process of approximating the precise value within a fixed number of digits, or bits, is called quantization.

In conceptual summary, a digital signal is a quantized discrete-time signal; a discrete-time signal is a sampled analog signal.

In the Digital Revolution, the usage of digital signals has increased significantly. Many modern media devices, especially the ones that connect with computers use digital signals to represent signals that were traditionally represented as continuous-time signals; cell phones, music and video players, personal video recorders, and digital cameras are examples.

In most applications, digital signals are represented as binary numbers, so their precision of quantization is measured in bits. Suppose, for example, that we wish to measure a signal to two significant decimal digits. Since seven bits, or binary digits, can record 128 discrete values (viz., from 0 to 127), those seven bits are more than sufficient to express a range of one hundred values.

Waveforms in digital systems

In computer architecture and other digital systems, a waveform that switches between two voltage levels representing the two states of a Boolean value (0 and 1) is referred to as a digital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.

The clock signal is a special digital signal that is used to synchronize digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.

Logic voltage levels

The two states of a wire are usually represented by some measurement of electric current: Voltage is the most common, but current is used in some logic families. A threshold is designed for each logic family. When below that threshold, the wire is "low," when above "high." Digital circuits establish a "no man's area" or "exclusion zone" that is wider than the tolerances of the components. The circuits avoid that area, in order to avoid indeterminate results.

It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and would occur only in a fault condition or during a logic level transition, as most circuits are not purely resistive, and therefore cannot instantly change voltage levels. However, few logic circuits can detect such a fault, and most will just choose to interpret the signal randomly as either a 0 or a 1.

The levels represent the binary integers or logic levels of 0 and 1. In active-high logic, "low" represents binary 0 and "high" represents binary 1. Active-low logic uses the reverse representation.

Implemented project sample

This is a digital A/D project sample that run in Addis Ababa Ethiopian Radio as follow;

After successul completion of the automation and digitization project of the library of institute of Ethiopian Studies in Addis Ababa University, Saramagd Int'l Co. participated in another tender for the automation and digitization of the Ethiopian Radio and due to the experience of the previous project and other activities in Ethiopia, managed to win the tender and started the implementation stages of the project. Saramagd Int'l Co. used some modern project management and value engineering techniques and methods to define the first phase of the project using 5 technical and managerial teams. The company hired and used some highly technical and young professionals with great experiences that have M.Sc. degrees in their fields of study. Saramagd Int'l Co. used the EPC model in this big and national project, It means the company used some modern and major project management methods that support the project from beginning to the end and at the final step, the company managed to implement the project and brought it to the exploitation level.

• Practical Scheme The project was organized in four practical stages:

Networking and Hardware Software A/D Conversion Transmission to Antenna (OnAir)

Networking and Hardware Prepration: Saramagd Int'l Co. anticipated 5 work stations (DAW) along with a server and an MSA storage system with the capacity of 6 terrabytes. A network with 1 Gbit/s speed was then implemented. In each of the studios that were selected for this phase, four nodes were aunticipated. Also a separate and proprietary power network was designed and implemented solely for being used by this project. In this network, a 3com switch with 1 Gbit/s speed was used and an appropriate earth well was designed and created for earthing the equipment used in the project. Designing this earth well, by itself, was a very good step, and the radio officials were happy to have a good and suitable earth well and design of this well by Iranian professionals has stimulated their admiration.

Software Prepration: In this project, a second version of the DGL software was designed and implemented for being used for broadcasting in radio staions. The software, like its predecessor, supports the Amharic language (official language of Ethiopia) and all the information fields are based on MARC 21 standards, and is compatible with both speech and music files. The software is able to ask the user to select some files and make a playlist and send the playlist directly to the antenna via itself. Detailed information about this software can be found in DGL 2 section of the software products section of this site.

Analogue to Digital (A/D) Conversion: This phase is consisted of 17 steps and is the longest part of the project. In this phase all the tapes which were selected by radio officials for the first phase, have been captured and two back ups from the digital files were created. All the information related to each file was also entered and saved in the DGL 2 software. All the files have been captured and saved in the wave format, and then for reducing the size of each file, all of them have been transformed to another format (which was a compressed one). All of the files then were seved on some high-capacity electromagnetic tapes which are designed for back up systems. Saramagd int'l co. Designed a special managment method for this project which is unique and helps keeping the old system along with the new one and creates a connection between the two. Also this system creates a way for data entry that prevents any kind of error for dara entry of over 200,000 files in this project.

Transmission to Antenna: The heart of the project is this part, which is the last step in implementation of the project. After capturing and data entry of all the tapes, and creating search and retrieval methods for the software, there should be a way to transmit the selected files, directly to the antenna using the some software. In fact, the real meaning of automation can be seen by this feature. For doing so, some interfaces have been created for attaching the comuter to the OnAir table, and also some software features have been added to DGL2 software so that the output of the sound card of the computer was connected to the input of the OnAir system, and also the control signal of the OnAir system was connected to the computer. In this way, the OnAir system was able to tell the computer to play the selected playlists.

• Technical Teams Software Team: This team designed and implemented the new version of DGL software (DGL2). This team worked in cooperation wih the sound engineering team and counsellors team to distinguish the required fields for the project and then design the software based on that. Also creating a playlist, getting order from the OnAir system, and sending files to antenna were some other features that are implemented in the software.

Hardware Team: This team was in charge of setting up the workstations, active and passive network equipment, MSA or the high capacity storage system, and the backup systems.

Electrical Engineering Team: This team was reponsible for creating a separate power network with an appropriate earth well and load distribution in the 3 phase power network.

Sound Engineering Team: This team was in charge of selecting appropriate hardware for the sound section of the project, like sound cards and required cables, introducing appropriate international standards for capturing, backup systems, and format conversion, and also training local Ethiopian staff, and implementation of the project.

Counsellors Team: This team was consisted of different specialized people. They were supporting and helping other technical teams - as mentioned above- during the whole process.

Implementation Team: This team was in charge of implementing the project. All the guidelines and solutions were handed to this team and they were supposed to put all that into action. The team was consisted of one Iranian manager and 5 technical and nontechnical Ethipian staff.

Managerial teams in Iran and Ethiopia has controlled all the other teams during the whole process and also officials of the Iran Embassy in Ethiopia, especially Mr. Fotoohi Ghiam, the honoured ambassador of I.R. Iran in Ethiopia, had a very close cooperation with Saramagd Int'l Co. and were very effective in successful completion of this project.

Examples of binary logic levels:

Technology	L voltage	H voltage	Notes
CMOS	0V to VCC/2	VCC/2 to VCC	VCC = supply voltage
TTL	0V to 0.8V	2V to VCC	VCC is 4.75V to 5.25V
ECL	-1.175V to -VEE	.75V to 0V	VEE is about -5.2V VCC=Ground

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See also

• www.saramagditco.com
• Digital signal processing
  • Digital image processing
• Nyquist–Shannon sampling theorem
• Whittaker–Shannon interpolation formula