(1A) Types of signal, modulation, transmission type and terminal equipment5 Type of signal (Digital verses Analogue)/Modulation. Understand the difference between encoding data as digital and analogue signals using simple diagrams or animations. Demonstrate the ability to encode sample data.

Defantions signal waves representation examples technology data transmissions response to noise flexibility uses bandwidth memory power cost impedance errors.   Digital/digital unipolar polar bipolar analogue/digital pulse amplitude modulation (PAM) pulse code modulation (PCM) digital/analogue bit rate and baud rate carrier signal amplitude shift keying (ASK) frequency shift keying (FSK) phase shift keying (PSK) analogue/analogue amplitude modulation (AM) frequency modulation (FM) phase modulation (PM).

Analog Vs Digital We live in an analogue world. There are an infinite amount of colours to paint an object (even if the difference is indiscernible to our eye), there are an infinite number of tones we can hear, and there are an infinite number of smells we can smell. The common theme among all of these analogue signals is their infinite possibilities. Digital signals and objects deal in the realm of the discrete or finite, meaning there is a limited set of values they can be. That could mean just two total possible values, 255, 4,294,967,296, or anything as long as it’s not ∞ (infinity).

Analog Signal Graphs Because a signal varies over time, it’s helpful to plot it on a graph where time is plotted on the horizontal, x-axis, and voltage on the vertical, y-axis. Looking at a graph of a signal is usually the easiest way to identify if it’s analogue or digital; a time-versus-voltage graph of an analogue signal should be smooth and continuous.

Digital Signals Digital signals must have a finite set of possible values. The number of values in the set can be anywhere between two and a-very-large- number-that’s-not-infinity. Most commonly digital signals will be one of two values – like either 0V or 5V. Timing graphs of these signals look like square waves.

Digital Waves Or a digital signal might be a discrete representation of an analogue waveform. Viewed from afar, the wave function below may seem smooth and analogue, but when you look closely there are tiny discrete steps as the signal tries to approximate values:

Noise When sending any signal down a wire or via radio waves (think WiFi) Noise can cause the signal to become corrupt. There are many causes of noise in a computer network. Some of these are listed… Internal noise Thermal noise (caused by heat created by the electrical signal moving through a wire) Imperfections (Any electrical circuits not made to a high standard) External noise Natural origins such as electrostatic interference and electrical storms Electromagnetic interference (EMI) – Current running through existing power cables Radio frequency interference (RFI) – from radio systems radiating signals Cross talk (from other cables separated by small distances)

Noise

Sources of Noise Typical sources of noise are typically devise which produce quick changes (spikes) in voltage or current. These could include… Large electrical motors being switched on Fluorescent lighting tubes Lightning strikes High-voltage surges due to electrical faults Welding equipment.

Reducing noise In order to reduce electrical noise you should try… Physical segregation of noise sources from noise-sensitive equipment Electrical segregation Harmonic current control Shielding/screening of noise sources and noise-susceptible equipment including use of shielded/twisted pair conductors.

Encoding digital data with an Analogue to Digital Converter Signals in the real world are analogue: light, sound, you name it. So, real-world signals must be converted into digital, using a circuit called ADC (Analog-to-Digital Converter), before they can be manipulated by digital equipment. Whenever we need the analogue signal back, the opposite conversion – digital-to-analogue, which is done by a circuit called DAC, Digital-to- Analog Converter – is needed. When you play an audio CD, what the CD player is doing is reading digital information stored on the disc and converting it back to analogue so you can hear the music. When you are talking on the phone, a digital-to-analogue conversion is also taking place so you can hear what the other party is saying.

ADC in action What the ADC circuit does is to take samples from the analogue signal from time to time. Each sample will be converted into a number, based on its voltage level. In Figure 2 you see an example of some sampling points on our analogue signal.

Sampling rate The frequency on which the sampling will occur is called sampling rate. If a sampling rate of 22,050 Hz is used, for example, this means that in one second 22,050 points will be sampled. Thus, the distance of each sampling point will be of 1 / 22,050 second (45.35 µs, in this case). If a sampling rate of 44,100 Hz is used, it means that 44,100 points will be captured per second. In this case the distance of each point will be of 1 / 44,100 second or 22.675 µs. And so on. Notice how the end wave has differed

Sample rate Vs Quality So, the more sampling points we use – i.e., the higher the sampling rate –, the more perfect will be the analogue signal produced by the digital-to-analogue converter (DAC). However, the more samples we capture more storage space is necessary to store the resulting digital data. For example, an analogue-to-digital conversion using a 44,100 Hz sampling rate will generate twice the number of data as a conversion using a 22,050 Hz sampling rate, as it will capture twice the samples from the original waveform. If you use a low sampling rate, the waveform generated at the DAC will be very different from the original analogue signal. If it is music, for example, the music you will play will have a very bad quality. So, we have this dilemma: if the sampling rate is too high, the output quality will be close to perfection, but you will need a lot of storage space to hold the generated data (i.e., the generated file will be very big); if the sampling rate is too low, the output quality will be bad.

PCM – Pulse Code Modulation The process of converting an analogue to digital is known as PCM. The process is describe here.

Sampling The analogue signal is sampled every T interval. Most important factor in sampling is the rate at which analogue signal is sampled. According to Nyquist Theorem, the sampling rate must be at least two times of the highest frequency of the signal.

Quantization Sampling yields discrete form of continuous analogue signal. Every discrete pattern shows the amplitude of the analogue signal at that instance. The quantization is done between the maximum amplitude value and the minimum amplitude value. Quantization is approximation of the instantaneous analogue value.

Encoding In encoding, each approximated value is then converted into binary format.

Digital data Vs Digital Signal The process for converting digital data into digital signal is said to be Line Coding. Digital data is found in binary format. It is represented (stored) internally as series of 1s and 0s. Digital signal is denoted by discreet signal, which represents digital data. There are three types of line coding schemes available.

Types of digital encoding

Uni-Polar Encoding Unipolar encoding schemes use single voltage level to represent data. In this case, to represent binary 1, high voltage is transmitted and to represent 0, no voltage is transmitted. It is also called Unipolar-Non- return-to-zero, because there is no rest condition i.e. it either represents 1 or 0.

Polar Encoding – Polar Non-Return to Zero (polar NRZ) It uses two different voltage levels to represent binary values. Generally, positive voltage represents 1 and negative value represents 0. It is also NRZ because there is no rest condition. NRZ scheme has two variants: NRZ-L and NRZ-I. NRZ-L changes voltage level at when a different bit is encountered whereas NRZ-I changes voltage when a 1 is encountered.

Return to Zero (RZ) Problem with NRZ is that the receiver cannot conclude when a bit ended and when the next bit is started, in case when sender and receiver’s clock are not synchronized. RZ uses three voltage levels, positive voltage to represent 1, negative voltage to represent 0 and zero voltage for none. Signals change during bits not between bits.

Bit rate and Baud rate Serial-data speed is usually stated in terms of bit rate. However, another oft-quoted measure of speed is baud rate. Though the two aren’t the same, similarities exist under some circumstances. Most data communications over networks occurs via serial-data transmission. Data bits transmit one at a time over some communications channel, such as a cable or a wireless path.

Bit rate & Bandwidth The speed of the data is expressed in bits per second (bits/s or bps). This says how many binary digits can be sent per second. Typically your home or school internet connection will be measured in Bits per Second (bps). Bandwidth is also is also a measurement of speed. Typically the higher the Bits per Second the higher the Bandwidth (but not always). Bandwidth is measured in Hz and Bits per second is measured in bps. Bit rate is how many bits are sent a second Bandwidth is how many times the signal changes a second (Strangely not always one bit!).

Baud rate The Baud rate is not often used. It refers to the number of times a digital signal changes in one second. This usually gives a lower rate than BPS as some method of sending Digital signals can allow more than one data bit to be transmitted per change state.

It may be required to convert a digital signal into analogue in order for it to be transmitted. There are several ways of doing this.

Carrier wave Typically in analogue signals a carrier wave is used to help transfer information. These are used as if you attempted to send a raw message over a wire or radio wave they would generally get disrupted with high levels of interference. To prevent this a high power “carrier” wave is used . This is typically a SIN wave. This carrier wave is manipulated and the changes to this wave show the binary or analogue information that is to be transferred. Below you can see an example of an unaffected carrier wave.

Amplitude shift keying (ASK) This is a simple method where a digital signal is converted into an analogue signal. Where a Binary 1 is required the amplitude of the wave is increased. Where is binary 0 is required the amplitude of the wave is reduced or not transmitted.

Frequency Shift Keying (FSK) As it names suggests, a frequency shift keying uses the frequency of the wave to convert a binary signal into a analogue signal.

Phase Shift Keying (PSK) Phase shift keying (PSK) is a way of conveying data by changing the phase of the reference signal. The carrier wave initially follows a typical SIN wave pattern like can be seen below. This carrier wave is used to transmit a digital signal but disrupting the pattern whenever there is a change from 1 to 0 or 0 to 1.

Analogue to Analogue In order to send an analogue signal over a large distance the signal must first be “modulated”. This is the process of impressing low-frequency information to be transmitted on to a high-frequency wave, called the carrier wave, by changing the characterises of either it amplitude, frequency, or phase angle is called modulation.

Amplitude modulation The method of varying amplitude of a high frequency carrier wave in accordance with the information to be transmitted, keeping the frequency and phase of the carrier wave unchanged is called Amplitude Modulation. The information is considered as the modulating signal and it is superimposed on the carrier wave by applying both of them to the modulator. The detailed diagram showing the amplitude modulation process is given below.

Frequency Modulation Frequency modulation (FM) is a different way of encoding information onto a carrier wave by varying the frequency of the wave. The carrier wave uses high and low frequencies to transmit the amplitude of the signal.

Phase Modulation To best understand Phase modulation you must first understand what a “phase” actually is. The phase of the wave is the starting point of the wave. In phase modulation you basically change either the starting point or starting time of the carrier wave and modify it as the signal changes. You should note that Frequency modulation is a type of phase modulation.