Maintaining Computer Systems

Task: Hardware upgrade of a computer system - RAM upgrade

Date 30/03/2009

People involved: Dilyana Manolova 159400

Used equipment:
Antistatic Wrist Strap
Screw driver – Phillips

Health and Safety procedures followed:

1. Cleared enough space for work on the desk.
2. Took a machine from the shelves and put it on the working desk.
3. Took out the Tool kit pack and prepared them for use.
4. Put an antistatic wrist strap to protect the equipment.
5. Checked the power before started work.
6. During the steps of taking out/ putting memory in the computer system was used antistatic wrist strap.

Description of the task:
RAM upgrade of a computer system.

Steps of work:

Step 1.
Unplug the power cable from the socket and all the peripherals from the ports. Unscrew the side panel of the computer case to open it.

Step 2.
Release RAM chips clips and check the RAM type.

Step 3.
Prepare bigger size of the same RAM memory type for upgrade.

Step 4.
Replace RAM memory chips with the new bigger size RAM chip.

Step 5.
Check whether the RAM memory is put on place.

Step 6.
Close the side panel of the computer case and screw it.

Step 7.
Plug back the power cable.

Computer Archtecture Task 2

1. Conversions between Analogue and Digital data

Analog signal can be any sound in the nature. But when this sound is needed to be transmitted into a computer system it needs to be converted into a digital form. Computer systems communicate only with digital data. A sound wave represented in digital form is series of pitch and volume values over the length of the recording device.

Data can be transmitted over a network in analog or digital form. It is cheaper if the data is transmitted in its analog form. But as the computer systems work only with digital data there is need of device which can convert the two forms of data: analog to digital and digital to analog form. These converters enable the computer systems to transfer data to other devices over the network.

Analog to digital converter (ADC)

ADC is a device that converts analog continuous signals to discrete digital numbers. The process of converting the data is carried out using an electronic circuit known as signal encoder. This is very complex process which involves sampling, quantization and encoding of the data.

An analog signal is a continuous wave with different values of its amplitude and its frequency. The amplitude is measured in decibels and the frequency in Hz or cycle per second.

The transmitted signal can be text, sound or mixture of them – video.

When sound signal is transmitted in its analog form it is represented in a form of waves. They can be with different amplitude and frequency.

These waves can be converted into electrical signals by a microphone, to spots on a CD disk and etc.

When sound is transmitted in the air it can be distorted by the surrounded environment. For example: it may interference with other sounds and to change into a combined signal; if it is transmitted on a long distance it can be lost or to become quiet, fade. This makes the analog signal very vulnerable to air conditions and to effect its degradation.

The dashed signal is received instead of the original sent signal.

We can use the analog waves to transmit numbers which converted into digital signal can be represented in bits which then can be transmitted inside a computer system.

The digital signal in the computer system can be represented with the physical property – voltage where the 0 represents the negative value and the 1 represents the positive value. To be able to transmit this signal the computer system transmits these voltage values or bits.

Every physical signal depredates with the time. But as the digital signal can be represented by 1s or 0s there is no problem to figure out its value.

This can not be done with the analog signals, as they have many more values that represent them.

Digital data is easy to be stored and transmit. There is a lot of error detection and correction techniques used after its transmission to make sure it is received in its original form.

Digitizing

This is a way of representing analog sound as a discrete bit values called samples. Digitization is performed by reading an analog signal at regular time intervals and representing its value at that point by an integer number.

There are two factors determining how close the data can be converted to its original form: sampling rate and the number of bits used to represent the integer numbers.

Sampling

Sampling is the process of measuring the amplitude of an analog signal at regular time intervals. It can be also said as reading an analog signal values at a specific time.

A sampled version of a continuous signal can be represented by the formula: s(T) ={ s(T), s(2T), s(3T),…s(nT)}, where s(T) is continuous signal and T is sampling interval. Then we can transmit this steam of numbers into the computer system. The image below shows each integer value circled at each sampling interval.

The most famous and used method of sampling the data is the Pulse Modulation Technique. This is a process of converting analog data into digital non-pulse integers which are not easily affected by distortions and noise.

Nyquist Sampling Theorem

In order to obtain an accurate representation of a time – varying analog signal, its amplitude must be sampled at a minimum rate that is equal to or greater than twice the higher frequency of the signal.

The sampling rate can be represented as Hz or sample per second (sps). The higher the rate is the better the quality will be. Low rates can lead to noise and jittering. If a signal is sampled with rate less than the Nyquist’s rate this will cause the signal to be distorted.

For example:

If the signal’s higher frequency is F, then the sampling rate should be 2F and the sampling time interval Ts=1/2F.

Quantization

This is a process of replacing the sampled values in terms of a discrete set of amplitude value known as a codeword. The number of bits used to represent a sampled value determines the accuracy of the quantization/coding process. The most significant bit of the codeword indicates the polarity of the sample: 0 – indicates positive value, 1 – indicates negative value

For example, if Vmax is the maximum positive and negative signal amplitude and n is the number of binary bits used, then the magnitude of each quantization interval Q is given by: Q=2Vmax/2^n

The difference between the actual signal amplitude and the corresponding nominal amplitude is called the quantization error or noise. The size of the quantization error depends on the number of bits used to represent the analog values.

The transmission bit rate can be calculated by the formula:

Bit Rate = Sampling Rate X Numbers of bits per sample

Bit rate represents the number of bits per second which is the size of uncompressed file or the length of sound measured in seconds.

The bit rate depends of two values – sampling rate (horizontal axis) and bit resolution (vertical axis). Doubling the sampling rate doubles the file size and the bit resolution.

bit rate (bits per second) = bit-resolution * sampling rate

file size (in bits) = bit rate * recording time

Digital to analog converter (DAC)

DAC is a device which converts the discrete digital numbers to continuous analog signals. The device that performs this conversion is called Decoder. The digital numbers or also called binary code which is represented by 0s and 1s. When a signal is sampled faster to be converted back into analog it reproduces better quality of the original signal but the size of the file increases accordingly.

When DAC is used the digital integer numbers are converted into a sequence of impulses which are then filtered by a reconstruction filter to reproduce a sampled signal according to the Nyquist frequency. This reproduction is never perfect as there are some errors due to the quantization technique.

Compression

This is a process of encoding bits of files according to some algorithm to reduce the file size.

Compression is very helpful for transmitting file between devices. It makes the transmission faster as the file size is smaller. This technique is also used to free space when the files are very big and occupy a lot of space on the computer.

Compressed images files are GIF and JPEG. Their techniques of compression manage to compress images to a fraction of their original size.

Compression is making the file size smaller. This is very important when the file needs to be transmitted or to be stored, as smaller file arrives faster and occupy less space.

Compression ratio is the ratio between the uncompressed file size and the compressed file size.

Types of compression:

Lossy Compression

Lossy schemes do not preserve all the original data. You can not recover lost picture information after compression. Lossy schemes attempt to remove picture information the viewer will not notice. As more and more picture information is removed, the picture quality decreases. In this scheme is achieved higher compression ratio. It can be used for compression of audio, static image files, video, where any loss of information may not be easily perceived. Lossless data compression is used in the ZIP file format.

Lossless Compression

Lossless schemes preserve the original data. There is no loss of data in this compression algorithm, so the original file size and quality stays the same and after decompression. This scheme is use for text compression where any loss of information could make these types of files useless.

The trade-off between compression ratio and picture quality is very important issue when we compress images. The compression ratio typically affects the picture quality. Generally, the higher the compression ratio is, the poorer the quality of the resulting image become.

The trade off between increasing compression ratio and distortion can be seen as blocks when you use the highest levels of compression or greatly enlarge the image.

The larger the image, the clearer it is with no perceptible distortion. As size drops (higher compression ratios) your size (hence load time) decrease rapidly. As the compression ratio is “the initial data size/the compressed data size” the trade-off between compression ratio and image quality depends on the image size.

JPEG (Joint Photographic Experts Group)

JPEG is an international standard for compressing still images. The JPEG standard specifies both compression - which defines how an image is compressed into a stream of bytes and decompression - converted back into the original image. The technique is based on the use of the Discrete Cosine Transform (DCT).
JPEG is "lossy," meaning that the decompressed image is not quite the same as the original image. JPEG achieves much greater compression than is possible with lossless methods. JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. JPEG uses 8x8 standard to give the best trade-off not only compression ratio and image quality, but also and load time.

When there is need to compress sound and video together other more powerful techniques are used, such as MPEG, MP3 and AVI compression algorithms.

MPEG (Moving Picture Experts Group)

MPEG video compression is used in many products such as: digital television boxes, DSS, HDTV decoders, DVD players, video conferencing, Internet video, and other applications. These applications benefit a lot of from video compression as they require less storage space for archived video information and less bandwidth for the transmission of the video information from one point to another.

MPEG compression is very complex as it combines audio and video data which are encoded together into a bit rate of a storage device.

MP3

MP3 is a digital audio encoding format, which uses lossy data compression. It reduces the amount of data of the original audio file by removing the sounds with frequency that the human ear can not hear. MP3 is used a lot for audio recording. This compression of audio can be made by using the MP3 standard with several bit rates: 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 192, 224, 256 and 320 Kbit/s. But as higher the bit rate become the better the sound quality will be and this will increase the file size.

AVI (Audio Video Interleave)

It is multimedia container format develop for Windows. AVI files can contain audio and video data that allows them to synchronize audio-with-video playback.

The data in the avi file is divided into blocks. They contain different type of data about the file. Some contain data such as the width, height and frame rate of the video, others the actual audio and visual data and so on.

WAV

This is an audio format file used for storing uncompressed data. The extension .wav comes from Waveform. It is mainly used for storing audio data on a computer with maximum quality.

1. Logic gates in a Computer System

Logical gates are used in digital circuits in any electronic system. The electronic circuits are specially designed to represent the different electric flows. The high voltage is represented by 1 and the low voltage by 0.

The task of the logic gates is to combine multiplexes multiple inputs and set or reset the memory bits. All these complex calculations are executed by simply process of adding and converting binary numbers.

Venn diagrams are used to represent the relation between the input and output binary numbers in the logic gates.

There are few types of logic gates:

The above logic gates can be combined and form a digital circuit called Half Adder

A half adder is a logic circuit that carries out one-digit addition. It needs two inputs, and the output is the sum of the inputs.

The simplest case of half adder is to add two one bit numbers.

For example, A and B are our input numbers. The truth table below shows what will be the result of that addition depending of the different input values of A and B.

A and B can be represented only by 0 or 1. In our case the executed operation between the inputs A and B is XOR. The output result is in the Sum column and can be called Y_0. This bit is also called Least Significant Bit. The Carry column represents the carry bit or Y_1. The above circuit can be used to build Half Adder.

When using Half Adder circuit as a binary adder disadvantage is that there is no provision for a "Carry-in" from the previous circuit when adding together multiple data bits.

For example, we want to add together two 4-bit bytes of data, any resulting carry bit would need to move across the bit patterns starting from the least significant bit. But as the Half-Adder has no carry input the final result would be incorrect. This is the reason of developing Full Adder to fix this problem.

Full Adder

It is more complex circuit. It can be used to add two four-bit binary numbers together (for example A, B and C_out which is the carry bit). Here is counted the carry and the final result should be correct.

Full adder can be made very complex by combining other full adder and half adder. This depends on how many bits are the numbers that need to be added. This can be 8bit, 16bit 32bit binary numbers.

The truth table of Full Adder of 8bit binary numbers is shown below:

SR Flip Flops

A Flip flop is combined logic gates which involve input values and output values which from the gates are going back into the circuit and form a sequential logic circuit

Set-Reset flip-flop is a combination of logic gates that maintains stable Output even if the Inputs are turned off. The input “Set” causes output of 0 and 1. The input “Reset” causes the opposite outputs.

Flip flop is controlled by one or two control signals or clock signal. When the flip flop is in storage mode the S and R inputs are both in low level (represented by 0). This keeps the two outputs Q and not Q in a constant state. If S (set) is high level and R is low level, then the output Q is forced to stay high level, even after S is changed into low level. It is similar and with R (reset) when it is kept in high level while S is held as low level, then Q output is forced to stay low level even after R returns to low level. All these can be seen in the table above.

How they are used in the memory circuit and in the multiplexor

Flip flops are mainly used to store digital data in registers in the computer system and to transfer this binary numerical data between its component.

The R-S flip flop is a building block from which other circuits can be constructed. This flip flop circuit can be considered to be a one bit memory circuit since Q can be set to one or zero. It is basic storage data device. It keeps the data until reset occur.

The RS flip flop circuit can be used as a controllable memory by controlling the Latch value. When the Latch value is 1 for D type flip flop the value set in the D is written to the flip flop as the output Q=D. but if the latch value is 0 then the Q value s kept.

The T type flip flops works in to modes. If T=1 the clock changes from 1 to 0 and the output changes its state. If T=0 the output remain the same. It can be constructed by any other type of flip flops. It is used for counters.

The J-K type flip flops are very flexible as it can be set up to replace the RS, D or T type flip flops.

Multiplexer

This is an electronic device performing multiplexing. The process of multiplexing is combining several input values/signals into one output value/signal. In computer system if the multiplexer has n input bits it is represented by 2^n, as we use binary digital data. This device is very useful as it replace many devices that can use only one or two input values to receive one output value. We can say it is a device with multiple inputs and a single output. Logic gates are represented with 2-1 multiplexer that performs Boolean logical operations over the input values.

The image below shows digital 4-1 multiplexor

Demultiplexer is a device which takes as an input only one value/signal and receives several output values at the other end. We can say it is a device with single input and multiple outputs.

Bistable and Astable flip flops

Bistable flip flop

This is a two state device called “multivibrator” and used in digital electronics. Bistable flip flops are the basic memory devices used in sequential logic circuit. It has two stable states. It is able to retain the two S and R states indefinitely. An external signal (latch) is required to change the output states. There are two existing designs for a bistable flip flops.

NAND gate bistable and set/reset with logic 0

NOR gate bistable and set/reset with logic 1

As they can remember their previous state they are used to build counters of registers. The counters are memory locations where the data can be increased or decreased with 1. And the registers are used to store data or control information.

Astable flip flop

Astable flip flop or also called astable multivibrator is used as an oscillator. Its circuit has no stable state. It has only two states and flips from one state to another continuously. When one transistor is on the other transistor is off. But it can not keep the transistor off forever, so it began to turn on. This will change and the state of the “on” transistor into off. It works as it keeps half of the circuit on and the other half off, and it keeps changing their state. Astable flip flop circuit is used for making flashers or generating square waves.