Section 3 - Data Representation - Part 2 Flashcards
What is a pixel?
The smallest identifiable area of an image. Each pixel is given a binary value which represents a single colour
What is a bitmap/raster image?
An image made up of many pixels
What is the equation for the resolution of an image?
Width in pixels * Height in pixels
What is the effect of an increased resolution?
Assuming the physical size of the image stays constant, a greater resolution means there are more pixels and therefore the image will be sharper
What is PPI?
Pixels per inch, a method of expressing resolution
What is color depth?
The number of bits per pixel. N bits give 2^N combinations
How can you calculate image file size?
Resolution * Color depth, however this does not account for metadata
What is metadata?
Data contained by a file, about the other data in that file.
What would the metadata of an image contain?
Image size and color depth
What are vector graphics?
Images made up of geometric shapes and objects. A vector file will store the properties of each shape in order to redraw the object when the file loads
What are vector drawing lists?
Where the properties of each vector image component are stored
What happens when an image is resized?
The computer will adjust the position and dimensions of the image properties to redraw the image perfectly each time. Regardless of the size the image will always be sharp and the amount of data required to store the image will not change
What are the advantages of vector images?
- They require less storage space
- They transmit faster
- They load quicker
Why are bitmap images used?
- Vector images cannot easily replicate an image with continuous areas of changing colors (e.g a photograph)
- Individual pixels can be manipulated within a bitmap image, making is easier to do slight modifications
What form are sound waves found naturally?
In a continuous, analogue form
How can sounds be represented in a computer?
The analogue sounds waves must be converted into a digital format. Recordings of the amplitude of the sound ware are took at given time intervals
What is frequency?
The rate at which samples are taken. It is measured in hertz, with the higher the frequency the more accurate the sound
What is the audio bit depth?
The number of points of amplitude. A greater audio bit depth results in a more accurate sound recording
What is the sampling rate?
The frequency with which you record the amplitude of the sound
What is the advantage of a high sampling rate?
The playback will sound smoother
What is the disadvantage of a high sampling rate?
The audio file size will be larger, taking up more storage space
How can you calculate the size of sounds samples?
sampling rate * audio bit depth * length in seconds
What is the difference between the signal of analogue and digital sounds?
- An analogue signal is a continuous signal that represents physical measurements
- A digital signal is a discrete time signal generated by digital modulation
What is the difference between the representation of analogue and digital sounds?
- An analogue sound uses a continuous range of values to represent information
- A digital sound uses discontinuous values to represent information
What is the difference between the data of analogue and digital sounds?
- Analogue technology keeps a wave in its original form
- Digital technology will sample an analogue waveform at intervals, turning it into a set of numbers to be stored on a digital device
How does an analogue to digital converter take sound inputs?
A microphone will convert the sound energy into electrical energy. The analogue to digital converter samples the analogue data at a given frequency, measuring the amplitude of the wave at each point and converting it into a binary value according to the audio bit depth
How does an analogue to digital converter output sound?
The binary values for each sample point are translated back into analogue signals to be sent to an amplifier connected to a speaker
What is Nyquist’s theorem?
To produce an accurate recording the sampling rate must be at least double the highest frequency in the original signal
What is MIDI?
A technical standard that describes a protocol, digital interface and connectors that can be used to allow a variety of devices to connect and communicate with each other
What is a MIDI controller?
A device that carries event messages between multiple devices
What are some examples of an event message?
Duration of the note, pitch, volume changes
What is the purpose of MIDI?
To allow a computer and a few skilled musicians to recreate the music of a much larger ensemble
What is a MIDI file?
A list of instructions that tells the computer to synthesize a sound based on pre-recorded digital samples
What are the advantages of MIDI?
- It allows a computer and a few skilled musicians to recreate the music of a much larger ensemble
- A MIDI file can take up 1000 times less disk space than a conventional recording
What is the point of compression?
To reduce the size of files, meaning they rake up less storage space and are quicker to transmit
What is lossy compression?
Compression by removing unnecessary details
What is lossless compression?
Compression where all information is retained so the original file can be replicated exactly
How are MP3 files compressed?
By removing frequencies too high for most people to hear and by removing quieter sounds played at the same time as louder sounds
How does lossless compression work?
By recording the patterns in data rather than the actual data itself. This means the computer can reverse the procedure to recreate the exact file with no lost data
What is the advantage of lossy compression?
It usually results in the smallest file size
What is the advantage of lossless compression?
No data is lost, which is critical for some uses where even minor altercations can be catastrophic
What is Run Length Encoding?
A method of lossless compression where data is converted into runs, with each run made up of the value that has been repeated and the number of times it has been repeated
What is dictionary-based compression?
A method of lossless compression where the compression algorithm will search through the text to find suitable entries. This means the recipient can reconstruct the file by matching up each reference to the location in the dictionary
What is the disadvantage of dictionary-based compression?
In short files the data required to send the dictionary can actually increase the size of the file being transmitted. However, for longer sets of text the size of the dictionary becomes insignificant
What is encryption?
The transformation of data from plaintext into ciphertext to prevent unauthorized third parties from being able to understand the plaintext
What is plaintext?
The original data sent in the file
What is ciphertext?
The encrypted data
What is the cipher?
The encryption algorithm
What is the key?
The information to lock or unlock the message
What is the Caeser cipher?
A simple substitution cipher that works by shifting the letters of the alphabet along by a given number of characters
What is the disadvantage of the Caeser cipher?
It is easy to break via brute force or some simple logic
What is the issue with ciphers that use non-random keys?
They can be broken by cryptanalytic attacks given enough time and resources
What is the issue with computer-generated random numbers?
They are not actually random, they just appear to be so. To get true randomness you must use a physical and unpredictable phenomenon
What are on-time pad ciphers?
Ciphers that offer perfect security if used properly. The encryption key must be equal or longer in characters than the plaintext, be truly random and be only used once
Why are one-time pad ciphers impossible to break?
As the key is random, so is the distribution of characters and therefore no amount of cryptanalysis will produce meaningful results
What is computational security?
A type of cipher that relies on the fact that there is no computer system powerful enough to crack the cipher in a reasonable amount of time.
What is the Vernam cipher?
A impossible to break encryption method that uses a 1 time-pad. It uses the XOR operation on the binary representation of each character in the plaintext and the key to get the ciphertext.
