5.6 Representing images, sound and other data Flashcards
what are the main difference between analogue and digital data? ( 2 separate points)
Analogue data is continuous, there are no limits to the values that the data can take. Digital data is discrete meaning that it can only take particular values.
The trace of a digital signals value against time is characterised by sharp edges and vertical drops as the signal changes value. While analogue signals tend to be smooth curves, they can sometimes feature sharp peaks.
How do you convert sound, digital to analogue and how does it work?
using a digital to analogue converter (DAC). This device reads a bit pattern representing an analogue signal and outputs an alternating, analogue, electrical current.
How do you convert sound analogue to digital?
The computer uses an analogue to digital converter (ADC) to convert analogue to a digital bit pattern. the device works by taking a reading of an analogue signal at regular intervals and recording the value in a process called sampling. Samples are taken at a specific frequency, given in hertz, which determines the number of samples taken per second. This is usually a high number as greater sampling frequencies result in a better reproduction of the analogue signal. Once the value if the analogue signal has been recorded it can be stored digitally as a bit pattern.
How do bitmap graphics work?
An image is broken down into pixels each of which has a binary value assigned to it.
What is the resolution of an image?
A resolution of an image is often expressed as a number of dots per square inch in an image where a dot is a pixel. Resolution can also refer to the number of pixels in an image.
what is colour depth, how can it be calculated?
Colour depth is the number of bits assigned to a pixel in an image is called its colour depth. It can be calculated by 2^n n being the amount of bits used
how do you calculate the storage required to represent a bitmap image?
Multiple the number of pixels (width * height) by the colour depth + metadata.
what are some typical examples of metadata?
Width, height, date created and colour depth
How do vector graphics work?
Vector graphics represent images using geometric objects and shapes such as rectangles circle and lines. The properties of each geometric object or shape in the image are stored in a list.
What’s the advantages and disadvantages of using vector graphics over bitmap images?
Because vector graphics uses shapes they can be scaled without losing quality unlike bitmap images.
Vector graphics are well suited to simple images which use shapes, like company logos however bitmap images are much better for storing photographs
Vector graphics frequently use less storage space than bitmapped graphics as information is stored for each shape rather than for every single pixel in an image.
what is sample resolution, what are the positives and negatives of a higher sample resolution?
the number of bits allocated to each sample is referred to as the sample resolution. Higher sample resolutions result in greater audio quality but also increased file size.
How do you calculate the size of a sound sample?
Multiplying the duration of the sample in seconds by the sampling rate in hertz and the sample resolution.
True or false there is metadata in an audio file?
True
What is the Nyquist theorem?
The Nyquist theorem states that the sampling rate of a digital audio file must be at least twice the frequency of the sound. if the sampling rate is below this, the sound may not be accurately represented.
What is MIDI?
MIDI stands for Musical instrument digital interface. It is used with musical instruments which can be connected to computers. Rather than storing samples of sound, MIDI stores sound as a series of event messages, each of which represent an event in a piece of music.
In MIDI what do the event messages contain?
the duration of a note, The instrument with which a note is played, how loud a note is, if a note should be sustained
what are the advantages and disadvantages of MIDI?
It allows for easy manipulation of music without loss of quality.
The instruments on which notes sound can be changed.
Notes can be transposed
Duration of notes can be altered.
MIDI files are often smaller in size than sampled audio files and it is lossless.
MIDI can’t be used for storing speech and sometimes results in a less realistic sound than sample recordings.
Why are files compressed?
Files are compressed in order to reduce their size and smaller flies can be transferred faster between storage devices or over the internet.
What is lossy compression?
Lossy compression is when information is lost in the process of reducing the files size.
what is lossless compression and the 2 methods?
Lossless compression is when there is no loss of information. the file size can be reduced without a decrease of quality.
This can be RLE or dictionary based methods.
How does run length encoding (RLE) work?
RLE reduces the size of a file by removing repeated information and replacing it with one occurrence of the repeated information followed by the number of times it is to be repeated?
How do dictionary-based methods work?
A dictionary containing repeated data is appended to the file.
in order for dictionary-based methods to work what must present?
A dictionary has to be present in a file. This will increase the size of the file.
When do dictionary-based methods and RLE work best?
They are most effective on files that contain a lot of repeated data
What are the advantages and disadvantages of lossy compression compared to lossless compression?
Information is lost in lossy but not lossless.
Quality is reduced in lossy but not lossless
There is no limit of compression in lossy but there is a limit to lossless
what is encryption?
Encryption is the process of scrambling data so that it cannot be understood if intercepted in order to keep it secure during transmission.
Describe what is meant by plain text and cypher text?
Unencrypted information is referred to as plaintext and encrypted information is called cypher text.
What is a cipher?
A cipher is a type of encryption method
How do caesar ciphers work?
caesar ciphers encrypt information by replacing characters. One character is always replaced by the same character. There are 2 types of caesar ciphers. Shift ciphers and substitution ciphers.
How do shift ciphers work?
When encrypting using a shift cypher, all of the letters in the alphabet are shifted by the same amount. The amount by which the characters are shifted forms the key.
How do substitution ciphers work?
Substitution ciphers are when letters are randomly replaced.
Why aren’t caesar ciphers secure?
they can be easily cracked because the frequency at which each character occurs can provide a clue as to which letter has been replaced with which. Once one character has been discovered a shift cipher can be completely crackers as the key can be found.
What does a one-time pad cipher mean, giving an example?
A one-time pad cipher means that each key should only ever be used once. For example the vernam cipher.
How does the Vernam cipher work?
Align the characters of the plaintext and the key. Then convert each character to binary. After apply a logical XOR operation to the two bit patterns. Finally convert the result back to a character.
Why is the vernam cipher so secure?
Since the key used with the vernam cipher is chosen at random the cipher text is also random and so the cipher is considered completely secure. It is the only cipher that is mathematically proven to be completely secure.
What is computational security?
The worth of all ciphers ever devised is based on computational security. In theory every cryptographic algorithm except for vernam cipher can be broken, given enough cipher text and time.
