Computer Science (15.12.22) Flashcards
What are Analogue to Digital conversions?
-Analogue sound signals are continuous
-Digital sound signals are discrete
-Sound is digitised by repeatedly measuring and recording the sound wave
What are Convertors in Sound?-
-Sound must be converted into digital form in order to be stored and processed by a computer
-An Analogue to Digital Converter (ADC) is used to convert inputs to digital signals
-A Digital to Audio Converter (DAC) is used to convert digital signals to outputs
What is Sound Sampling?
-Sound is sampled using bit depth and sample rate
-SS is a measurement of the amplitude of a sound wave at a given time
What is the Sample Resolution?
-The number of bits (audio bit depth) used to record each measurement
-More bits used per sample enables the height of the wave to be more accurately measured but increases the file size
-Determines how closely the wave is sampled on the y-axis
What are Hertz?
-Sample rate is usually measures in Hertz (Hz)
-1 Hz = 1 sample per second
-Hz to KHz = divide by 1000
What is the Sampling Rate?
-The greater the frequency, the greater the accuracy, and the file size
-The number of samples taken per second
How do you Calculate the File Size?
-File size (bits) = sample rate x resolution x duration
-6 sample per second x 4-bit resolution x 3 seconds = 72 bits / 8 = 9 bytes
What is Lossy Compression - MP3?
-Removes sounds that we can’t easily hear or that least affect the playback quality
-It leaves out some data from the original so can negatively affect the sound quality
-Useful for storing, downloading or streaming
What is Lossless Compression? (sound)
-The formats are able to reduce the file size when compressed but do not lose any information
-FLAC (Free Lossless Audio Codes)
-ALAC (Apple Lossless Audio Codes)
-WMA (Window Media Audio)
What are other file formats?
-WAV and AIFF files are uncompressed audio files
-Lossy file formats are still very popular as they reduce the file size well
-MP3 (MPEG)
-AAC (Advanced Audio Coding) -> typically higher quality than MP3
What are examples of Bitmap image file formats?
-BMP
-JPG
-GIF
-PNG
-TIFF
What is an example of a Vector image file format?
-SVG
+editing softwares such as Illustrator (AI) and Photoshop (PS)
What is the bit depth of images?
more bits per pixel = more colour combinations
1 bit = 2 colours
2 bits = 4 colours
3 bits = 8 colour
What is BPI and Pixel?
BPI = beats per inch
Pixel = picture element
What are Bitmap (or Raster) Images?
-Made up of PICture ELement or pixels
-a pixel is the smallest identifiable area of an image
-a pixel’s colour can be changed by changing this value
What is the Image Resolution
-Resolution is the number of pixels in the image
-High definition (HD) television: 1920 x 1080 pixels
How do you create an image?
-Each pixel is given a binary value
-Each value represents a different colour
-Using one bit per pixel allows only 2 values, 0 and 1
1 = Black, 0 = White
How does increasing the image resolution affect the file size?
It increases the number of pixels in the image which would increase the file size
Two factors that affect the file size of a bitmap image:
- More pixels, it increases the file size
- More colours, more pixels needed-needs more file size
What is Serial Transmission? (USB)
- Bits are sent one signal at a time over a single wire
-Once the bit has been received, the next bit is sent
What is Parallel Transmission?
A number of wires are used so that many bits may be sent at the same time
Parallel VS Serial Transmission:
If the same speed is used for one serial wire or eight parallel wires then parallel transmissions will be faster
What is Lossless Image Compression?
-Will not lose any of the original data
-When data is uncompressed it is restored exactly as it was in the original
What does RLE stand for?
RLE = Run Length Encoding
-uses frequency/ data pairs to encode each run length of the same coloured pixel
What is RLE Frequency/Data Pairs?
-This is an uncompressed format where 1 = white and black = 0:
11110 11110 0000 0000 0000
It would be encoded as: 4 ‘1’, 1 ‘0’, 4 ‘1’, 16 ‘0’
What are Parallel cables?
They are able to send more data than serial cables if the same data rate is used
What is Crosstalk?
-May occur when a signal on one line creates a disturbance in another line
-This may a cause a bit to be reveresed in a parallel line
-longer cable= more chnce of crosstalk happening
What is Skew?
-Each wire in a parallel cable has slightly different properties, signals arrive at a different time
-The receiver must wait until all lines are received until the next set of bits may be sent
-Transmission needs to be over very short distances to reduce the problem
What are the USB features?
USB= Universal Serial Bus, is a serial cable that is commonly used to connect external devices to computers,laptops,table and smartphones
-The cable may be pluggled in or removed whilst the device is turned on
Basics about compression:
1 MP3 = 4 floppy disks
data compression = trasmission
lossy -> lossees info. + smaller storage
lossless -> 2 main methods: RLE (most known), Hifman encoding (text)
What is Simplex Transmission?
-Data travels in one direction only
-This would be used to send data from a keyboard to a computer
What is Half-Duplex Transmission?
-Data is able to travel in both directions along a single connection, but can’t travel at the same time
-The full bandwidth is available to both the sender and receiver, can’t send at the same time
What is Full-Duplex Transmission?
-Data is able to move in both directions simultaneously using 2 cables
-Ethernet network cables make use of 2 twister pairs of cable-one for sending and one for receiving
-Fibre optic cables use 2 fibres-one for transmitting and one for receiving
What can cause a Transmission Errors?
-Electrical interface
-Power surges
-Synchronisation issues
-Wear and tear on the cable or connectors
What can Transmission Errors cause?
-Data to be lost
-Data to be gained
-Data to be changed
What is Error Checking?
-Computers need methods to check for data transmission errors caused by interference
-These methods include the use of:
-Parity bits
-Check digits
-Echo checks
-Automatic Repeat Requests (ARQ)
What are Parity Bits?
-When sending a byte of data, one bit is used as a parity bit
-Used to check if there is an error in transmission
-Even parity-> total number of 1s transmitted are an even number
How do you use Parity for Error Detection?
-When data is transmitted, the parity bit is set at the transmitting end and parity is checked at the receiving head
-Wrong number of bits found (doesn’t know which bit was incorrectly transmitted) = error has ocurred
-If a transmission error is found, the receiving device notifies the transmitting end and the data is resent
How do you Check the Data Entries?
-Barcodes and other numbers have check digits which are used to make sure a number has been entered correctly
-A barcode scanner may misread the number and request it to be scanned again
-A checkout assitant or customer may mistype the number if they have to enter it manually
What is a Check Digit?
last digit in barcode
What is Encryption?
-the process of changing data into another form or code so that only people with access to a secret key or password can read it
-may not be written in readable form
What is the Ceaser Cypher?
-A basic encryption algorithm where the alphabets are displaced by a known amount
eg: Caesar code wherein the letters are displaced by 5 places
How does the Caesar Cipher work?
-The original message is called plaintext. The encrypt text is called ciphertext
-Only the sender and the receiver know the encryption key. Based on the encryption key, the scrambled message received is decrypted
What is Decryption?
the process of converting the encrypted message to plaintext
What are some disadvantages of CC?
-Can be easily cracked by unintended users
-Cannot be used for highly confidential info.
What is the Reverse Cipher?
A message is encrypted in reverse cipher by reversing the message
What is the Substitution Cipher?
Denotes encryption algorithms wherein each charcater is replaced by another character
What is the Pigpen Cipher?
Alphabets are placed in grids and the characters are encrypted using the shape of the section of grid it represents
What is Modulo 2 Encoding?
XOR operation: output is 1 only when both the inputs are different.
What are Transposition Ciphers?
The plaintext is rearranged in a new order to form cipher text. Two method:
-Rail fence method
-Route method
