Unit 3 Flashcards
Binary number system
- Computers use a binary number system consisting of only 0s and 1s
- Everything that a computer needs to process must be converted into binary format
- This format is used for storing numbers, text, images, sound and program instructions
Bitmap image file formats
- BMP
- JPG
- GIF
- PNG
- TIFF
What is sample resolution in terms of an image?
The number of bits per pixel
What is sample resolution in terms of a sound file?
The number of bits per sample
What is the typical screen resolution?
72 PPI (Pixels Per Inch)
Increasing the number of colours
More bits per pixel = more colour combinations
- 1 bit = 2 colours
- 2 bits = 4 colours
- 3 bits = 8 colours
- 4 bits = 16 colours
What is the relationship between colour and bit depth?
- Each pixel can represent a finite number of colours
- A pixel is attributed a number of n bits
- The number of combinations (2^n) dictates the bit depth and therefore the number of colours that can be represented
- A higher bit depth gives a greater range of colour and a better quality of image
How does the number of colours affect the file size?
An increase in the number of colours that are used in an image (the bit depth) will increase the file size
How does the size of the image affect file size?
- Size has two meanings
- An increase in the number of pixels in the image will increase the file size
- If an images physical size is increased by making each pixel larger than there is no change in the file size
Difference between analogue and digital sound signals
- Analogue sound signals are continuous
- Digital signals are discrete, this means the sound wave is sampled at regular intervals
how is sound converted into digital form (what is used to convert the sound waves)?
- Sounds must be converted into a digital from 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
ADC –> Processing –> DAC
What is a sound sample?
A measurement of amplitude of a sound wave at a given time
Sound sampling
- Sound is sampled using a sample resolution (bit depth) and sampling rate
- The sample resolution determines how closely the wave is sampled on the y-axis
What is the sampling rate?
- The sampling rate is the number of samples taken per second. It is measured in hertz (Hz)
- The frequency or sample rate per second affects the level of detail in the digital representation
- The greater the frequency, the greater the accuracy and file size
What is the sample resolution?
- The number of bits 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
Lossless compression
Lossless compression formats are able to reduce the file size when compressed but not lose any information
Examples of lossless music file formats
- FLAC (Free Lossless Audio Codec)
- ALAC (Apple Lossless Audio Codec)
- WMA Lossless (Widows Media Audio)
Lossy compression
- Lossy compression removes sounds that we can’t easily hear or that least affect the perceived playback quality
- Lossy compression leaves out some data from the original so can negatively affect the sound quality
- Useful for storing, downloading or streaming
What is the formula for calculating the size of an uncompressed music file?
sampling rate x bit depth x duration
Examples of lossy file formats
- MP3
- AAC (Advanced Audio Coding)
What is compression?
Reducing the size of a file
Why is compression software used?
Reduces the size of a file so it takes up less disk space, and is quicker to download over the Internet
How many bits are in a byte?
8 bits = 1 byte
How many bits does ASCII use to store each character?
7 bits
How many characters can ASCII encode?
ASCII encodes 128 characters
Advantages of using Unicode instead of ASCII
- Can represent a wider range of characters
- Can represent characters from a wider range of languages
What is the result of a binary shift to the left?
Doubles the original number
What is the result of a binary shift to the right?
Halves the original number
What is a pixel?
Short for picture element, is a single point in an image
What is the colour depth of an image?
The number of bits used to represent each pixel
How do you calculate a bitmap image file size (in bits)?
size = width x height x colour bit depth
How do you calculate bitmap image file size (in bytes)?
size = width x height x colour bit depth / 8
What is meant by sampling rate?
The number of samples taken in a second, and is measure in Hertz
How do you calculate the size of a sound file (in bytes)?
size = sampling rate x sampling resolution x duration / 8
What is image resolution?
- Refers to the clarity of an image as it appears on a screen or on paper
- Can be expressed as the number of pixels that an image contains per inch
Describe the affect of colour depth on the file size
- When colour depth is increased, the file size increases also
- The more bits that are used to to represent colours, the larger the storage space required to save the image
Describe how analogue sound waves are converted into digital sound
- Analogue sound is received by a microphone
- This is converted into an electrical analogue signal
- The signal amplitude is measured at regular intervals
- The values are rounded to a level (quantisation)
- The values are stored as a series of binary numbers
Sound waves are analogue, what does this mean?
Continuously changing
Why is file compression used?
- To reduce the amount of storage needed on a computer to save files
- Allow large files to be transmitted as an email attachment
- Allow a file to be transmitted in less time
What is lossy compression?
A data encoding method where files are compressed by removing some of the detail
When is lossy compression used?
To compress images, audio files and video files
What is lossless compression?
A data encoding method where files are compressed but no data is lost
When is lossless compression used?
To compress data and text files
What is run length encoding?
RLE is a simple form of lossless data compression in which runs of data are stored using frequency/data pairs
When is RLE not useful?
RLE is not so useful with files that don’t have many runs, and in fact increase the file size
When is RLE most useful?
On simple images such as icons that contain many pixels that are the same colour
What is Huffman coding?
- A compression technique used to reduce the number of bits used to represent each letter
- The more frequently a letter appears in the text, the fewer bits are used to represent it in a text file
State two reasons why data are compressed
- To reduce file size
- To reduce downloading time, typically used for streaming
