Representing images sound and other data Flashcards
(21 cards)
Analogue data
continuous
no limits to the values that the data can take
can take any values and can change as frequently as required
Digital Data
Discrete
can only take particular values
a digit signal must always take on a specified range of values can only change value at specific intervals
Digital to Analogue conversion
a device called an DAC (digital to analogue converter)0 is used
reads a bit pattern representing an analogue signal and outputs an alternating analogue electrical current
Analogue to digital conversion
ADC device takes 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 in Hertz (determines no. of samples taken per second)
usually a high no. as greater sampling frequencies result in better reproduction of analogue signal
once recorded, can be stored digitally as a bit pattern
Sample Resolution
no. of bits allocated to each sample
higher sample resolutions = greater audio quality
- increased file size
Size of a sound sample
duration of sample (in seconds) * sampling rate (in Hertz) and sample resolution
Nyquist Theorem
states that sampling rate of a digital audio file must be twice the frequency of the sound
MIDI (Musical Instrument Digital interface)
MIDI is used with electronic musical instruments which can be connected to computers
MIDI stores sound as a series of event messages (each representing an event in a piece of music)
What event messages contain
duration of note
instrument with which a note is played
volume of note
if a note should be sustained
Advantages of using MIDI over a sampled recording
Allows easy manipulation of music without loss of quality
notes can be transposed (can change places)
often smaller in size than sample audio files
no information loss when music is stored using MIDI (lossless)
Disadvantage of MIDI
Cant be used for storing speech and sometimes result in a less realistic sound than sampled recordings
Bitmapped Graphics
an image is broken down into pixels each of which has a binary value assigned to it
resolution of an image
number of pixels in an image OR number of dots per square in in an image
Colour Depth
Number of bits assigned to a pixel in an image is called its colour depth
(e.g. if a pixel has been assigned one bit it can represent 2^1 different colours)
calculating storage required to represent a bitmap image
no. of pixels (width x height) * bit depth
Metadata
describes information about other data
e.g. width, height, date created
Vector Graphics
Represent images using geometric objects and shapes
Vector VS Bitmapped Graphics
Vector - use shapes rather than pixels
can be scaled without losing quality
+well suited for simple images
Bitmap - enlarging results blurry or pixelated images
used for storing photos
Data Compression techniques
Lossy - some info is lost in process of reducing files size (e.g. resolution of a image is reduced)
- no limit to how much file size can be reduced
-quality of file is reduced
Lossless - no loss of info when using lossless
- size of can be reduced wi/out decreasing its quality(with limits)
Run length encoding
RLE is a type of lossless encoding
Reduces sizes of file by removing repeated information & replacing it with one occurrence of repeated info followed by no. of times it needs to be repeated
NOT all data is suitable for RLE (if theres no repeating data its useless)
Dictionary Based Methods
type of lossless compression
a dictionary containing the repeated data is appended to the file
Results in a sig reduction in size but dictionary used to compress has to be present in the file (in order for file to be reproduced, increasing the file size)
