Representing Digital Media Flashcards
What is a microphone?
A transducer, converts audio (sound waves) into an electrical signal
What are some ways of creating digital audio?
Sampling and synthesis
What is the anatomy of audio?
Sine wave (S(t)=a sin(2pi f(t-theta))
a is amplitude
f is frequency
t- theta is the phase
What does amplitude, frequency, and phase relate to sound?
Amplitude relates to the loudness of the sound
Frequency is related to the perceived pitch
Phase can depend on the relative location of the sound
Can any periodic function be expressed as a sum of sine waves? What about non periodic?
Yes
Yes just make the period as long as the signal
What is dB-SPL?
Sound pressure level
A relative logarithmic unit relative to the 20 micropascals
=20log(sound/threshold)
What are the hearing frequency limits!
16 Hz to 20 kHz
How does the upper bound limit for hearing change?
Varies between people and steadily decreases with age
What does digitizing sound involve?
Taking samples at a fixed rate and recording them
What happens when the sampling rate is too slow or too fast?
Too slow means it’s inaccurate
Too fast means the file is very large
What does the Nyquist-Shannon sampling theorem say?
The sampling interval must be less that or equal to half a period
Should sample periodic signal at least twice the frequency
If the frequency of a periodic signal is 10 Hz what should the sampling be at minimum?
20 Hz minimum
What is bit depth? What does a high bit depth mean?
determines the number of possible amplitude values we can record for each audio sample
higher means more accurate sample
What is a bit depth of less than 8 used for?
Can be used to record physical processes like blood pressure, heartbeat, motion due to walking or running (not sound)
What is a bit depth of 8 used for?
telephones, sometimes quantization noise can be heard
What is a bit depth of 16 used for?
High quality sound
CDs, MP3, DAT etc
What is a bit depth of 24 used for?
DVD-Audio, DTS
often used before or during mastering (sound processing/editing)
Combining bit depth and bit rate gives you what?
bit rate
What format is uncompressed audio typically saved in?
.wav
What is lossless vs lossy audio compression?
lossless is similar to text lossless compression
lossy removes imperceptible sounds, and reduces bit rate
What is codec? Where did the word come from?
Compression and decompression algorithms for audio (and video)
“compressor” and “decompressor”
What are some examples of codecs? Are they lossy or lossless?
FLAC, lossless (using a combination of run-length and Huffman encoding), used for archiving high quality audio, 62% comrpession
MP3, lossy, 13% compression, uses psychoacoustics, Huffman encoding, and lower bit rates (mobile devices)
AAC, lossy, 14% compression, Uses psychoacoustics, Huffman encoding, and lower bit rates, better quality than MP3 (apple and modern smartphones)
What are the 4 types of cells in human retina?
1 rod (low light vision)
3 cones (short/medium/long wavelenght sensivitve)
WHat happens when all three cone cells are stimulated by light?
you see white
What is the RGB colour model?
three integers (usually in the range of 0-255) describe the amount of red, green, blue required to produce the colour
What is white in RGB model, what about black, red, green, blue?
white: 255,255,255
black:0,0,0
red:255,0,0
green:0,255,0
blue:0,0,255
What are some other colour models?
CMYK (cyan,magenta,yellow,black)
HSL (hue,saturation,luminence)
YUV: (luminence,blue,red)
What is CMYK usually used for?
colour printers, start with white paper and subtract individuals components
What are HSL and YUV similar to?
colour wheel principle, better maps how the human brain interprets colour
How many hexadecimal digits are used to represent a colour?
6
RRGGBB
Where does the word pixel come from? What are they?
picture element,dots of colour on an image
What does resolution mean?
the number of pixels on an image (or dispaly)
size of image measured in pixels, sometimes reffered to as pixel density
What are vector graphicss? How are they created?
image that is defined using mathematical equations representing lines, curves, and polygons
Using drawing applications or text editors
What vector graphics be enlarged?
yes, without loss of detail or an increase in file size
What does SVG stand for? What are they?
scalable vector graphics
are text based files that can be compresseed accordingly
What are raster graphics? How are they created/edited?
Imagine that is comprised of a matrix of pixels
Painting applications
What is a drawback for raster graphics?
Suffer from pixelation
When enlarged the shape of the pixels becomes really obvious
What is indexed colour?
Popular way to compress images
Similar to keyword encoding, only saves important colors
What does GIF stand for? What are they?
Graphics interchange format
Allow for transparency and animation, has a compression rate of 10%, lossless encoding and uses indexed colour and coding related to huffmans (max 256 colors)
What does PNG stand for? What are they?
Portable network graphic
Lossless compression that uses ibdexed colours and run length encoding, compression rate of 7%, allows for transparency and more modern
What are png and gif best for?
Line drawings, logos, diagrams
What does JPEG stand for? What are they?
Joint photographic experts group
Good for photos, bad for images with text or sharp lines, lossy and has compression of 1-10 depending on quality, divides images into blocks of 8x8 pixels
What is virtual reality?
Immersive technology, images change with head motion to always show the proper perspective
What are some challenges for virtual reality?
Processing power, needs more complex hardware, images need to be generated faster than for movies, cannot be used for too long
What are videos?
Comprised of frames of still images combined with audio
What are some typical frame rates?
24-30 frames per second to 60 frames per second
What are spatial compression techniques? Example?
Use information from within the same frame to reduce file size (intra-frame technique)
M-JPEG
What are temporal techniques? Example?
Use data from nearby frames to reduce the file size (inter-frame technique)
MPEG
What are key frames?
Typically compressed with spatial techniques, can be independently reproduced, inserted automatically at scene changes and/or regular intervals to preserve quality
What are P and B frames?
Encoded by saving differences between it and key frames
P is predictive B is bidirectional
What happens if the nitrate of a video is too slow?
Becomes pixelated
What is CBR?
Constant bit rate,same but rate throughout the video
What is VBR?
Variable bitrate , can vary up to a maximum, higher nitrates during fast paced scenes and lower when there is no motion
What are some extensions associated with file containers?
.avi (audio video interface)
.mp4 (MPEG-4)
.mkv (matroska)
What are some example codecs for videos?
HuffYUV (Huffman, YUV colour space), Lossless, Huffman encoding, compression ratios around 47%
MPEG-2(Lossy,temporal and spatial compression,Huffman encoding Compression ratios around 3.3%, Highly dependent on bitrate and other encoding options
H.264 (Lossy,temporal and spatial compression, frame prediction, Predicts the next frame using spatial and temporal information, compares with
the actual frame, saves only the difference,Compression ratios around 1.6%)
• H.265(HEVC)
• successor of H.264: further 50 % reduction of size, much more complex!
• Often drains battery faster
EECS1520 Digital Media
A sound wave with a frequency of 2 kHz has a period of what? What is the general formula to solve this?
1/2000 sec
1/frequency
What is not rate determined by?
Sampling rate and bit depth
